Carburetors

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Theory

At the risk of atomizing a little more fuel onto the fire (a little carb humor there...), let's examine the basic function of a carb. Air flows through the venturi, and, thanks to the discovery by G.B. Venturi (1746-1822), is accelerated. This accelerated airflow causes a pressure drop, which in turn causes fuel to be drawn from the fuel bowl through a series of jets and passageways. Small venturies are by nature more responsive to small changes in airflow, with the result that an increase in flow rate will result in a more rapid increase in the rate at which fuel is drawn from the bowl. Unfortunately, a small venturi also poses a limit on the total airflow through the passageway. The traditional carb (Holley 4150/60, 4GC, et al) has a single booster venturi inside the primary bores. The Q-jet actually has multiple nested booster venturies. This, IMHO, is the secret to the greater sensitivity of this carb in part-throttle driving. Naturally, this additional restriction also compromises airflow (some Holley carbs which are clearly designed for W.O.T. only don't even have booster venturies, but use annular fuel discharge from the perimeter of the main venturi - which actually is a clever idea).

Now, the secondary side of the Q-jet is clearly designed for W.O.T. only. Not only are there no booster venturies, there isn't a venturi at all - just a couple of huge, irregularly shaped holes. The secret is the air valve. I'll go out on a limb here (seeing as how I usually _never_ have strong opinions...), and proclaim that while the vacuum secondary Holley carbs clearly have better driveability than double pumpers, the "mass airflow" sensing nature of an air valve (which includes the 4GC, AFB, AVS, and T-Quad carbs also) provides more accurate control of airflow based on engine needs than does an actuator controlled by manifold vacuum. Again, the ultimate goal here is to maintain a sufficiently high airflow (and resulting pressure drop) to properly draw and atomize the fuel without imposing an excessive airflow restriction.

This is true. However, the vacuum secondary Holleys don't derive their vacuum source from strictly manifold vacuum, as in port being plumbed directly to the manifold. The vacuum source is a small port that opens to the constriction in one of the primary venturis. So, as air flow through the venturi accelerates, an ever increasing pressure drop (increasing vacuum signal) will be realized at the point of maximum constriction--another discovery of our friend, Venturi.

So, the vacuum signal applied to the secondary diaphragm housing in a Holley is directly related to the airflow through the primaries. Consequently, both air-valve carbs and vac secondary Holleys respond to airflow. However, the Holley's response to airflow is an indirect response, whereas, the air valve carbs respond directly to airflow. Hence, I would give the air valve carbs an edge in this area. Nevertheless, don't think that vac secondary Holleys are simply responding to manifold vacuum, they aren't. They respond to VENTURI vacuum, which changes with airflow.

Of course, all of this is rendered moot by EFI systems. Electronics (much as I distrust them) replace the need for this delicate airflow balancing act, and fuel atomization is controlled by injector nozzle design, not a booster venturi or air valve. This, more than anything else, is the reason for the incredibly high outputs of today's motors.

All the systems in nearly all downdraft carburetors, whether 1 bbls. 2G's, Q-Jets, Holley, Carters, etc., utilize an "up and over" system that effectively prevents leaks -- for idle, off-idle, main metering, and secondary fuel. I am simplifying quite a bit here, but suffice it to say that all fuel in the carb must first travel through the jets at the bottom of the bowl, then turn UP through a well (either idle, main, or secondary well) to a point slightly higher than the fuel level in the bowl. As it travels through the well it is emulsified with air. Once it reaches the top of it's well, it makes a 180 degree turn and travels back down.

For the idle mixture, it travels back down the idle channel to the idle discharge port. For main system and secondary fuel it doesn't make an abrupt 180 turn but makes a sideways turn of some angle until it gets to the fuel discharge tube, from there it exits the fuel discharge nozzle in the center of the booster venturi and makes it's downward trek through the carburetor bore, itself. Secondary fuel likewise, exits the secondary discharge nozzle. The point is, you could take the jets OUT of the carb and it wouldn't leak.

To see for yourself, the next time you have a Q-Jet apart, gut the main body, take the jets out and everything, and fill the bowl with water. You will have no leaks, unless some plugs are bad or something.

Another thing to consider. The metering rods do not seat in the jet. When it is not running, the rods are up, in their richest position, not seated. When the engine is running at an idle, engine vacuum pulls the rods to their leanest position, which still allows sufficient fuel for operation. Even secondary rods do not seat in the jet.

The secondary orifice in a Q-Jet is a stainless steel disc with a .135 hole. Other brands use a material of the same type as the primary jets. But Q-jet is the only one with secondary metering rods.

[ Thanks to Joe Padavano, Thomas Smith for this information. ]

Calculating Flow (CFM) / Sizing

It's easy to overestimate the CFM requirements of an engine. You can calculate the cfm your engine flows using the formula everyone on this list has probably seen many times:

engine CFM = (engine CID x max rpm x volumetric efficiency)/3456

Now for a stock 455, I'll assume you won't go above 5500rpm, and stock heads with stock exhaust manifolds is probably going to keep your volumetric efficiency no higher than 85% at 5500 rpm. So your engine needs - only 615 cfm!

Here is where the universal misunderstanding begins. Usually people ignore the formula above, or use a rule-of-thumb like "use 1.5 times the airflow predicted by the formula". The fact is that 4-bbl carburetors are rated for their cfm when there is 1.5" of vaccuum sucking on them. If you use a 600cfm carb on your 455, there will be about 1.5" of pressure drop across it when your engine is at max rpm. Since atmospheric pressure is about 30 inches of pressure, you just took away 1.5" from 30", leaving the engine about 5% down on power compared to a much bigger carb. If your engine makes 300 real-world horses with those exhaust manifolds and stock heads, using the 600 cfm carburetor costs you only about 15 hp, at max rpm. Everywhere else in the rpm band the engine will be more responsive with this small carb, the car will feel better in traffic, get better gas mileage, etc. Unless the car is a drag-race only car, the 5% power drop at max rpm is really not very significant - how many minutes a day does your car spend at 10% throttle? How many at WOT? How many at WOT and 5500 rpm?

So I'd say pick any carb that flows at least 600 cfm (I believe all the true 4-bbl Quadrajets do), tune it to match your car and driving style, and you'll enjoy life with it. On the other hand, bolt on a 1050 cfm Dominator, and you'll probably be miserable with the result.

You probably know that the airflow through a carburetor is proportional to the square-root of the pressure drop across it, so 3" of vacuum will flow (square-root of (3/1.5)) times more than 1.5" of vacuum across the same carb. Since 3/1.5 = 2, and sqrt(2) = 1.4142, that means that 500 cfm at 3" is equivalent to only 353.5 cfm at 1.5" of vacuum!

Here's a neat one. I used a 600 vac secondary on my 1995 NON-roller cammed 502 chevy engine in the truck while I worked on the 750 vac secondary. VERY RESPONSIVE off the bottom end! Makes a difference with the smaller bores at lower rpms, hence, showing that even the biggest cubic inch engines can run better on the street with a smaller cfm rated carb. HOWEVER, there's a trade off with this. Top end was off considerably with the smaller carb on the big engine (instant restrictor). Sometimes, you can just go TOO small.

This is where I say the Q-jet when properly calibrated, or even Holley's spread-bore, gives you the best of both worlds very economically. (as long as it isn't those high $$ electronic doobies). Of course, once repairs were made to the 750, top end power was restored.

Very simply, the problem with using a "big" carb on a smaller motor somewhat equates to using gas with more octane than your engine requires. Sure, it'll run, but you don't NEED it, and you're gonna waste money. The major concern with the intake system is velocity. If you can keep the velocity up, the more responsive it is. Smaller ports/valves, carbs make this possible. Again, this is simplified. Intake runner size, port size, cam lift/duration and valve size have a LOT to do with it. You can go huge with everything, but lose on the bottom end. The best running engines have MATCHED components working together to make the most power per stroke. Bigger isn't necessarily better for a street engine. I've personally had strong running, properly built Chevelle 327's make me look bad in my stock 69 W-30 400 years ago. (still though, they didn't kill me by a bunch) About the only time bigger is better IMO, is fuel line size.

Ultimately, the way to build power is to move more air and fuel (by weight) in and out of the engine. Since the inlet side is driven by vacuum, huge ports and runners will kill part throttle power due to the inability to get that large volume of air moving. Of course, small runners provide too much of a restriction at high rpm. This is the reason that some newer cars have variable geometry induction systems. A set of small diameter, long runners is used for part throttle driving, with a set of short large diameter runners for W.O.T.

Anybody remember the old Offy Dual Port manifolds? These had two sets of runners, small ones connected to the carb's primary bores and large ones under the secondaries.

I've read an article not to long ago in Hot Rodding if I remember right. They were comparing the 600cfm - 850cfm with a typically built street performance 350 engine. They got the most horse power and torque out of the 750. They pretty much said the formula isn't quite always true but is a good rule of thumb. GM recommends the 750 Holley carb on their 350 ZZ create engines. My ZZ4 just absolutely rips with the 3310 750 Holley. A magazine back in '90 dyno tested a ZZZ create engine with a 750 cfm carb and then changed to the Edelbrock quad setup using 2 600 cfm carbs. The horsepower and torque increased. That's 1200cfm!! It was only rev'ed to 6,000 rpm. It mainly depends on the setup. Big roller cams really like the bigger carbs.

I had a 350 Olds built to the Edelbrock RPM specs in my H/O for 3 years. I ran a 3310 750 vaccum secondary spread bore Holley and it was very responsive and had tons of bottom end, mid range, and upper. The 600 cfm will give you better gas mileage. The engine package is totally streetable. Edelbrock recommends a 750 cfm carb with their Olds RPM package.

[ Thanks to John Carri, Mike Rothe, Joe Padavano, Chad Schwartz for this information. ]

Firsts

All the talk about Holleys and Q-Jets prompted me to do a little research. What I found is, I think, rather interesting in a trivial pursuit sort of way. Not really significant when it comes to selecting a carb, however. Now, while I maintain my assertion that I like and use Q-Jets, let's just say I'm just doing a little bit of sacred cow tipping!

Things for which the Q-Jet is revered:

[ Thanks to Thomas Smith for this information. ]

Flow Rates

It depends on the size of the venturi. Refer to the info below.

Venturi size CFM
1 3/32 278
1 3/16 352
1 1/4 381
1 5/16 423
1 3/8 435

Keep in mind, 2 bbls and 1 bbls are rated at a 3.0 in/HG pressure drop an 4 bbls are rated at a 1.5 in/Hg pressure drop.

[ Thanks to Thomas Smith for this information. ]

Nomenclature

For a squarebore carb, the primaries are significantly smaller and typically do not share the same centerline as the secondaries in a spreadbore.

In a squarebore, the bores are closer to the same size, though, in some squarebore carbs, like 390 cfm Holleys, the primaries may be noticeably smaller than the secondaries. However, even then the bores share the same centerline.

Many aftermarket intakes, such as mine, and the Edelbrock Performer and RPM, etc, are "dual pattern" intakes (the area where the carb bolts on looks like a butterfly). Either a squarebore OR spreadbore can be physically be mounted on them with no adaptors.

Carter AFB's, AVS's and Holley 4150's, 4160's, and 4010's are squarebores. Carter Thermoquads, Rochester Quadrajets, and Holley 4165's, 4175's, and 4011's are spreadbores.

If the intakes are true squarebore intakes (four individual holes) or true spreadbore intakes (again, four individual holes, though of different size), you cannot really bolt a squarebore carb on a spreadbore intake and vise versa.

In the case of putting the squarebore on the spreadbore intake, the primary bores of a spreadbore, though small, are spread further out than the primaries of a squarebore. The squarebore primaries of the carb would not line up well with the holes in the manifold. Also, most spreadbore primaries are smaller than most squarebore primaries, so even if they did line up, the manifold would present a sudden, constriction.

In the case of putting the spreadbore on the squarebore intake, the secondaries of a squarebore intake are smaller than those of a spreadbore carb, so in that case, you'd have significant flow problems. Again, you'd have the problem of the primaries not lining up right. HOWEVER, Edelbrock does make an adaptor for installing a spreadbore on an OEM squarebore manifold, and vice versa.

Most 3310 Holleys are variants of the 4160 family of Holleys and are 750 CFM. It has a metering block in the primary side, which USES JETS and a metering plate in the secondary side which does not use jets. They are vacuum secondary carbs so you can use your secondary diaphragm kit.

