I've been doing some research cuz I'm thinking about rebuilding or buying a carb. From what I can tell, the numbers on a Quadrajet really don't matter; they're mostly for identifying.

So, does it matter? Do I need to look for a 7040207 or will any old qjet work?

 

As I understand it, there are two significantly different versions of the Q-jst carb: the standard car carb that is used and a "truck" version of that carb. Some of the parts are not interchangeable and there are tuning differences between them. If you can determine that the Q-jet you buy came from a car (preferably a Corvette, so some of the linkage hardware is useable), then it should work just fine.

Lars Grimsrud is the 'guru' for info on Q-Jets. If you find a candidate carb, e-mail him with the numbers on it and tell him what car/engine/transmission you have, and he should be able to advise you of how to alter it appropriately. V8FastCars@msn.com

 

I am not an expert but I will pass on what I have learned in a nutshell.

Some of the numbers refer to California or manual transmissions. This may mean that it will have more vacuum ports than you need or fewer than you require, depending on your set up. From the factory these are set up different but you can make adjustments for your needs.

Later Rochesters have more emissions equipment with them. These can be removed if required for your application or lack of interest in emmissions.

In short, yes, you can take any old Rochester and get it to work for your application (insert disagreement/discussion here) The bodys are basically the same (except for additional ports) and it was the set up from the factory for each that changed.

If you want the carb that was on your car then get the numbers that match. If not then numbers that are closer (in terms of date)will help

 

Hi Z,
I agree that Lars is the Q-Jet MAN.
I'll add that I believe some q-jets had the fuel line fitting at the front while some have the fitting on the side. I'd think that would be an issue to watch out for in finding a carb.
Regards,
Alan

 

I think that the more time you spend around the Quadrajet and study it, the more you will appreciate the effort that went into creating such a carburetor and the degree to which it achieved the goals set for it. I would recommend that you spend a little time at Cliff's High Performance website and I think you'll better understand why so many folks seem to have such a dislike for them. It is not a simple carburetor by any means, but this site will take a lot of the mystery out of them, or frighten you to death!!
http://www.cliffshighperformance.com...carb_ID_2.html

If you feel inclined, you might want to pick up a copy of Cliffs book, How to Rebuild and Modify Rochester Quadrajet Carburetors by Cliff Ruggles It is well worth the price. It will not make you a Quadrajet Master, but you will definitely feel much less intimidated by it.


Cliff is one of the folks that Lars Grimsrud speaks highly of and they share a lot of their Quadrajet knowledge with each other.

To answer you original question, Yes and No... Yes, it does matter and no, any old Quadrajet will not work, at least not well. There is much more of a difference between Quadrajets than you might think, However if you at least attempt to stay within the series that came with your generation of car, you will find life is much simpler.

Good luck... GUSTO

 

I was doing some looking on Cliff's site about the numbers, and that's kinda how I was sort of thinking that maybe it's not too big of a deal. I image that if I were to get the carb for the 1969 L46 that it would be the same. I also thought, maybe I would want to stick with a manual transmission one and a non-Kalifornia one.

Other than those I was hoping that they'd be pretty similar. I've thought about just dropping the $240 at Eckler's for one cuz I feel I could be pretty confident that I'd get the right one.

If only I lived in CO (unless Lars has moved since I was on here last). It's been a while since I ed around this place.

 

Sorry, can we stop quoting a whole message?
IMHO the number does matter. My 71 Cadillac came with a Q-jet. Which means a
472 cubic inch. I doubt that
carb would work well on a
350/383.
So numbers do matter. Although I think a "Cadillac" q- jet cam be rebuild to work well on a 350/383. No doubt more experienced people will come around.

 

One of the remarkable things about a Q-jet is how tunable it is. This can be a downfall if you have say... a 1968 327/300 and find a killer deal on a 1978 C-10 carby since the jetting specs differ so much between the engines. Buy at least one book and research things like accelerator pump stroke, main jets, choke options and like subjects before you decide that any old donor carb will work on your engine/combination.

 

Got 4 of those in my basement.

 

I just had Lars go over a truck carb, originally for a 350 engine, now set up for my LS5. It's a temp carb, use until I can get my other one worked on by Cliff. The carb which Lars fixed up works pretty well. Cliff has a LONG backlog right now.

Piece of advice from Lars. DO NOT buy a commercial rebuilt QJet. If you don't have one already, the best ones are usually the ones nobody has messed with, covered in grease. Buy a few for parts. Problem with the commercial rebuilts are they are often modified, parts from different carbs mixed together, and some internal parts changed, impossible even for Lars to tell if the internals are correct since their not documented or numbered.

Based on my recent experience it seems the jets and metering rods can be changed to get the carb in the ballpark for your application. Whether it will be optimal I don't know.

I've seen the fuel inlet in different locations, thought the ones which weren't on the side were non Chevrolets?

 

Hi,
It's funny that the pic on Cliff's book has the front inlet.
Was that actually more typical for q-jets?
Regards,
Alan

 

Just to clarify an earlier point: A Q-Jet coming from a big block car (454 Chevy or 5xx Cadillac) can still be used on a 350 SBC or 327 or 265, for that matter. The carb manages airflow to the secondaries...as needed. If the engine has a small displacement, the secondaries only open as much as that displacement requires. The same carb can [and will] open more for a BB engine. The rods and jets would need to be swapped for the right sized units so that fuel mixture is correct for the specific engine, but that's a relatively easy process. Other than that the carb would work fine for either large or small engines. You can't do that with a Holley!

 

Hey Alan. From what little I know, only Chevys had the 90* fuel inlet fitting. Pontiac, Cadillac and Oldsmobuick had a straight out the front fitting. Ford also ran a Q-Jet on some Mustangs/Cougars. They are out the front also. I have never seen one of those in person. I would think there are more Chevys than the rest combined. mike...

 

Yes, the front inlet was quite typical of the B-O-P cars, while the Chevrolet's almost always had the side inlets on them.

To address 7T1vette's comments about swapping parts and coming up with a carb that works for your application. I believe that he is correct.
Someone with a good understanding of the Quadrajet, can no doubt do this. However, I believe one of the reasons that the Quadrajet has developed such a nefarious reputation is that typically someone that doesn't understand the Quadrajet well tries that very thing and doesn't get it right. The result is a mix-master of parts that very often will never work well together.

General Motors adapted the Quadrajet to operate on such a broad spectrum of cars with different weights, transmissions, driving and even emissions requirements by changing various aspects of the Quadrajet to accomplish this. Frankly they did a great job and as a result there are many, many Quadrajets out there that all appear the same on the outside, but internally they are really quite different.

As I mentioned earlier, if you get access to a copy of Cliff's book you will better understand just how GM did this and why the Quadrajet numbers can be so important. Can Lars or Cliff or someone with their experience swap out a bunch of parts and make it work well, sure. But can the average backyard mechanic delve into a Quadrajet he finds at a swap meet (that turns out to come from a Buick station wagon ?) and get it to perform like an original carb on his 1975, L-48, I doubt it. And that gentlemen is why so many folks abandon their Quadrajet for something that has less overall capability.

Good luck... GUSTO