Quadrajet Carb question

[TMA62]

Guys:

Each digit within the serial number on the Quadrajet carbs contained information on the carb and what it was used for. The last digit was used to differentiate manual or automatic transmissions. Physically, what was different on the carb? If one placed a QJ carb for manual use on a car with an automatic tranny (or vice versa), would there be any adverse effects? If so, what would those be?

Thanks in advance!

Ted

[WayneC]

Not an expert here, but the serial numbers may denote differences in adjustments made at the factory, jet sizes, and differences in the throttle linkage and/or an extra vacuum port to accommodate a method of downshifting the automatic transmission when you stomp on the gas pedal (like a throttle valve cable).

Also, when a car maker produces many thousands (or millions) of cars, minor differences between carbs can save them money by eliminating things not needed on some models, like extraneous vacuum ports, while providing those things on other models.... so for example, a carb intended for an automatic may come from the supplier (ie, Rochester or Holley) with a cable clamp on the accelerator linkage that is not present on the otherwise identical manual trans carb, which has an empty hole where that clamp would go (may be a bad example, the clamp might come on the transmission cable assy instead, or the hole may be absent on the carb intended for a manual tranny).

I suspect a carb meant for a car with an automatic transmission can probably be used on the same model car with a manual transmission, but the opposite could be problematic. It should be possible (don't ask me how, though) to find the specs for jets/accelerator pump, float levels, choke, etc and purchase correct parts for the carb variation you choose, thereby transforming that carb to match the specs of the carb that was originally used on the car you have.

Each new model car is tested exhaustively and tweaked/altered by carb experts to get best balance of driveability and economy under many driving conditions before the car goes into production. If the tuning items are anywhere close to correct, most of us would never know the difference (we'd just ignore the stumble that only occurs when we goose the throttle while going about 45mph up a hill). A Rochester carb for a Silverado probably needs very different calibration than one for a Chevy Nova, even though the same basic 4-barrel carb model is used for both, and both vehicles may have a 350ci engine. If you buy a carb with different serial numbers than the one used on your car, you ARE the carb test engineer, since the carb was not tuned/configured specifically for your vehicle.

Carburetor overhaul companies, like recarbco ( http://www.recarbco.com/) can probably find the correct carb and/or rebuild yours to factory specs based on the serial number. I had them rebuild my '71 Avanti carb some years back and was pleased with the work.

[Gunslinger]

I'm one who believes carburetors are voodoo...I can screw up a one-barrel (and have). Since automatic transmissions of that time period had vacuum modulators for shift points, a carb intended for a manual transmission car may not have the necessary vacuum connection or have it blocked off. Install the manual transmission carb on a car with an automatic without making the necessary connections and you won't have the transmission shifting properly.

Quadrajets are excellent carburetors and the same basic carb is used for countless applications. Its air on demand design and small primaries can squeeze a lot of economy out of the engine. It does take someone familiar with them to get them to work properly...especially after so many years. Few techs today know much about carburetors.

[Devildog]

As usual, Gunny has it right. It is often difficult to find younger tech to rebuild a carb because they they think it is VOODOO magic. Get the ignition and timing correct first. Then get a vacuum gauge and it is easy to adjust any carb.

If they are clean inside, accelerator pump works, no air leaks; then the only real adjustment is sizing the jets and metering rods which is actually sort of fun. Holley, Edelbrock, etc. all have simple little primer instruction posted on-line to help you.

Do not be afraid of them.

Joe

[TMA62]

Thanks for everyone in posting. To make a long story short, I have a 1971 Avanti II. But the model number on the Quadrajet indicates that it is from a 1978 Chevy with a manual transmission. I think what could have happened is the when my Dad had work done on the car in the mid-80's, he had the carb replaced. I bet the mechanic told him that he had a rebuilt one on the shelf and could just drop it in (as opposed to rebuilding Dad's original carb). I am guessing that the carb came off of a 1978 Chevy pick-up since most manual trannys were on trucks as opposed to Corvettes and Camaros. This could be a source of the problem. The good news is that I bought on Ebay an old Quadrajet carb from a 1971 Camaro with automatic trans. The model number checks out.......it probably is very close to what was originally on Dad's car. Plus, I got it for only $30. It needs to be cleaned and rebuilt. I am looking forward in dropping it in the Avanti and testing it. It may clear up some things.

I will keep you posted.

[CaffeineRacer]

Really the only thing I can think of that would be different would be an attachment for something like a TV cable on an automatic transmission. Other than that there is not communication between the carb and the tranny. The carb doesn't care what the transmission is doing. I only cares what the engine is doing and how much air/fuel it needs. Buy Cliff Ruggles or Doug Roe's book on quadrajets if you really want to understand them. Doug's much more thorough, but harder to understand IMO, while Ruggle's seems to miss important info so I bought both.

[wildfelr]

I recently replaced the electronic Quadrajet on my 81 rqb3263....with a non elect.qjet remanned in Jax, fl by American carb.i also removed the computer (ecm) and all associated wiring an relays etc...very simple ...also required replacement of elect. advance distributor with a vacuum HEI (coil in cap) aftermarket distributor....not a big deal either.....and of course plug wires........Qjets work fine when you simplify the rest of the motor........My opinion if you take ALL the crap off of a 305.....ALL OF IT........ittl run as good as a 327....but thats just my opinion....and its a lot of work ....once...im done and mine runs great (use mid grade gas).....BILL RQB3263

[wildfelr]

If you fail to correctly connect and ADJUST the TV cable form the trans to the carb you will burn up 3rd gear bands in trans very quickly !!!

I know this from expensive experience.....on a previous hot rod....there are plenty of instructions on the web on how to adjust it ...its simple....when the carb is wide open the TV cable should be pulled tight..(or at the end of its travel)..most have a simple length adj. on the cable ...Its small but important $$$...rqb3263

[chuck]

A number of years ago I looked up the number of the carb on my 82 to determine what the carb was built for-a Chevy Van with an automatic!

chuck

RQB-3553