[ Thanks to Thomas Smith for this information. ]


Rochester, Holley, Carter, etc Carbs

What is the difference between Rochester, Carter, Holley, and Edelbrock, Barry Grant, et alii.

The first three brands have been available on factory production cars. Edelbrock and Barry Grant, among others, are aftermarket only, but are based on, and thus functionally similar to certain carbs from Rochester, Carter, and Holley.

Holley carbs were found on some production cars through the ages, but certainly not to the extent of the proliferation of Rochester or Carter carbs. please let us not forget the eminently forgettable Holley/Weber progressive (yes, progressive, as in one primary and one secondary) 2bbl carb found on such notable cars as the Vega, Pinto, and a slew of offshore 4 bangers... There were a whole bunch of these "OEM Holley" carbs produced. In truth, Holley carbs became much more popular after they became available only as aftermarket carburetors.

In point of fact, the real rival to Rochester over the years was really Carter. The Thermoquad was Carter's answer to the Q-jet. It is also a spreadbore and has the added advantage (or disadvantage) of having a phenolic fuel bowl. This was intended to prevent heat transfer to the fuel with the higher underhood temps of emissions-controlled engines, but the plastic had a tendency to warp and split.

Sadly, Carters are quite obscure nowadays due to the proliferation of Q-Jets and Hollies. They are still available, but fetch a premium, as do new Hollies or Edelbrock carbs.

Edelbrock has two basic carb models, one based on the Carter AFB (square bore) [right guys???], and the other based on a Carter spreadbore that was much more similar to the Q-jet. The difference between these late-model Carters and Quadrajets was that the Carter had two Power Pistons, one for each primary venturii. This made the Carter eminently tunable: imagine boring only cylinders 1,3,5, and 7 0.030" over (one bank of cyls with bigger displacement), and leaving the other 4 alone, but still being able to tune the carb perfectly for both banks!

Barry Grant carbs are based on the typical Holley squarebore design, but are much more tuneable than Holleys are (and you pay for the advantage!). This because you can change the size of the venturiis by adding or removing sleeves to/from the venturii.

There are other carb brands available, one of which is Predator. The Predator (and the Kendig before it) use 2D venturies - basically a square passageway which narrows in one dimension only. Obviously this makes the variation in the venturi much easier to implement, as you only need to move a linear surface, not a circular one.

Which One????

This is a subjective question. The Holley guys will scream Holley. The Q-jets will cringe and support their local carb.

Whichever route you go, be prepared to expend time and energy learning how it works. There is nothing worse than tweaking a mechanism about which little is known.

My Holley doesn't bog. It can be susceptible to bogging if either the accelerator pump linkage has too much play or if too light a spring is installed in the secondary diaphragm housing (in the case of vacuum secondary carbs).

I don't know for sure what it was "designed" for, but I do know the still very popular 4150 series was introduced in 1957 for production vehicles, and that it and the 4160 model were used on production vehicles for some time after that. Also, I have had my Holley on my car in anything but WOT conditions. Take start-and-stop Washington D.C. traffic, for instance. Gotta love the downtown area around 15th and Constitution when the tourists are in full force! I even have good idle with it.

Now, one thing about Holley that separates it from Q-Jet is that Holley is a company that makes many different carburetors. The Q-jet is, itself, a carburetor. So, to blanketly say a Holley is optimized for WOT would be much more accurate if you're referencing the huge Dominator. However, that would not necessarily be true of a 390 cfm or 450 cfm Holley 4160 with vacuum secondaries. These would provide good economy and part throttle driveability even on small V8's. With enough tuning, I've gotten my 600 cfm squarebore to be acceptably efficient.

For a pure drag car, I dunno. I don't know from experience, but I've read some accounts that claim the Q-jet can be made to provide stellar performance even in drag racing. See, I can say nice things about Q-Jets, too :-)

I will sorta agree, that for any car that sees street driving in real traffic, the added sophisitcation of the Q-jet is unmatched. I will say that even though I've gotten acceptable efficiency from my Holley, it is not as forgiving of lead-footedness as the Q-Jet. However, here again, the carb under consideration is but one model: the 4160, part 1850-3. Another Holley, say a 4175 spreadbore with vac secondaries, (which has primaries almost as small as a Q-Jet) may provide much better economy. Some of the later Holleys even allow full electronic hookup to a car's ECM to maintain OEM emissions requirements.

It is true that Holley carbs are "high performance", in the sense that they are very good for drag racing, and they are almost always the carb of choice on dedicated drag-race cars. At wide open throttle, Holley's work very well, and this is what they are designed to do best. They are also modular, which makes them easy to rebuild, and relatively inexpensive.

However, if you drive your car on the street most of the time, you probably drive at full throttle for no more than a few seconds per day, if that. Under these conditions, there is relatively little airflow through the carburetor; for a carburetor to work well on the street, it has to have sensitive boosters that respond to small amounts of airflow. This is where the Holley carbs are at their worst. They have the least sensitive booster venturis of any carburetor I know of, which makes them poor low-speed performers; the usual result is poor gas mileage and increased exhaust pollutants. In the US where gas is relatively cheap this may be liveable with, in other countries where gas costs as much as US $5 per gallon, a Holley is probably not a great choice for a mainly street-driven car.

Both Carter carburetors and Quadrajets are designed from the start to be very responsive to part-throttle, low-airflow conditions. They use double or triple booster venturis, vacuum-responsive metering rods, and air-valve secondaries, all of which make the carb adapt better to the engine under the varied loads and speeds of street driving. Carter carbs are elegantly simple designs and easy to understand, Quadrajets are quite complex and it takes a little more effort and knowledge to tune one of them correctly, but once you get it done you will get better gas mileage, less exhaust pollution, better part throttle response, and just as good full-throttle performance. In my opinion, for a mainly street-driven car a Carter or Qjet is considerably better suited than a Holley. Between the Carter and Qjet it comes down to personal choice - each has advantages, especially if price is a factor and the Qjet is already on your car! The Qjet is a spreadbore carb (small primaries, big secondaries) which helps it perform well at both part-throttle and full-throttle.

For my money the best street carb of all is one that is no longer available - - the last and best Carter design, the ThermoQuad, which replaced the Carter AVS, which in turn replaced the Carter AFB that Edelbrock and Carter still make to this day. Like the Qjet, the ThermoQuad was a spreadbore design, and unlike the Qjet, it is a very simple and yet very effective design.

Unfortunately, mainly because of nostalgia and the association with 60's muscle cars, people buy older, less efficient carburetor designs in preference to newer ones. Holley found that out with their new-design 4010 and 4011 carbs which no one would buy in enough quantity to be profitable, and the only Carter that attained cult status is the ancient AFB design that Carter had already improved upon even before the muscle car era ended - it was already obsolete in the 1960's!

Some cynic once said something like "a carburetor is an incredibly ingenious and complex device designed to provide exactly the wrong air/fuel ratio under all conditions of load and speed". That's definitely not true, but sadly there's an element of truth in it, which is why every new car uses EFI.

Here's a little idea of mine to think about. A carburetor is supposed to sense airflow and meter fuel in response, which is how it "adjusts" to varying loads and engine rpms. An ideal carburetor would be so sensitive to air-velocity that it not need an accelerator pump at all (all the accelerator pump does is compensate for the carbs inability to respond to quick changes in airflow). It turns out there is at least one type of carburetor that *is* so responsive to air-flow that it does not, in fact, need an accel. pump - that is the Weber carb and its derivatives (Japanese copies , etc).

Carrying this line of reasoning a little further, a carb that gets by with a smaller accelerator pump (a smaller squirt of gasoline is adequate to cover up the carbs deficiencies) is a better street carb than one that requires a bigger accel. pump shot to cover up its flaws, i.e is more responsive to airflow and more likely to give you good mileage and good throttle response. A little investigation shows that ThermoQuads, Qjets, AFB's, AVS's, etc, all have smaller accel pump volume than Holleys. Worst of all are double-pumper, mechanical secondary Holleys, which, while great race carbs, don't really do a very good job of sensing the engines requirements at all - they need not one but two massive accelerator pumps to cover up their poor response to airflow.

While I don't doubt that someone who know what they are doing can make any of the above carbs work on their car, I think it is clear that by design the ThermoQuad, Qjet, AVS, etc, are better suited to an engine that works over a wide range of rpm and loading, i.e., any engine that runs mainly on the street.

While I have never used a Predator carb, I am fascinated by it because it is the only carb I know about that apparently needs almost no calibration to run on any engine. It is supposed to be truly responsive to airflow through the engine, so much so that just about the only tuning needed is to swap one part (a fuel cam) that matches the fuel curve to the engines likes. No jets, needles, air bleeds, secondary vacuum diaphragms, air valve spring tension, booster venturi's, power valves, choke pull-offs, etc, etc. to adjust, because the carb actually responds to what the engine wants, rather than trying to guess it by using a series of complex mechanical do-dads to approximate the desired fuel curve. Unfortunately Predators are very expensive, may cause hood clearance problems with many cars, and don't have many intake manifolds designed to work with them (they have a different fuel discharge pattern from other more common carbs, and the wrong intake manifold will turn that into poor air/fuel distribution to the different cylinders).

Of course other factors such as availability, cost, and reliability go into any carb decision too. But if someone laid all these carbs out in front of me, all brand new, shiny, and functioning correctly, and asked me to pick one for my daily-driver car, the Holley - any Holley - is the last one I would pick. I would pick the ThermoQuad first, the Qjet next, the AVS after that, then the AFB, then vacuum secondary Holley's, and finally mechanical secondary Holley's.

Anyone preparing to vent their displeasure with my choices, remember, this is just my opinion! There's room in the world for a lot of different ones!

I've tried the Q-jet and Holley Double Pumper on my 71's 455. I'm happier with the 750 vacuum operated secondaries. I've slightly modified mine, but it works like a CHAMP. Either should do fine, as long as it's set up correctly for the application.

Difference: double pumper MAKES the engine take fuel, cutting down mileage, the vac secondaries ALLOW the engine to take on more fuel as needed. The DP gets away with the mechanical secondaries opening RIGHT NOW by adding a second pump shot to cover the big air hole it created. Which the engine MAY or MAY NOT be ready to deal with. It will respond, but with a lot more wasted fuel. With the vacuum secondary, the engine "tells" the secondaries when to open by the vacuum signal. Loose metaphoric comparison: try to make a baby eat without making a mess. Won't happen.

Weight of vehicle makes a big difference on vac secondary carbs. Generally, trial and error is the only REAL method when deciding on diaphragm springs. You want to set it up to where you can't even feel the secondaries kick in. A bog or slight hesitation could mean that you need a stiffer secondary spring. Go as light as spring as you can, starting with the stock spring, then go one step lighter at a time until a bog appears. Then move back up to the next stiffer spring. That should do it. This is similar to adjusting the Q-jet secondary air valve "wrap", only more time consuming.

Motor size doesn't make as big a difference as Volumetric Efficiency. If your engine can't move a lot of air, (restrictive exhaust, small ports, etc.) then a smaller CFM carb will do. I think a 70% efficient (generous here) 455 requires something along the lines of around 720 CFM at 5000 rpm. So a 750 VAC secondary seems good. I've seen too many people overdo it with huge CFM carbs. Not good. Even if you used a 600, your street driveability is greatly enhanced due to the somewhat smaller bores, meaning more intake velocity.

Generally speaking, treat carb CFM as you would octane. Don't recommend using significantly MORE than you're engine requires.

Well, Matt is right about the effect a spreadbore has on gas mileage when running on the primaries - and yes the sound is nice when the massive secondaries kick in. However, there are several carbs that Holley has produced, such as the one I will be running on my 468 BBC, which have the same design. It's list number is 6211. It is an 800cfm spreadbore double-pumper. This type of carburetor would give you the advantages that the Q-Jets do. Some people say that even so, the Q-Jets are easier to tune for economy, but if you want a good "race" engine go with Holley. Perhaps. However, like you said, John, there are many more books related to tuning Holley carburetors than there are to Rochester carbs. I have 2 excellent books on Holley carbs which I would highly recommend myself. Both are published by S-A Design. The two books are:

SA08 - Super Tuning and Modifying HOLLEY CARBURETORS
SA27 - HOLLEY Rebuilding & Modifying (A Guide To Holley Modular Carburetors)

Both books have a chart in them with all the Holley list numbers and the specs of the carb. Although all the pictures are in black and white, I never really had to strain to figure out what the pictures were - very detailed descriptions, very well written books.

HOWEVER.....if you did want to stick with Rochester's Q-Jet carbs, there are books on rebuilding and tuning them too. I just picked up Haynes TECHBOOK 10230 (2068) - ROCHESTER Caburetor Manual. It covers all the Rochesters, one and two barrels as well, not just the Q-Jet, with sections on tuning, repairing, rebuilding and modifying them. I have not yet read this one through since I just got, but the Haynes manuals I have used in the recent past have been very good quality. Haynes also publishes a manual on Holley carburetors, but I don't have the number of it.

[ Thanks to Karl Aune, Thomas Smith, John Carri, Mike Rothe, Trevor Lee for this information. ]


Rochester Carb Specifics

ROCHESTER ID: First note that thru '65 nearly all Rochester carbs had ids on tags, and '66-up were stamped on the side of the carb. Probably the most noteable Olds exception to this is the numbers on tags on the '66 Tri-Carbs. I agree something must be mixed up. Of course *anything's* possible! Generally there are exceptions, but the following applies: The next to the last digit must be a 5 to be an Olds carb. 1 is Ch**vy The last digit: odd is manual, even is auto Third digit is year, 2 is '64-'69 Fourth digit is last digit of model year. Fifth digit is carb type Kurt W-MACHINES@prodigy.net Actually, the Q-Jet wasn't released until 1966. I assume you mean a Rochester 4GC. Joe Padavano Regarding the ID on those carbs, I'm puzzled by those numbers. Not just the fact that they don't show up in the books *at all* (and the book lists all Rochester #'s from 1951-1990), but the fact that these numbers don't match the usual patterns. Here's what you'd expect from reading the two carb #'s: >7029683 >7031575 70= These first two digits simply designate a Rochester product The third digit is the series of the carb, followed by the last digit from the year of the date of the application; Thus, the 2 and 3 designate a carb from the '60's (0 and 1 were the Fifties, into the early Sixties; 4 and 5 were the Seventies). The fourth digit (9) in the first carb # makes this to be a Rochester carb for a 1969 something. The fourth digit in the second number, however, is odd, since the "3" series carbs usually designated the early emissions-controlled carbs used in the '65-'67 cars, when things like A.I.R. and C.C.C. required specially-calibrated carbs, and a "1" wouldn't fit in with that. Then again, Rochester often deviated from the standard practice of the fourth digit being the last digit of the year. Now, the last three digits usually specify the application of the carb within a particular year, and usually, the second-to-last digit correlates to the manufacturer (but not always, and not consistently, but most all of the time). The chart that Year One reprints is pretty accurate: 1,2=Chevy 3=Cadillac 4=Buick 5=Olds 6=Pontiac (although sometimes a 7) Now, a 7 or 8 in the second-to-last position usually indicates a non-GM application (like Checker, Chrysler, Ford, marine and the like), so I'd be inclined to think the carbs you list are something like late-60's Rochester carbs not produced for GM applications. One other possibility; service carbs usually employed the numbering schemes from the years they were introduced, not the years they were actually intended to be installed on as replacements. It's possible that the two carbs are late-60's service carbs for earlier applications. Sorry I can't be of more help; can you describe what the carbs look like, and double-check those numbers? Are there any other ID tags or numbers, or part numbers on any of the carb components? The books really don't even have anything close to a range of numbers like the ones you provided. Robert Barry CA CAR: I would look at the Carb number, of course it may not be original but = the Quadrajets had a specific number for California only cars. This = could at least tell you if it was originally sold in CA. steven@purplemonkey.com (Steven Saraceno) YEAR OF CAR FROM CARB YEAR: >What is the number on your carburetor? That will helpo identify engine >year. Not necessarily; the last few years Rochester got lazy, and used the same carb with an identical number, unadjusted for years. I believe the number was 17086553 or something like that. I can check this tonight. Bob Barry QJET: However, the Q-Jet is a little bit of a different animal. Witness, for example, GM's usage of them on 307's, and 305's. The secret is in the small primaries (smaller than the primaries in a 600 CFM Holley) which optimize low and mid range. The secondaries are important, as well, in the Q-Jet's applicability to engine sizes that, according to the CFM charts, are way too small for the Q-Jet's CFM rating. The key here is in the mechanically operated throttle and the air valve that responds to engine demand, and is not directly linked to the secondary throttle. The fuel flow in the secondaries responds to air valve position, not secondary throttle position. These features make the secondaries, in theory, very much like a variable-venturi carburetor. These two things: small primaries and the air valve secondaries, have allowed the Q-Jet's usage on a wide variety of engine sizes. Now, to be sure, on a 350, let alone a 307, the engine demand may never get to a point where the secondary air valve completely opens. But this is the beauty of the Q-Jet: it effectively limits the carb's CFM to whatever the engine needs. All this assumes proper adjustment and calibration, of course. Thomas P. Smith ELECTRONIC QJETS: The electronic Q-Jet receives commands from your car's on-board computer (ECM -- electronic control module) to constantly vary it's fuel metering to maintain as close to ideal an AF mixture as possible. > and is that correct. Most likely. Q-Jets went electronic in California in 1980 and the rest of the states in 1981. > What's the difference? Nonelectronic carbs don't use a computer. They respond soley to airflow and engine vacuum. Electronic versions do this plus respond to computer inputs, which come from continuous analysis of your AF ratio via the oxygen sensor. For this reason, an electronic carb has the potential to be more accurate and economical. As for the rest of your questions, check everything else, i.e., timing (though the computer probably keeps your timing pretty close, you can still check your base timing), spark plugs, wires, distributor cap and rotor, vacuum leaks...the works. Many times symptoms of other problems are blamed on a carburetor. Also ensure the carb is plugged in. There are a couple small wire harnesses on the carb. One comes from the ECM and and the other plugs into the throttle position sensor. They must be plugged in for proper operation. I'd also ensure the choke is operating properly...and is plugged in, too. A carb rebuild and adjustment may fix your problem, but eliminate other variables first. Thomas P. Smith smithtp@ix.netcom.com EGR MODIFIED CARBS: >Exhaust Gas Recirculation. Basically runs exhaust through the intake to >take up airspace that could be used by the air/fuel mix. This extra >space can cause more horsepower, when used correctly, and Detroit wanted >to avoid that possibility. Well, to be honest, they *did* use all the cubic inches of an engine, but only at wide-open throttle. Under that, however, the EGR valve was in effect, and driveability suffered. You can see how they calibrated the carbs for this: pre-EGR carbs had a very gradual taper on the primary and secondary rods, but the EGR carbs tend to have real fat rods, except the last 15% or so, where it tapers radically; mixed with bigger jets, these fatter rods gave leaner operation when the EGR was working, but richened enormously at WOT. Theoretically, it allows you to have full power at WOT, but in effect be driving a "smaller" engine at all other throttle openings. Unfortunately, driveability with the pre-FI cars suffered, and the response from such a jetting setup was an off/on switch effect; full power at WOT, but *much* less power at lesser throttle openings. The power from my non-EGR 455's is much more linear than the power from my 403 with it's EGR-era carb. Gotta rejet that thing one of these days... >OK, OK, so maybe it didn't happen ~quite~ like that, but it does cool >the combustion process, (can't burn exhaust very well!) which reduces >NOx, allows more advanced timing, and one more item for technicians to >diagnose! No kidding about that advanced timing thing; many EGR cars have initial timing in the 16'-20' range! Bob Barry Since the EGR isn't active at Wide Open Throttle, you won't gain anything at WOT by disconnecting the EGR. At part throttle you may gain some power, but if you want more power at part throttle you just open the throttle more :-) Not having an EGR will require rejetting the carb and adjusting the timing. The EGR helps prevent detonation and allows a bit more timing to be run. That extra spark advance helps make up some of the power loss from the EGR. Tom Lentz, tlentz@ior.com Subject: Re: Plugging my EGR valve? ><< You can either replace your current carb with a pre-EGR carb, or [snip] >well now, i am running a '80 quadrajet on my '72 350 with an open element and >no egr. so im thinking maybe i can get some jets out of an older carb? is it >possible to just swap them out? if i can, it will be done! i have had a >little taste of detonation every now and then and its very annoying. now Replacing the jets with ones from an older carb will not be the answer, most likely (though they'll swap out), since the jets in the emissions-era carbs were pretty big to begin with (in fact, and older carb might have smaller jets, making matters worse if they were swapped in). The jetting on a Q-jet involves not only the jets, but the tapered metering rods that ride in and out of that jet. With the EGR carbs, GM tended to go with larger jets, and even *larger* diameter rods, so the effective area of the jet at most points in the metering rod's travel was smaller, but could be very rich when the thin end of the rod was in the jet. What you'd need is a set of rods that will give you richer operation under part-throttle load, so you're looking at a set of rods with a thinner tapered section. Here's the bad news: the pre-EGR carbs ("70xxxxx"-series), with the rods you're looking for, used a different style of primary metering rods than the '80 carb you've got (which is a "170xxxxx" series carb); those are the only tuning items that don't interchange (jets and secondary rods do). Now, there are thinner-taper "170xxxxx" series rods available, but you've got to do your homework first as to what rods and jets you've got in there now, and what carbs might have the rods you're looking for. A couple other options: 1) You could start with a "70xxxxx" series carb as a baseline, as its fuel curve would be calibrated for the pre-EGR setup you've got. Or 2) get the Edelbrock tuning kit that they've got for the '75 and later Q-jets; it's about $100, but would have a range of primary rods with which you could fine-tune your carb. that >i put on headers, i cant tell the difference between the exhaust and pinging. >so im always a little uptight as to whether or not its detonating. that knock >sensor msd makes looks pretty sweet, so i might give it a try too. Pinging costs hp, as well as damaging your engine, that knock sensor would be a nice addition; is it universal, or is there one designed for the specific frequencies detonation would produce inside an Olds V-8? Bob Barry Actually the '75-'80 (car) carbs are different than the pre '75, and post'80 car carb castings. The 800CFM's are very common after 1974, which will hurt around town mileage. The primary rods/hangars are different on the pre '75 carbs. The 1974-earlier carb castings are essentially "flat" across the front of them. '75 later more angled. Don't forget, if the carb was designed for EGR, not running EGR will cause lean condition. Good thing about post '74 carbs are the availability. You could easily get enough primary rods /jets to solve the non-EGR problem. Dorian Yeager '75's had REALLY lean jetted Q-jets. Not just Olds but all GM. '75 was the year Rochester changed everything and then later worked out some bugs in the design. Bob Blanchard '73 CARBS: Can't help you on the distributor, but that carb is an Olds carb from 1973; you're missing the final digit on it, though- something should come after the final "5". If it read "7043250" it would be a regular 350 carb; if it read "7043251", it would be a regular 455 carb. If it read "7043252", it would be a Toronado 455 carb. If the final number were higher than "2", then it might be from something special. If there really is no final number on there, then it's either mis-stamped, or a poorly re-stamped replacement carb. If you do find a final digit on there, post it and I can look up the jets and rods for you, and see how far off from your original '71 carb's settings you are. Bob Barry 800 CFM QJET: From: Michael Ecker Subject: 750cfm vs. 800cfm Q-Jets For those of you out hunting for the bigger Q-Jets, here is an easy way to tell. Look down the primary bores with the throttle held wide open. The 750 models have a ridge all the way around the bores, just below the booster venturis. The 800's only have two raised bumps, opposite each other, sticking out from the sides of the bores - also just below the booster venturis. This may be hard to visualize at first, but once you find an 800 you will know it immediately compared to the 750. By the way - I've found 3 of the 800's recently in the local junkyard, all in 1976-1980 Caddy's. One nice bonus is that all three had electric chokes. Two of them came off 425 Caddy engines, and these had much richer secondary rods than the other (which came off a Caddy 368). I checked my 1976 Cadillac 500 (1706_363) carb and my 1980 Turbo 301 carb. They are the 800 CFM variety. I have a generic Ch@vy one, that is 750 cfm. If I find my 7039273, 69 428 HO carb, I will check its size, but I do not believe there were any 800 cfm units until 1971, but I cannot confirm this. If anyone can, please let me know. Easy check: Look down the Venturi, observe a raised area around the outside of the Venturi (this is shaped specific, don't grind it!!), if it goes around and is the same all around, it is a 750, if it is fairly thin, and has a small bump near where the Venturi booster is supported at, you have a 800 cfm. If you measure the area inside that ring, it measures 1 3/32 for a 750 and 1 7/32 for a 800 cfm. The butterflies on the manifold size appear to be identical in size, the change is in the Venturi. So you can't just bore it out. The 80 Turbo Trans AM is probably the last production car from Detroit with a 800 CFM carb, and it is a special Q-Jet. It will work well on any other GM product that can use the fuel inlet "normal", ie, not like a Ch#vy. It goes straight in. The only significant difference is the vacuum under the APT metering rods is EXTERNAL, not internal. No problem! Add a vacuum line to it!! (Else you will KILL mileage!!) This was controlled by the PEVR, Power Enrichment Vacuum Valve. This is not a computer controlled carb!! 81 was 80 was not, but the 81 carb should be a great hi performance carb for any computer controlled car. This Q-Jet also came with the richest secondary metering rods ever, DX, near needle sized at the tips, and a very rapid rate to get there. It has the absolute richest 70% to 100% secondary operation. No other is richer from 70%+ secondary operation. This is needed for the Turbo boost at 10 PSI and no computer to control it. ESC is part of the distributor, but is stand alone. (hint, make that 11:1 compression live on pump gas??? Works for me!!) AND, they also have pull over enrichment for the primaries, something to deliver even more fuel when under high load/high rpm situations. This is a special feature found only on large engine carbs. Ok, the drawback? It has 51 primary metering rods, sorta lean, so, you change it to be richer based on whatever car you have. I was designed for a single plane intake on a 301. No mine is not for sale!! But I can dig up the carb numbers if anyone needs them, just email me directly. Thomas Martin I beg to differ on this one. I have removed tons of Q-jets from late model cars that were 800 cfm. The bigger carb was more common in the later models than the earlier ones. My CCC carb of an 85 Olds 307 is 800 cfm. All I did was drill the vacuum hole to send the signal to the power piston and get a proper float bowl and air horn and it now makes my 350 run great without a computer. I am now waiting for my 403 short block to arrive and I will make this same carb work fine on it also. Q-jets are the best carb around if you understand them. But you must know them to make them work as you wish. Mrmagic442@aol.com Sorry but Joe is correct (as usual! :)) The only 800 cfm Q-jet ever produced was put on 455 Buicks from '71 to '76 period. The increase in 50 cfm comes from a larger primary venturi. The secondary side flows the same as 750 cfm units. Tony Waldner According to Doug Roe's book on Q-jets, it's the primray venturies that are bigger on the 800 cfm models, 1 7/32" dia vs. 1 3/32" dia for the 750 cfm. Dude retrorockets@snet.net Seem to be most common on Olds 75-8(?) Even CCC ones off of the 307's can be (mine was) no use to a non CCC person, but interesting. Stamped numbers, unless you have Karls big book, are probably worthless for you, as every year the number changed for the most part. WAY too many combinations of Q-jet numbers to look through to find the same number. The ones I have came off of the following vehicles: '77 Cutlass w/350, '78 Vista Cruiser with 403 (800's rumored to be on most every 403) '75 Buick Apollo w/350, and '86 Cutlass w/307 (CCC though) If you can stand dealing with the EGR/smog nature of the mid 70's carbs, look in those years. Most Chev's don't seem to have them, plus due to the different fuel inlet, not very useful. To spot one, look down the primary venturi, and look for the "bump" in the lip down an inch or so. If there is a solid "lip" then it's 750. Single bump, 800. Get Doug Roe's Q-jet book. Perfect picture on one of the pages comparing the top down view. Dorian Yeager A "Tech Tips" page I got from Dick Miller Racing says pre-EGR 800 cfm q-jets are "relatively rare," and found on 1971-1973 Buicks & 1973 Pontiacs. I have a carb from a 73 Buick, & it is NOT 800 cfm. Hitson, Roger HITSOR@mail.dmh.state.mo.us TRI-CARB: ..I thought the tri-carb set-up was for the 394's, not the newer style blocks (455's, etc.). Is this an aftermarket manifold, or an original tri-carb set-up (thought you've said before that you had an original tri-power, but could be wrong)... This is the relatively rare L-69 tri-carb setup offered only on the 1966 442s. Something like 2000 of these units were made, with 54 of them serving as the basis for the W-30 option that year (when fitted with the O.A.I. setup). It is based on a cast-iron manifold with progressive, fully mechanical throttle linkage (didn't the J-2 operate the outboard carbs with vacuum diaphragms?). Three individual small chrome open element air cleaners were used, but were replaced by the large chrome O.A.I. shroud on the W-30. The other unique feature of this manifold is the block-off plates for the carb heat riser. A small removable plate on either side of the carb flange can be unbolted and repositioned to block or open the center carb heat riser passage without removing the manifold or resorting to epoxy or aluminum filler. Joe Padavano Drew Senko wrote: ..Also, I am told, GM made some sort of rule after 66 that you couldn't install tri-power on a factory car (except C*rvette). If this is true, this would explain why the option was for 1966 only, and wasn't available in 1967. You can treat this all as heresay, if you are worried about receiving incorrect info. It is correct to the best of my knowledge... You are absolutely correct, Drew. This is also why the GTO was limited to a single Q-jet after the 1966 model year. By the way, the 66 W-30s cleaned up in C/Stock in NHRA that year, so that tri-carb O.A.I. combination really made some HP. I meant to add this to my other post and forgot. Thanks for finishing the story. Joe Padavano evans1@serv01.net-link.net Subject: Tricarb dilemma (Was Re: Rochester 2-Jet I.D.) Actually, I have two #7029683 and one center 7031575 residing on a '66 tricarb setup that I purchased in 1984 *not as cheaply as Chris Witt's, but close*, and the numbers puzzled me as well. And, yes, I've checked, rechecked, and rerechecked the numbers, and those are correct. No, the Tri-Carb isn't original to the car. I just found it odd that these carbs aren't even original to this intake, and I am trying to get a little background on them, is all. Mostly in case there may be some specification oddities that I'd just as soon not be surprised with if I have to do any work to them in the future. The center carb appears to be a typical run-of-the-mill 2-Jet, but the end carbs have no idle circuits and, doggone it, certainly look like they came on there. Mike Evans Mike- the Rochesters for the Tri-Carb have the triangular "tin" tags held on to the carb by one of the cover screws. The tag has the carb number on it. Kurt & Rose wmachine@en.com >Actually, I have two #7029683 and one center 7031575 residing on a '66 >tricarb setup that I purchased in 1984 *not as cheaply as Chris Witt's, but >close*, and the numbers puzzled me as well. And, yes, I've checked, >rechecked, and rerechecked the numbers, and those are correct. > >The center carb appears to be a typical run-of-the-mill 2-Jet, but the end >carbs have no idle circuits and, doggone it, certainly look like they came >on there. This setup *in fact, the entire car* has been in storage for the >last ten years, and I only routinely checked the numbers as I was cleaning >them. Frankly, I couldn't care less about the originality or the value of >this "custom" setup, as I have no intention of ever selling it, but now I'm >intrigued as to the origin/birthdays of these carbs. The fact that the end-carbs have no idle-circuits certainly argues for them being factory tri-carb units; do they have mechanical or vacuum linkage on the end carbs? >I also was leaning toward the service replacement reasoning, though I have >no way to check that.....any further thoughts? One problem with the "service replacement" idea is that Rochester utilized the numbering scheme of the year that the service replacement was produced, not the year of the application it was produced for, so, for example, service carbs for early-70's cars that were produced in the late-70's bear the "170xxxxx" numbering scheme, so even if they were service replacement carbs, they would have been produced in the late 60's. Actually, the #7029683 carbs are somewhat less mysterious than the one center #7031575 carb, as they seem to be 1969 service replacement carbs for some tri-power application (though the "6" in the fifth digit there is mysterious, as the 2-jets invariably have an "0" or "1" there, and it's only with the later emissions carbs that the higher numbers enter in). The 7031575 is mysterious because there is simply nothing like a "7031xxx" carb in the books; it wouldn't be a 1971 service replacement carb, as that would bear a "7041xxx" number, and the only "703xxxx" carbs are the '66 and '67 emissions carbs, which all read "7036xxx" or "7037xxx". Are you sure you can't make a scratch so it reads "703_7_575"? It still wouldn't match anything in the book, but at least it would seem to be a carb from within our universe... ;) No, these are real mystery carbs you've got there; they don't even fit in with the oddball variations that you find on some Rochester carbs. Maybe you can claim they're "experimental" carbs??? :) Bob Barry From: "Brian Lorway" Subject: Re: '66 tripower carbs (L-69 and W30) front unit = 7026055 (L-69 and W30) center unit = 7026056 (L-69 and W30) rear unit = 7026057 Info. was found in book "4-4-2 by the numbers" from Supercars Unlimited. Problem here is that there are something like 5 different venturii diameters for 2-Jet carburetors. Pontiac could have used a different venturii carb, in which case the carb could be jetted differently. _________ In some cases, they did. Early Poncho tripowers used the small venturi carb in the center position, with the larger ones on the ends. I believe the 66 Pontiac tripower does use the same size carbs in all three locations; in fact, I seem to recall that they are effectively identical to the 66 Olds carbs, though with different tag numbers. These carb numbers are unique to the Olds tripower application. More importantly, the numbers are on a separate tag held with one of the air horn screws. The tags get torn off and can be easily switched. A better approach would be to get the jetting specs and modify a set of large venturi 2GCs to work. Yes, you'll need to remove the choke mechanism from the end carbs. Alternatively, as I noted, the Pontiac carbs are functionally equivalent and much more prevalent at swap meets. Joe Padavano I'm not much up on Pontiac stuff to say for sure, but I believe they use a different throttle linkage than the 4-4-2. Although the Pontiac carbs and linkage could probably be adapted if you are not concerned with originality. The 66 4-4-2 Tri-Carb used #68 jets in the front and rear carbs. And #63's in the center carb. These are the 60 degree, long taper style of 2 bbl jets. You'll also need to do something about the idle circuits on the front and rear carbs. Greg Rollin ANEROID CAVITY: While "aneroid cavity" does sound suspiciously like a naughty body part on an alien female, the words rang a bell from my childhood, when I was reading a long obsolete physics textbook that one of my older siblings had once studied. The book talked about an "aneroid barometer", which was a barometer which used a small metallic enclosure evacuated to high vacuum to measure atmospheric pressure - like squishing a sardine can with your fingers, the vacuum enclosure flexes slightly as atmospheric pressure on the outside changes, and a linkage detects that motion and displays it on a dial. The evacuated metallic ocntainer was called an aneroid cavity or aneroid chamber. So what's the connection to carburetors? I know that many smog-era carburetors attempted to provide altitude compensation. As those of you who have driven a carbureted car through Denver or drag-raced at high altitude tracks know, the carb jetting needs to change with altitude to compensate for the thinner air at high altitudes. The reason is that x molecules of gasoline need y molecules of oxygen to burn properly, meaning a given mass of gasoline needs a matching mass of air to burn properly. But that given mass of air has less volume at low altitudes than up in Denver, and unfortunately carburetors respond to the volume of air flow through them rather than to the mass of air flow through them. The "aneroid cavity" in the quadrajet, by Dorian's description, moved a metering rod in and out of a jet in the air horn; the purpose of this was undoubtedly to try to modify the jetting as required to track changes in the altitude at which the car was being driven. Probably so the car could pass smog checks in Denver as well as Los Angeles. John Carri aneriod cavities are not found on CCC carbs, as the computer takes over this function with the M/C solenoid. Jeff Newman newsance@netzero.net

GM apparently sold Rochester Product Division (or just the carburetor producing portion) to Magnetti Marelli. Karl mentioned this previously.

Apparently, MM bought the capital tooling equipment (Karl reads this as heavvy stuff like CNC lathes....) from GM and in the guise of Weber U.S.A. is producing Q-Jet parts for GM's Service Product Order department. Also the finer tooling seems to still belong to GM.

Further; Q-Jet part production will apparently cease in 1999.

So Technically, one could say that new Q-jet parts are still being made, but are being used for carbs being serviced.

[ Thanks to Karl Aune for this information. ]

Factory / Stock Information

Identification / Decoding

As for a quick decode, conside Q-jet model number 17056259. Q-jets all begin with either "70" or "170", indicating a Rochester product (the "170" carbs are the newer ones, usually post 1974, though trucks used the "70" carbs until the 1980's; big difference is that the primary rods don't interchange between these two series carbs).

The next two digits indicate the year. Actually, the "6" is the last digit of the year, and the "5" is just different from the "2" that you would find in that place on a 1966 Quadrajet (I believe the 1986 would probably have a "7" in that place)

That leaves the last three digits, which usually distinguish the particular application of the carb for that year. The second to last digit usually indicates the manufacturer, with "5" being Olds (but just like that i before e, except after c thing, there are exceptions, especially in the mid-'60's). For example, most 350 4bbl Olds carbs ended with "250", and the Toronado engines ended with "252" for a long time. The standard big-blocks had the "251" carb, and the 442 and W-31/W-30 carbs would end in "254" to "259". No, this example is not a 1976 W-30 carb, though.

A good decoding explanation can be found at http://www.oldengine.org/unfaq/leadfoot/qjet1.htm.

After 1974, to meet the more stringent emissions requirements, makers specified specific carbs for engines in particular models. So while every 1971 350 4bbl carried the carb # 7041250, in 1976 there were different numbers for the Cutlass carb, the Delta carb and the Omega carb.

As to whether it's a good choice for a performance buildup, it's not a *bad* choice, though I know I have built up a better selection of the "70" series primary rods than of the "170" series rods. Whatever you use it for, you'll likely have to rejet it and use different rods; with the "170" series carbs, GM seemed to favor larger jets, but thicker primary rods, which netted less fuel flow. Jets and secondary rods will interchange between series.

A few Carter Spreadbores ended up on some Cutlasses in the early 70's. Rumor has it that Rochester could not produce the amount of Q-jets needed so they subcontracted Carter to "pick up the slack" . You can id one of these pretty easily. On the side of the carb it will say Quadrajet by Carter instead of the usual Rochester Quadrajet.

[ Thanks to Jeff Easton, Bob Barry, Chris Smetana, Steve for this information ]

Jets and Metering Rods

Here is a list of the carb #'s, along with the #'s for the jets, primary rods and secondary rods. These are the #'s from the AC-Delco manual, so they are only as accurate as that is. Also beware that I am just copying what is in the book(s); thus any errors or inaccuracies there are here as well.

Especially interesting are the 1970 W-30 and W-31 with manual transmissions; it looks like those Quadrajets used the 2-bbl jets with no metering rods! (this has been confirmed by some original W-30 owners; note this is only the 1970 M.T. applications) 

Year & Application   Carb #   Main Jet Primary Rod   Secondary Rod
1966-
400/425 M.T.         7026250  7031971  7031844- "44" 7033655- "AU"
330 M.T.             7026254  7031971  7031844- "44" 7033658- "AT"
400 A.T. w/o A.I.R.  7026256  7031971  7031844- "44" 7033655- "AU"
330 A.T. w/o A.I.R.  7026255  7031971  7031844- "44" 7033658- "AT"

1967-
330                   7027036  7031971  7031844- "44" 7033658- "AT"
330 w/ C.C.C.         7027135  7031971  7031845- "45" 7033658- "AT"
330 w/ A.I.R.         7027153  7031971  7031845- "45" 7033658- "AT"
400/425 w/o A.I.R.    7027156  7031971  7031844- "44" 7033655- "AU"
400/425 w/ A.I.R.     7027157  7031971  7031845- "45" 7033655- "AU"
400/425 w/ C.C.C.     7027032  7031971  7031845- "45" 7033655- "AU"
400 M.T. w/O.A.I.(5)  7027156  7031971  7031841- "41" 7033655- "AU"
425 Toro w/o A.I.R.   7027131  7031971  7031844- "44" 7033655- "AU"
425 Toro w/ A.I.R.    7027130  7031971  7031845- "45" 7033655- "AU"
425 Toro w/ C.C.C.    7027132  7031971  7031845- "45" 7033655- "AU"

1968-
350                   7028250  7031971  7034849- "49B" 7033658- "AT"
350 w/ W-31           7028255  7031974  7034849- "49B" 7033658- "AT"
400/455               7028251  7031971  7034849- "49B" 7033655- "AU"
455 Toro              7028252  7031971  7034849- "49B" 7033655- "AU"
400 W-30              7028254  7031975  7034849- "49B" 7033655- "AU"
400 "442"             7028253  7031972  7034849- "49B" 7033655- "AU"

1969-
350                   7029250  7031970  7034849- "49B" 7033658- "AT"
400/455               7029251  7031970  7034849- "49B" 7033655- "AU"
455 Toro              7029252  7031970  7034849- "49B" 7033655- "AU"
400 M.T.              7029253  7031972  7034849- "49B" 7033655- "AU"
400 W-30              7029254  7031975  7034849- "49B" 7033655- "AU"
350 W-31              7029255  7031974  7034849- "49B" 7033655- "AU"

1970-
350                   7040250  7031970  7040701- "52C" 7033658- "AT"
455                   7040251  7031970  7034849- "49B" 7033655- "AU"
455 Toro              7040252  7031970  7040701- "52C" 7033655- "AU"
350 W-31 (6)          7040255  7002657     -           7033655- "AU"
455 "442" MT          7040253  7031969  7040699- "48C" 7033655- "AU"
455 W-30 M.T. (6)     7040256  7002658     -           7033655- "AU"
455 W-30 A.T.         7040258  7031969  7040701- "52C" 7033655- "AU"
455 W-33, 442 A.T.    7040257  7031969  7040701- "52C" 7033655- "AU"

1971-
350                   7041250  7031970  7034849- "49B" 7038256- "AS"
455                   7041251  7031970  7034849- "49B" 7033104- "AK"
455 Toro              7041252  7031970  7034851- "51B" 7033549- "AX"
455 M.T.              7041253  7031969  7034849- "49B" 7033655- "AU"
455 A.T.              7041257  7031969  7034849- "49B" 7033658- "AT"

1972-
350                   7042250  7031969  7034850- "50B" 7045778- "CG"
455                   7042251  7031969  7034850- "50B" 7045779- "CH"
455 Toro              7042252  7031969  7034850- "50B" 7045779- "CH"
455 "442"             7042253  7031969  7034847- "47B" 7045779- "CH"

Notes:

The following abbreviations are used:

A.I.R. - Air Injection Reactor
C.C.C. - Climatic Combustion Control
O.A.I. - Outside Air Induction
M.T. - Manual Transmission
A.T. - Automatic Transmission
Toro - Toronado
W-30 - High-performance big-block engine
W-31 - High-performance small-block engine

(1) Except for the applications otherwise noted, the carb listed was used in all applications of that motor. The usual exceptions are the specific engines used in the 442 and Toronado models.

(2) The last two digits of the part number designate the size of the jet orifice in thousands of an inch. Thus, a jet with the part # of 7031971, has an orifice of .071", and is stamped with a "71".

(3) The last two digits of the part number usually designate the size of the largest portion of the tapered shank of the rod in thousands of an inch; Thus a primary rod with the part # of 7031844 has a taper of .044" at its largest point. This two-digit number is also stamped on the rod. After 1968, the metering rods had a double or triple taper, and had a "B" or a "C" suffix stamped on the rod, respectively. These triple-taper rods, however, do not have a part # corresponding to the size of the taper, although the stamped number on the rod does correspond to its diameter.

(4) The secondary metering rods are stamped with a two-letter code. A guide to the comparative dimensions of these rods can be found on pp. 152-153 of Doug Roe's Rochester Carburetors.

(5) The O.A.I. carburetor had the same part # as the standard manual transmission 442 engine carburetor, but had different primary metering rods.

(6) Note that the W-31 and W-30 carburetors apparently used 2-bbl jets with NO primary metering rod, and thus no power piston. If any owner of one of these carburetors could confirm this, I would appreciate it (I thought it was a typo at first, but it is specifically listed this way for both W-31 and W-30 applications).

Triple Taper Primary Metering Rods

The Doug Roe book on Rochesters, in the chart showing all the primary metering rods, shows the only triple taper rods ever made were rods 7040699- 48C and 7040701- 52C, and these were used on 1970 Oldsmobiles only. Olds was also the only one to use the triple rods.

The triple taper rod was and attempt to fine-tune the fuel curve more precisely than the dual-taper rods, introduced in 1968, allowed. I guess it didn't work well enough to justify the expense, since they went back to dual-taper rods the very next year. It seems that the carb section of the Olds engineering department was getting a lot of overtime calibrating the 1970 units, what with the triple-taper rods and the no-rod "W" units. Apparently, not everybody went to the dual-taper rods in 1968, however.

Mondello's tech guide states that the replacement of the primary metering rods is unnecessary, since they all have the same size small-end. While this is true, it is only applicable for WOT use. At part-throttle, the taper of the rod is somewhere in the jet, and it's this taper that determines part-throttle response. You know, when you want to merge or accelerate like a responsible citizen; not too cool to drive at WOT all the time when you're taking your family out for dinner. Therefore, for street use you do want to select a primary metering rod that will complement your setup.

A source for OEM carbs is at http://www.blake.com/oemcarbs/.

"70" and "170" Q-jet Metering Rod Differences

There are two kinds of primary rods (all secondary rods are interchangeable); the rods from the "70" series carbs (up to about 1974) don't interchange with the "170" series rods ('75 and later; the carb number stamped in the side of the carb will either begin with "70" or "170", which indicates which series it belongs to).

The "170" series rods are .080" longer. I've compared them myself to verify this. In general, the "170" rods are thicker for much of the taper.

Well, I was overgeneralizing; some "70" series carbs were used for a couple years on EGR carbs (on Cadillacs through 1976), though I don't know of any non-EGR "170" carbs.

Usually, though, the large diameter of the rod is thicker for more of the rods travel, to make the mixture leaner when the EGR is in operation. The thin diameter is the same as the non-EGR rods, as the EGR cuts out at WOT, so you have a sharper taper near the tip, compared to a non-EGR rod. The same holds true for the secondary rods.

The 70xxxx rods are longer than the 170xxxx rods. I swapped a set of 70 rods into a 170 series carb and it ran fine, but idled poorly and was down on power because the rods only come up a set amount, and the longer rods didn't allow the tapered end to come up fully into the jet.

[ Thanks to Bob Barry, Jeff Easton for this information ]
======================================================================

Diagnosis / Repairs

1966 QJETS: Yes, a '67 carb is a better (the best) carb for your '66. However, they are getting harder and harder to find. Especially one that has not been hosed or worn out. But, even '67 well plugs are prone to leakage. A '66 carb will do just fine though. Theorectially, all the very early Patty Melt type '66 carbs were recalled by GM or already burned up and none should still exsist. Very doubtfull if you have one. The Patty Melt variety used a very funky needle and seat deal and the front plug (no longer used) popped out. There are fixes for that too if one should ever turn up. The MAIN reason (to this day) that any Gen 1 Toro carbs burn up (no fault of the carb either) is the use of a rubber fuel line bewteen the fuel pump and carb (high pressure side), and the lack of use on an inlet filter COMBINED with an inline filter BEFORE the fuel pump (low pressure side). Those two things alone will give you 99.99% protection from burning up. I have had very good success with '66 carb rebuilds and see little reason why yours should not turn out good as well. Greg "Sparky" Kalkhoff Peter--if your '66 carb has a plunger and diaphragm thingie under the float instead of a needle and seat, it's the bad kind. Either way, I always keep a fire extinguisher in the floor of the back seat (not in the trunk!). Have never had to use it on any car, but it's force of habit. I put it by the fender whenever messing around with the fuel system. Mark Pruett markpruett@hotmail.com Since the 'Quadraflame' problem was *supposedly* fixed by Jan. of 66 according to Ralph Braun, you could also find a carb that dates from a later 66 model in lieu of one off a 67. Lisa The patty melt thing just refers to it burning up. Yes, the very early '66 QJ's burned up with regularity. I have been told that NONE are in exsistance anymore. If you have one, it already buned up your car, so this should be a moot discussion. But, you never know! I have to believe there are a few lurking about yet somewhere. Let's just hope it's not on any of our cars! But to be serious for a moment. As far as I'm concerned, the early '66 QJ's are not really an issue as they really should be all scraped by now. But, the two biggest reasons why Toros STILL burn up to this day (and you hear about it every now an then) is the use of a rubber fuel line where the steel line should be and putting an external filter with rubber on both sides between the fuel pump and the carb (cutting the steel line in two). I know I have said this before, but it BEARS repeating, and will keeep doing so every now and then. THROTTLE SHAFT BUSHINGS: I did this in mine (got sick of paying $40 for it) I got the bushings from "carburetors unlimited" in Kent and Fife(? maybe auburn) Washington (just down the street from me..think they have a web page). The bushings were, if I remember correctly, $5 for two of them. Personally, I think there is no reason to bush anything BUT the primary throttle shaft, and only on the drivers side, as that is the only one that has a substantial load on it (it will always move side to side, but front to back or top to bottom movement should be minimal) Even the the carb shop does only that one. If it's really bad, you will have problems keeping your idle speed steady, as the shaft will be real sloppy. It's a "J" drill bit also I think, which was a bit hard to find (and pricey for a stinking drill bit) Of course I promptly lost it after doing the carb. Dorian Yeager LEAKING QJET: The smell of fuel can sometimes be attributed to the baseplate of a Q-jet leaking into the intake manifold, if so that can be easily repaired with a carb overhaul, if not you can get jets from any GM dealer. As to black smoke during acceleration you may need to adjust the secondaries on your carb. To do this with the motor off on the carbs right (as seen from drivers seat) there is a screw head pointing at you from the rod that runs across the carb that the upper secondary butterflies rotate on. Push down on the secondary butterflies (upper) to see the how quickly they rebound, there is a small allen head screw which is the "lock" for that rod which controls the spring tension and the rate at which the secondaries open. Loosen the allen screw the end of the rod is slotted for a regular screwdriver, tightening this makes the secondaries come in slower, loosening it does the opposite, hold the rod where you want it and retighten the allen screw. But be careful, only make minor adjustments, 1/8 of a turn will make a difference in how quickly the secondaries come in. I adjust mine to where the car no longer bogs (throws up a split second) when I stand on it and I'm done. A lot of people tell me there is no secondary adjustment on a quadrajet, I simply smile and ask if they wanna race. Ade Lindsay Doug Roe mentions grinding the plugs only if you're going to be removing them altogether to drill out the lower secondary channels. If you're merely epoxying the plugs for "just in case" purposes during a routine rebuild, all that's necesary is cleaning (Roe actually shows a small file being used to clean, not grind, the area) and epoxying the plug area. Clean with solvent to remove gas buildup. BTW, the thick plugs that are used in later Q-Jets rarely leak (I didn't say it could NEVER happen). The early style cup-type plugs are the susceptible ones. If the carb is, indeed leaking, it is very practical to repair. The offending location is usually the secondary well plugs. With the throttle body removed, look on the underside of the main body and you'll see a large protrusion directly ahead of the secondary bore. This protrusion fits into a recess in the throttle body and looks like two enclosed cylinders joined at the sides. These are part of the secondary well and they are sealed with a couple of stainless steel discs, which are visible. This is often the point of leakage. To repair, clean the area and rough it up slightly, then apply a fuel-proof epoxy to seal it. I've used JB Weld successfully. Just for good measure, seal the primary well plugs, too. These are ahead of the secondary well plugs and are near the front of the main body. They are very recognizable because they approach each other at an angle and are smaller than the secondary plugs. Once the epoxy has thoroughly set up, reassemble the carb. I've even done this to carbs that had no signs of leakage at the well plugs. It's cheap and easy insurance. Be sure to get the surface thoroughly clean with no gas, oil, or other types of residues and slightly roughed up with a file or fine sandpaper, as I said. If not the epoxy may not adhere tightly and you'll be doing this again later. BTW, get yourself a carb rebuild kit when you do this. You'll want to replace the gasket between the main body and the throttle body. It is not necessary to completely disassemble the carb, but this might be a good time to do so since you'll have it off the car, anyway. Thomas P. Smith SIDE INLET PROBLEMS: >I pulled off the air cleaner assembly and saw a small puddle of gas on the >intake manifold. This was coming from the round "plug-like" area at the >very front of the carb. - just to the driver's side of where the fuel >line enters the carb. and the fuel filter is housed. > >Questions: >Is this the area of the Q-Jet that is notorious for developing a leak? Scream mode on: this is a Design Flaw of the early Qjet with the side inlet design. After a carb fire, I eventually switched to Holley for precisely this reason. The 66 factory service advisories tell tales of this problem, so they knew about it. I don't know if there is a permanent fix, but I thought it best to get the h*ll away from these carbs before they really blew up the car. The reason the plug is there is that the casting has to be drilled and you can't drill around a corner. So they drilled, then plugged the hole. >What is the best fix? A later model Qjet, or , if you prefer, a Holley, Carter, etc. >What is an inexpensive temporary fix so that the car can be driven safely? I've tried various kinds of goo, but I don't trust them, so I switched. Perhaps others can help you on this. Fix this as soon as you can for your safety and that of your passengers, from someone who's been burned (literally). cf STRIPPED FUEL FILTER THREADS: From: GChris1511@aol.com I had the same problem with my quad and NAPA fixed me up. the threads on the old one were around 7/8 the self tapping replacement was 1" boring out the hole was the difficult part. The best approach is a new float bowl. The second choice is a heli coil, though the Quadrajet inlet fitting is a bastard size that is hard to find and expensive. The thread cutting oversize fittings can be used, but BE VERY CAREFUL. The only way to use one of these safely is to take the carb apart, screw the oversize fitting in, FLUSH THE INLET VERY WELL and never touch it again. These fittings tend to break off all sorts of bits of the casting which flow downstream at unexpected times. Since this is past the filter, they usually foul the needle and seat resulting in a very dangerous condition. Tomco sells a fitting known as "deepsieze," this is a compression fitting using "O" rings. These work OK in my experience, but you must keep an eye on them and replace the "O" rings every few years. My only experience on this issue is with Quadrajets, so I can't comment on the application to other carbs. Good luck. Two other options I forgot to mention: 1. There is some kind of kit out there that uses epoxy and a release agent to recreate the threads. 2. Some carbs can be drilled and tapped to the next larger size. I have seen a rebuilt Quadrajet that had been repaired like this. It would not accept a standard fuel filter housing. Fred Nissen I was the one who mentioned the Helicoil kit. Its size is 7/8-20, P/N 5549-14, roughly $135 IIRC. I believe it'll fit practically any Q-jet in existence; not sure about Holleys or two-barrel Rochesters or WCFBs or AFBs. Oh....you'll also need a 57/64 (.891") drill, and it's about another $25! Extra inserts (7/8-20 x .330 are P/N K318-14-1. You know this is a serious kit, 'cause instead of a cheesy vinyl plastic envelope like smaller Helicoil kits, this one comes with a custom-fitted latched metal case. Helicoil is now owned by another outfit, so check with a machine tool jobber. Phil Brandt f111a@prismnet.com CARB FUEL BOWL DETERIORATION: Actually, it's the oxygen that causes the flaking, hence, the term oxydation. It's similar to rusting. The water just accelerates the process. Remember high school chemistry class? Oh well... By the way, carburetor castings are made of zinc. I don't know if what is commonly referred to as "pot metal" is the same as zinc, however. In most carburetors, the air horn and main body are zinc castings and the throttle bodies are aluminum. A notable exception to this is the Carter AFB, which is made entirely of aluminum. According to Muscle Car Review, the "AFB" actually means Aluminum Four Barrel; not too exciting, huh? As for coating the carb, which has been kicked around on this thread, the gold-brown color of the zinc castings, is a dichromate finish. My understanding is that it is supposed to inhibit casting oxydation. Holley offers a re-dichromating service an any carburetor. Check out their website to read more about it. Joe Padavano's idea that newer fuels are part of the problem is backed up in another Muscle Car Review (MCR) article a few issues back. I recall a statement that oxygenated fuels contributing to deterioration of older fuel systems. I assume this can include the carb. Finally, the latest issue of MCR warns that some carb cleaners, fuel additives, octane boosters and lead substitutes can deteriorate paper fuel filters. Thomas P. Smith smithtp@ix.netcom.com Q-JET BOG: From: Thomas Martin Get the Q-Jet book from HP-Books, "Rochester Carburetors", worth it!! You can shorten the pump shaft up to 3/32 in or so, as long as it does NOT come above the pump fill slot. You can also bore it out and put in a 2GC pump rubber seal. This will bring you up to 50cc. Also, make sure you are in the inner slot of the arm. This is for racing, or HP street. Normal engines don't need. Make sure secondary fuel supply tubes are not blocked or missing (mine had fallen out on my E4MC, overtorqueing the carb) Drill 4 #68 holes in the secondary fuel supply tubes will enhance fuel delivery to secondary fuel shooters. Start 3/32 up, 1/32 apart. See book for details. (First try to find a #68 bit!! 1/32 is the preferred size, #68 is acceptable) As for bog, 1976 Olds says to set the air valve spring to 1/2 turn, as measured by the air valve doors just enough to hold closed, (tap several times to make sure you are right) then add 1/2 turn. This goes for 85 307's (442 and "Y") as well. You can tighten up to 1 full turn to slow secondaries. Look for fuel spewing. Make sure the fuel is staying in the carb venturies. Could be a strange pressure condition causing other problems. Make sure the front dashpot/choke pulloff is good, press pulloff in, put finger over vacuum source hole, and release, it should stay in for no less than 10 sec. If not, replace. Make sure that the airvalve when opens that it does lift the metering rods up, I had one that the cam broke and the valves would open, but no fuel could come in. The obvious, look for vacuum leaks everywhere. A major leak will bog at lower RPM. Best of luck with it! Q-Jet is still the most refined carb ever made! From: "Michael Johnson" Subject: Re: QJET hes.....itation Secondaries are spring loaded. There is an allen head screw on the underside of the top plate on back left corner of carb. This screw sets spring tension on the upper flapper thing. ( What is the proper name for this piece?) Loosen this set screw and turn the small common headed screw next to it to reduce the amount of tension. This will reduce the delay you experience on full throttle accel. You will not be able to completely eliminate this bog but this adjustment helps alot. Hope this helps! From: Miguel Morales The Q-Jet Bog can surface from using one on an engine that has a long duration Cam loping the idle and causing low vacuum situations from Low RPM. Similar problems are cured on Holleys by switching to a stiffer secondary close spring. The lower manifold vacuum when the Bog begins after you floor it will allow the rear dashpot to release way too early. Actually tighter Air Valve spring tension up to a point, will help to alleviate the Bog in these low vacuum cam situations. That's one reason a blower helps maintain engine breathing once the pedal goes to the metal. I'd even guess that hooking that rear dashpot's vacuum hose to an alternate source, above the primary throttle plates (metered vacuum?) would help keep this vacuum up to the minimum needed much above what manifold vacuum can supply. Another problem that usually may not be cured is to use a Q-Jet with way too rich secondary metering rods once the Air Valves start opening. I agree, the Q-Jets are reliable, giving impressive throttle response when rebuilt correctly with few leakage problems, even on the electronic ones. From: "Dan Mann" Both of the problems that you found could have been responsible for causing the bog. I had an Olds 350 in a 3420 pound Nova that started out with the same problem. The car had 3.90 gears, posi, a medium duration cam (224 in, 224 ex duration at .050 lift; .501 lift on both sides), approx. 2600 rpm stall,performer 350 intake manifold, stock 7a heads, .030 overbore with cast dished pistons(with a very approx. compression ration of 9.0 to one),turbo 350 tranny, some really cheap headers with 1 3/4" primary pipes (supposed to fit early cutlass with big block, fit nova with no modifications)that ALWAYS leaked, mondello roller tipped rocker kit, and last but not least a big Quadrajet that came off a Pontiac station wagon equipped with a Poncho 400. The car ultimately ran 14.00 at 97.75 mph. I rebuilt the quadrajet when I rebuilt the motor, and it worked fine (I originally used a performer cam:204,214 at .050 lift and I can't remember the exact lift but it was mild). With the quadrajet the car had awesome throttle response and pulled hard throughout the throttle range. Many people were running chevy motors in their cutlasses around here with holley's carb, and most were going slower than I was with my QJ. My problem arised when I switched to the wilder cam. I place a lot of blame on the cam manufacturer for this. There advertisement stated "biggest cam you can run with a stock torque converter". Yeah Right! With that cam and my QJ and those gears and stall, the car was a DOG off the line! No way would that cam do anything with a stock converter! I had to advance my cam timing four degrees to regain some of the low end power. I should have put a different cam back in and taken that one out but for various reasons I was stuck with that one a least for the summer. So, I got a QJ book and tried to tune out the bog. I learned alot about QJ carbs that summer! Those tubes that are back by the secondary air valve are there to feed fuel to the secondaries and if you didn't knock that tube off when you disasembled the carb than that could cause major lean problems either for half the engine or the whole engine depending on your intake (open plenum or divided). The dashpot you might be describing could be the one (you didn't say what vaccum assembly it was exactly) that slows the secondary air valve from opening. This can be very important also in that if it isn't calibrated correctly you might bog. I always ran mine disconnected and just adjusted the tension on the secondary air valve spring (visible with the carb upside down looking at the secondary air vavle area in the air horn casting and adjustable with a small allen wrench and a small screw driver). If you really want to fine tune then go to a pet store and buy a aquarium gang valve for about 2 dollars and put it in the air line between the vaccum dashpot and the vaccum source and open the valve to cause a very small vaccum leak. This will take some tension off of the secondary air valve and let it open sooner. Another area is the secondary 'accelerator pump'. There is a small set of holes (one on each side of the carb) either at, above or below the secondary air valve. They very in size and position from Qjet to Qjet depending on factory specs. As you open the throttle and open the secondary throttle blades, a vaccum is created in between the throttle blades and the secondary air valve (SAV for short). This vaccum sucks a small amount of fuel into the intake and then into the engine before the SAV opens to cover up a lean spot. If the holes are the wrong size or position, or your SAV opens to soon or to late you run into rich or lean problems. I guess expierementation is the only cure here, but even if your car does not bog you may be able to drop your E.T. if you mess with these things. I wouldn't recommend drilling holes bigger though unless your really sure it will help and you have alot of spare parts and time. Eventually, I tried everything to help my bog...even drilling with pin drills.. and to no avail. One day I tried a K&N air filter and that did it! Apperently my paper filter was restrictive enough to cause to much of a vaccum drop and I was running rich (the engine was sucking and the air filter was restrictive so the vaccum was causing a bunch of fuel to get sucked out of the carb into the engine. With the better filter I picked up .2 seconds in the quarter mile just because I could launch again......I blame the cam company not the carb for the problem in the first place because I think that a higher stall would have solved the problem too...just don't believe everything you read I guess. Has anybody out there blown up a small block Olds? How? Mine refuses to blow and I need an excuse to build a big block! D. Mann From: Knightvd Subject: Re: Cutlass Hesitation (very long - should be FAQ) In a message dated 98-03-23 18:54:43 EST, you write: << I have a little problem with my Olds. Or so I hope its little. Well my problem is when I floor the gas pedal when I'm in drive and sometimes when its in park it sounds and feels like its going to shut off but then it takes off. Any help with my problem would be greatly appreciated. >> OK, I'll take this one since so many were gracious enough to help me with the same problem. If its an '87 I''m assuming its a four barrel QuadraJet. The list just addressed this question quite thoroughly when I recently posed the same question. Here are some snips of responses I got and the things that I found that fixed the problem.... "Get the Q-Jet book from HP-Books, "Rochester Carburetors", worth it!! You can shorten the pump shaft up to 3/32 in or so, as long as it does NOT come above the pump fill slot. You can also bore it out and put in a 2GC pump rubber seal. This will bring you up to 50cc. Also, make sure you are in the inner slot of the arm. This is for racing, or HP street. Normal engines don't need. Make sure secondary fuel supply tubes are not blocked or missing (mine had fallen out on my E4MC, overtorqueing the carb) Drill 4 #68 holes in the secondary fuel supply tubes will enhance fuel delivery to secondary fuel shooters. Start 3/32 up, 1/32 apart. See book for details. (First try to find a #68 bit!! 1/32 is the preferred size, #68 is acceptable) As for bog, 1976 Olds says to set the air valve spring to 1/2 turn, as measured by the air valve doors just enough to hold closed, (tap several times to make sure you are right) then add 1/2 turn. This goes for 85 307's (442 and "Y") as well. You can tighten up to 1 full turn to slow secondaries. Look for fuel spewing. Make sure the fuel is staying in the carb venturies. Could be a strange pressure condition causing other problems. Make sure the front dashpot/choke pulloff is good, press pulloff in, put finger over vacuum source hole, and release, it should stay in for no less than 10 sec. If not, replace. Make sure that the airvalve when opens that it does lift the metering rods up, I had one that the cam broke and the valves would open, but no fuel could come in. The obvious, look for vacuum leaks everywhere. A major leak will bog at lower RPM. Best of luck with it! Q-Jet is still the most refined carb ever made! Thomas Martin "The Q-Jet Bog can surface from using one on an engine that has a long duration Cam loping the idle and causing low vacuum situations from Low RPM. Similar problems are cured on Holleys by switching to a stiffer secondary close spring. The lower manifold vacuum when the Bog begins after you floor it will allow the rear dashpot to release way too early. Actually tighter Air Valve spring tension up to a point, will help to alleviate the Bog in these low vacuum cam situations. That's one reason a blower helps maintain engine breathing once the pedal goes to the metal. I'd even guess that hooking that rear dashpot's vacuum hose to an alternate source, above the primary throttle plates (metered vacuum?) would help keep this vacuum up to the minimum needed much above what manifold vacuum can supply. Another problem that usually may not be cured is to use a Q-Jet with way too rich secondary metering rods once the Air Valves start opening. I agree, the Q-Jets are reliable, giving impressive throttle response when rebuilt correctly with few leakage problems, even on the electronic ones." What fixed my problem: "Well I took the top off the carb, inspected the accel pump plunger. It looked good, checked the air valve operation, that was OK. Then I noticed that one of the brass tubes that is fitted in the top of the carb had come out and was laying in the bottom of the carb. I have never seen this happen before. I don't know when it might have come out. It could have been when I took it apart or the last time I reassembled it. Anyway, I put it back in it's place and reassembled the carb. I also replaced the dash pot with one from another carb." Follow-up "Both of the problems that you found could have been responsible for causing the bog. I had an Olds 350 in a 3420 pound Nova that started out with the same problem. The car had 3.90 gears, posi, a medium duration cam (224 in, 224 ex duration at .050 lift; .501 lift on both sides), approx. 2600 rpm stall,performer 350 intake manifold, stock 7a heads, .030 overbore with cast dished pistons(with a very approx. compression ration of 9.0 to one),turbo 350 tranny, some really cheap headers with 1 3/4" primary pipes (supposed to fit early cutlass with big block, fit nova with no modifications)that ALWAYS leaked, mondello roller tipped rocker kit, and last but not least a big Quadrajet that came off a Pontiac station wagon equipped with a Poncho 400. The car ultimately ran 14.00 at 97.75 mph. I rebuilt the quadrajet when I rebuilt the motor, and it worked fine (I originally used a performer cam:204,214 at .050 lift and I can't remember the exact lift but it was mild). With the quadrajet the car had awesome throttle response and pulled hard throughout the throttle range. Many people were running chevy motors in their cutlasses around here with holley's carb, and most were going slower than I was with my QJ. My problem arised when I switched to the wilder cam. I place a lot of blame on the cam manufacturer for this. There advertisement stated "biggest cam you can run with a stock torque converter". Yeah Right! With that cam and my QJ and those gears and stall, the car was a DOG off the line! No way would that cam do anything with a stock converter! I had to advance my cam timing four degrees to regain some of the low end power. I should have put a different cam back in and taken that one out but for various reasons I was stuck with that one a least for the summer. So, I got a QJ book and tried to tune out the bog. I learned alot about QJ carbs that summer! Those tubes that are back by the secondary air valve are there to feed fuel to the secondaries and if you didn't knock that tube off when you disasembled the carb than that could cause major lean problems either for half the engine or the whole engine depending on your intake (open plenum or divided). The dashpot you might be describing could be the one (you didn't say what vaccum assembly it was exactly) that slows the secondary air valve from opening. This can be very important also in that if it isn't calibrated correctly you might bog. I always ran mine disconnected and just adjusted the tension on the secondary air valve spring (visible with the carb upside down looking at the secondary air vavle area in the air horn casting and adjustable with a small allen wrench and a small screw driver). If you really want to fine tune then go to a pet store and buy a aquarium gang valve for about 2 dollars and put it in the air line between the vaccum dashpot and the vaccum source and open the valve to cause a very small vaccum leak. This will take some tension off of the secondary air valve and let it open sooner. Another area is the secondary 'accelerator pump'. There is a small set of holes (one on each side of the carb) either at, above or below the secondary air valve. They very in size and position from Qjet to Qjet depending on factory specs. As you open the throttle and open the secondary throttle blades, a vaccum is created in between the throttle blades and the secondary air valve (SAV for short). This vaccum sucks a small amount of fuel into the intake and then into the engine before the SAV opens to cover up a lean spot. If the holes are the wrong size or position, or your SAV opens to soon or to late you run into rich or lean problems. I guess expierementation is the only cure here, but even if your car does not bog you may be able to drop your E.T. if you mess with these things. I wouldn't recommend drilling holes bigger though unless your really sure it will help and you have alot of spare parts and time. Eventually, I tried everything to help my bog...even drilling with pin drills.. and to no avail. One day I tried a K&N air filter and that did it! Apperently my paper filter was restrictive enough to cause to much of a vaccum drop and I was running rich (the engine was sucking and the air filter was restrictive so the vaccum was causing a bunch of fuel to get sucked out of the carb into the engine. With the better filter I picked up .2 seconds in the quarter mile just because I could launch again......I blame the cam company not the carb for the problem in the first place because I think that a higher stall would have solved the problem too...just don't believe everything you read I guess." D.Mann "Yeah Vince, I seem to remember a smaller tube on the driver's side popping out of mine. They just fall out. But from what I remember a mechanic saying, the smaller ones are primary air bleeds. It didn't affect performance all that much but I did take small pliers to delicately twist it back in. Overheating seems to be an wasy way out for them. I know the larger ones feed the secondary venturies and can be drilled like someone on the list suggested. It's a delicate job and you should have spares on hand. That same suggestion among others appeared in a 1983 Car Craft (November) and also is in the book. Other suggestions were to place foam the bowl in the throttle plate area to avoid leak down and to not enlarge the secondary bowl reservoir holes in the float area since this would be innefective and is an easy way to YYK the carb float bowl." Vince Knight A bog when stomping on the carb is usually due to the mixture going suddenly very lean. The most likely causes are (1)The accelerator pump isn't working, or (2)the secondaries are opening too soon, due to the spring tension adjustment being mis-set. Assuming you don't mind a quick carb primer, here's a quick explanation. When you stomp the throttle abruptly open, the throttle blades get whacked wide open, but the engine is still turning at low rpm and isn't really pulling much air through the venturis. As a result the venturis don't create enough suction, and don't feed enough fuel. The cure is the accelerator pump: it simply squirts a teaspoonful of raw gas into the venturis when you stomp on the throttle. The extra gas feeds the engine, letting it rev until the airflow increases for the carb venturis to start working as they are supposed to. If the accelerator pump doesn't squirt out its teaspoon of gas at the right time, the engine bogs. The second problem - too loose a secondary air door - usually happens because of age weakening the spring, or because of an enthusiastic but uninformed previous owner. The result is similar to the first: the air door flops open while there's insufficient airflow for the big secondary barrels on the carb to work as intended. The cure is to adjust the secondary air door spring to its correct tension. John Carri SECONDARIES OPENING UP: > I don't think the 4-barrel on my > Q-jet is opening up. Is there any way to find out whether or not my guess > is correct? Here's the best I can offer: Warm the engine up good, then shut down and take off the air cleaner. With no vacuum, the secondary air valves should easily open with finger pressure. If not they could be binding, or the tension spring may be set too high. Pull the throttle to wide open while watching the linkage and secondary throttle plates. The secondary throttle shaft should start to move at about 45 degrees of primary throttle rotation. There's a metal tang on the linkage that determines this spot. At WOT both throttle shafts should be open full. If the secondary shaft doesn't turn, check the lockout pin on the other side (choke side) of the carb and make sure it's not stuck (it's there to keep secondaries from opening until the motor warms up). The choke has to open all the way before that pin is free to move. Dave Cullen Q-jets do not use vacuum operated secondaries. They are mechanical, with an air valve above that's operated by air flow rate, not vacuum. Second, the secondaries don't open at idle, they open at W.O.T., so idle vacuum is irrelevant even on a car that _does_ have vacuum secondaries. You might get better auto info from Dear Abby than this joker. Yes, low manifold vacuum could be one possible problem, though a more likely answer is that the air valve wrap spring is set too loose. The air valve is the pair of large metal plates located in the air horn above the secondaries. These plates open as required based on the amount of air flowing through the secondaries. With the engine warmed up, you can try pushing on the air valve and should sense some spring resistance trying to close them. This spring can be adjusted. The easiest way is to remove the carb. Look at the driver's side of the air horn, where the air valve shaft exits the housing. You should see a small slotted screw in the housing (you may need to rotate the air valve shaft to see it). Now look directly underneath this screw on the underside of the air horn and you should see a small coil spring and an allen head set screw. Loosen the set screw and back off on the slotted screw until the end of the spring no longer contacts the pin coming off the air valve shaft. With the air valves closed, rotate the slotted screw until the spring just touches this pin, then go 3/4 of a turn more (to add preload). Holding it there, tighten the allen head set screw and you're done. Joe Padavano Clarification, please. On ~my~ Q-Jet, the secondary air plates are held closed by a vacuum diaphram. At WOT, vacuum drops, the diaphram releases, allowing the rear air valves to be pulled open by air flowing thru the secondary throttle bores. The throttle shafts are connected by spring loaded mechanical linkage, but the secondary air plates are definitely vacuum operated (more correctly, "lack-o-vacuum" operated). Isn't this the case with all Q-Jets? Dave Cullen No, some very early Q-jets used a small damping piston in the air horn that extended into the main body to dampen the air valve opening, which reduced oscillations when the air valve opened. It was soon abandoned for the simple dual purpose vacuum diaphragm that served both as the choke pull-off (vacuum break) and air valve damper. True, without a decrease in vacuum, the air valve will be held shut, so your point about vacuum playing a part in overall secondary operation is well taken. But technically, the diaphragm is there to control valve oscillations and prevent secondary operation when the car is just "goosed" momentarily. Another linkage, the secondary lockout, which is either at the secondary throttle itself or at the air valve, prevents secondary operation when the engine is cold (choke not fully open). Now, if the vacuum break diaphragm has a hole in it, it will neither perform its choke pull-off function, nor it's secondary air-valve damping function, and may give a nasty little vacuum leak. To ensure it works, just see where the shaft sits when the engine is off, start the engine and see if it retracts. If it doesn't retract fully, it has a leak and must be replaced. Thomas P. Smith smithtp@ix.netcom.com> I think what is being referred to here is that the secondaries don't "truly" engage until there is a significant amount of air movement. Because the quadrajet is designed to have the secondaries engage with air movement not WOT. Matthew Bremmer Actually, that's the air valve dashpot, which serves primarily to prevent or delay the secondaries from opening under light engine loads. The actual rate of air valve opening is governed by the air valve wrap spring. I quote from the Roe Q-jet book: "1967 and later Q-jets have an air valve dash pot operated off the choke vacuum diaphragm unit. The air valve is connected to the vacuum break by a rod. Whenever manifold vacuum is above approximately five to six inches Hg, the vacuum break diaphragm is seated...and the air valve is closed. During acceleration or heavy engine loads when the secondary throttle valves are opened, the manifold vacuum drops. A spring in the vacuum break diaphragm overcomes the vacuum pull and forces the plunger and link outward, permitting the air valve to open when the combination of depression under the air valve and velocity through it combine to force the air valve open against the tension of its torsion spring." Now, Roe does go on to point out that the a calibrated orifice in the inlet to the vacuum diaphragm controls the rate at which the diaphragm releases and thus can control air valve opening rate also. Dave brings up a good point, and this diaphragm does often go bad, which can obviously allow the air valves to open too soon. I stand by my original comment, however, that idle vacuum level has no effect on the operation of the secondaries or the air valve. Joe Padavano I'm not a pro, just a hobbyist.Fixing secondaries is actually an easy job, but it could be one of a couple of different problems from what I know. Lots of trial and error. Couple of opening comments. Get Doug Roe's book on Rochester carburetors. On your next boneyard run, scoop up all the quadrajet secondary rods and hangers you can for maximum tuning choice. These need only a screwdriver to remove. The most desirable secondary rods have very thin tips. thin tip = rich WOT mixture. On the hangers, an early alphabet number is leaner, a later alphabet number is richer. The average on the hanger scale is about J or K and where most Olds qjets seem to start. Goal: Basically you want the airvalve (the top butterfly plates) opening smoothly and completely on demand. (i.e. when you put your foot in it.). The way they work is that vacuum from the engine is supposed to pull them open, then resistance from the screw/spring combo is supposed to meter the gas along with the air that gets pulled in. You don't want much of anything else preventing them from opening or performance suffers and the motor can get hot from running lean when the secondaries are supposed to giving gas & air, but aren't. Tools: Tools you'll need to fix or fiddle with qjet secondaries: 1) a flat blade screwdriver, smallish one 2) a hex key (Allen wrench) set. Look for a 3/32" 's , I think... Problems: A couple of things I've found that can bind up the secondary air valves or prevent them from opening smoothly: 1) Too tight, malfunctioning, or bent wrap spring. 2) butterfly plates expanding just enough to interfere with sides of the back two barrels. 3) Interference from a secondary lockout tang 4) Interference from the vacuum break Fixes: Warning: Do not tighten the resistance screw more than one turn or you might need a new resistance spring. Not easy to buy. Numbers below correspond to problems above: 1) You solve this with the Allen wrench and the screwdriver per Doug Roe's instructions.Underneath the airhorn (the top piece of the carb, on the passenger side, at the rear, there's a hex head screw. When it's tight it secures the position of the wrap spring to provide the correct amount of vacuum resistance to hold the air valve closed except on demand. Stick the allen wrench up under the carb,into the screw and loosen it. Then you'll find that the flat head screw pointing horizontally toward the drivers side will change positions, counterclockwise. The "clock" position of this screw is what determines when your secondaries open if everything else is working o.k. To begin the fiddly part, Use the flat blade screwdriver to hold the set resistance screw in place just enough to hold the airvalve closed, then tighten the allen screw to hold it. Then go out, drive and get on those secondaries. If they moan, they're opening. If the motor bogs they're opening too soon. No moan, secondaries are not opening. To fix the bog, turn the resistance screw an eighth of a turn clockwise, then repreat the test. Do this till you have the bog out. If they're not opening, check for the bind below or a too tight resistance screw. Modest pressure with your finger ought to be able to open the airvalve, and open it smoothly. They shouldn't be real loose though, or you'll bog for sure. 2) This is the easy one -- just get the car good and warmed up. On mine this was a fairly exuberant 20 minute combined freeway/secondary road drive. When I pushed on the airvalve plates, I found they were binding and I knew they moved freely with the car cold. Then I just loosened the plate screws a little, let the plates find their new home (i.e. where they didn't bind), and retightened the screws. Then mine moved freely with the only resistance coming from the wrap spring. That's how it should be. The factory peens the end of these screws which makes it darn near impossible for them to fallout, unless you take them _all_ the way out for servicing for some reason. Just loosening and retightening shouldn't be a problem from what I know -- unless you remove them completely and ruin the peening which acts as a hard stop to a loosening screw. In fact retaining the factory peening is exactly why I didn't remove them completely. Moreover, just getting the plates to sit properly simply didn't require complete removal. 3) When the choke is closed some qjets have a tang that keep the secondaries closed. This keeps the engine from taking on too much demand as it's warming up. You may find this tan mechanically preventing your secondaries from opening. Just use a little paper clip to drive out the roll pin it pivots on. Problem solved, but now you have to be responsible and not beat on a cold motor. 4) I don't fully understand this part of the carb, but I think it keeps the secondary air valve from flopping open, sort of an extra resistance mechanism tied to the choke. You should see some sort of rod arrangement on the passenger side of the carb which slides back & forth in a linkage that controls the air valve. It's tied to the amount of vacuum in the diaphragm on the front passenger side of the carb. When this falls to zero or loosens up the secondaries are free to open, subject to control by the aforementioned resistance screw/spring setup. You can take off this vacuum break deal, but it will eliminate part of the choke. I took mine off, then reinstalled. In no way do I claim knowledge or guru status. I just happened to fix mine in a way I'm happy with. cf You can see if the secondaries are working if you pull the throttle by hand (engine off) all the way. Look inside the carb and if you see down in the back barrels thru the carb, you are all right... you can see the secondaries open, is what I mean. George I'm a Q-jet newbie, will someone explain this to me? My understanding is that the secondary throttle butterflies open because of the mechanical linkage when the primaries are open about 40-50 degrees, (YES-drc) after which the secondary air valve is sucked open by manifold vacuum. Manifold vacuum is highest at idle, so I would have expected that the secondaries would suck open faster with no load on the engine than when under load! Could it be that the dashpot which controls secondary opening rate slows down the secondaries enough that they simply don't open under a quick blip of the throttle? (YES-drc) Of course you can't hold the throttle open for long with no load on the engine, either! Is that why you folks said that the secondaries won't open except under load? - -John Carri Your reasoning is correct, but the flaw is that the air valves are above the throttle plate, so they are not seeing manifold vacuum. With the throttle plates closed, the space above the throttles is at atmospheric pressure. As the throttles open, air sucked into the venturies causes a low pressure area under the air valves. Joe Padavano The dashpot (front vacuum break unit) holds the sec air valves shut tight. Vacuum needs to drop low enough for long enough time to allow the VB to release its hold on them. drc On my 85, the lockout pin acts on the throttle shaft, not the air valves. Secondary throttle linkage is spring coupled, so as not to bust when the pin is engaged and the shaft won't move (cold choke). This spring can lose its tension, and not provide enough "push" to open the secondary throttle plates against engine vacuum. No "bauuuugh". drc removing the pin will not effect cold start / driving...just dont get on the car until she is warmed up...have some sense...also , I dont know all of the replys but just disconnect the secondary arm that connects to the vacuum diaphram on the side of the carb....if you are wondering if they are opening or not......they open alot faster that way and the car will run just fine if you have 3.73 gears like my 83 Hurst Olds does....you can get fancy and adjust the tention on the secondaries but most likely there is no need for that......( if you want to there is an allen head screw under the secondary butterfly shaft where the arm is connected and a screw on the side, loosen the allen screw and have the screw driver in the screw head and to loosen the secondary tention go counter clockwise with the screw and to tighten the tention go clock wise, then tighten the allen head screw underneath ....dont make it too loose or they could open when you are just driving normal and of course you dont want to do this at all if your car has something like 2.78 gears...or worse , bog city...talk to you soon, Hot Rod Harry All the early carbs I have (pre 74 hehe) act on the air valves, while my 77 and later carbs are all on the secondary throttle shaft, on the passenger side carb. Dorian The Edelbrock Performer carb, which is a Carter AFB clone, utilizes an air valve over it's secondaries very much like a Q-jet (in fact, Carter, not Rochester, invented the air valve idea). Anyway, try not to think of it as a "vacuum secondary" as much as an "air flow" secondary. That is, it opens when airflow through the secondaries is sufficient to force the air valve open. The primary difference between the Carter AFB/Edelbrock carbs and Q-Jets is that the opening rate of the air valve in the AFB/Edelbrock is controlled by a counterweight. In the Q-Jet, it is controlled by an adjustable spring and the choke pull-off. Why do you even need this air valve? Well, in the absence of a secondary accelerator pump (as in the Holley double pumpers) if the secondary throttle blade is suddenly opened mechanically, without the air valve, you will suffer an incredible bog until fuel can get started through the secondary discharge nozzles. The air valve gradually allows airflow through the secondaries so the fuel has time to get started. Even with the Q-Jet's secondary accelerator well thing, the air valve is essential. The only way you'll get around it is to have a Holley double-pumper. That is if you REALLY like buying gasoline. Thomas P. Smith Leaving the primary side alone is probably the best idea. You can create driveabliity problems for yourself that make a car miserable to drive, regardless of its power. Trust me, I know ; ) Loosening up the Secondary Air Valve spring is the first step to more responsive secondaries. Some say to remove the link to the secondary pulloff pot. I disagree. If you can feel the secondaries come open, they probably come on too fast. Check the vacuum pot that pulls the secondaries shut. It should have a pinhole orifice in the neck (my GM one was past the bend, so I couldn't get at it. I bought a Hygrade replacement that had the orifice visible from the end of the vacuum c