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Carburetors used on 1971 Avantis


TMA62
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I have a 1971 Avanti II and realized that it did not have the orginal carburetor installed on it. Apparently, it has a 1978 Quadrajet on it now. I would like to replace it with the original model but which one? If any of you have a 1970-72 Avanti II and believe it has the orginal carburetor on it, please reply. It is likely a quadrajet and the model number will be on the drivers side on the outside of the secondary chamber. It should start with the number 7.

Thanks in advance!

Ted

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My 1971 Avanti (T400 trans) carb reads 7028208 DB.

The "28" says it is a 1968 ("41" would be 1971); this carb would have been correct for a 1968 Corvette (Avanti Motors used base Corvette engines).

I believe it to be the original carb, but I'm the 2nd owner of the car (purchased about 1979). I did send the carb away for rebuilding in 2006, and I was unable to find any old records to determine whether I'd recorded the number I found on it before I shipped it to the rebuilder... I seem to recall that I sent it out because it had a leaking/stripped fuel filter inlet (I normally rebuild my own carbs), and I did find an email from the rebuilder referring to having tested my carb on a test engine and that the filter housing helicoil didn't leak during the test, which bolsters my belief that the carb returned to me is the same one I sent him.

This link has a code breakdown that might be helpful:

http://home.comcast.net/~chadwick.robert/How_to_tune_a_Qjet.pdf

Corvette carbs (base engine with automatic trans) for:

1968... 7028208

1969... 7029202

1970... 7040202 (and later in the model year: 7040212)

1971... 7041212

Probably any of the above would work fine.

Edited by WayneC
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Thanks Wayne. The carburetor in my Avanti is 17048228 which I think it from 1978. This was my Dad's car. I do not recall him ever replacing the carburetor but it apparently was done. What would be the differences between a Quadrajet from 1970-1 and one from 1978? Would the latter have improved gas mileage potential? That may have been the reason why Dad made a switch to a newer carb.

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Avanti Motors supposedly purchased engines in something like six months of production quantiles for the best discounts. If that's true, it wouldn't be uncommon to find one model year Avanti with a prior year production GM engine...and that's if they were pulled and installed in the same order received...oldest inventory gets installed first...something like how a grocery store rotates merchandise.

Pretty much all Quadjrajets can be built to suit any GM engine. A good carb guy can match the jetting, etc., to the cam profile and other specs of the engine. There will be some choke differences between different year and application carbs. Some will have a hot-air (divorced) choke while some will have the more familiar bi-metallic spring choke or an electric choke.

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I can't tell you much about the differences, except to say that all quadrajets are not the same, even for the same make and model car. There are many variations, even among Corvettes, because the carbs were tuned/tweaked by engineers from Rochester for the specific vehicle configuration during extensive driving at the GM test track prior to production. Variations include transmission type, jetting and adjustments, mixture rods, type of choke, fuel entry point, vacuum take-offs, etc, and I recall that some early smog-era carbs (1973 and later) even had plastic caps over the mixture screws that had to be broken to change the mixture (so smog station inspectors could spot carb-tuning violators). The stamped number reflects all the specifics of that particular variety. I never associated better gas mileage with post-smog carbs; my recollection is that mileage degraded rather than improved, as did driveability, but much of that may have been attributable to added smog equipment rather than the carb itself.

I will try attaching some photos of my carb, as it looked when returned by the rebuilder:

post-308-0-21127500-1413996514_thumb.jpg

Edited by WayneC
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I also owned a 1971 Avanti ll which I sold several years ago. it had the original Quadra-Jet carb on it, and I believe that I have that carb number written down at work.....If I do have it, I'll post the number tomorrow.

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Even though the idle adjustments were made tamper proof in the mid 70's, they were a better base for a performance carb. The float chamber and primary boosters were tweaked. Look for carbs with the dual venturi boosters on the primary side Some larger Buicks and Pontiacs had single boosters, and don't have as good low speed throttle response. Smaller engines have stops in the secondary air valve, and don't open fully. These can be reworked to open all the way. It's mostly moot as the smaller engines can't pull enough air to open them anyways. It is usually good practice to replace the float when rebuiding these carbs, as they saturate with gas over time, and no longer float properly. Check for leakage between the base, and main body of the carb, as the well plugs in the center (underneath) can leak. Seal them with JB Weld and let it set up. You must use new gaskets between the base and main body. Also the manifold base gasket has anti crush washers built in, and won't reseal properly, so replace that every time you remove from the car. Late electronic carbs have to usually have the throttle position sensor replaced. The mixture solenoid has to have 1/8" movement of the main metering rods. Also you have to adjust the carb with a dwell meter hooked up, and the solenoid adjusted to get 30 degrees, as it affects the computer settings.....hence the timing. If you remove the idle adjustment plugs to set the idle screws, they are not the same as Carter or Edelbrock settings, and take 4 turns out from seated.

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The 'Computer Command Control' system, with its fussy electronic Quadra-Jet carb, is bad news. Don't throw away money trying to fix a system that was unreliable when new. If you're having issues with the 'CCC', replace the carb with a pre-electronic Quadra-Jet, or an Edelbrock,..along with an earlier Chevy V8 distributor with its advance controlled by engine vacuum. You will never look back!

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  • 9 months later...

I've got a CCC quadrajet. I had it rebuilt by a knowledgeable local guy last year and absolutely love it. It performs greats and I get 20 mpg. They aren't junk if you know how to massage them. In general the quadrajet is a wonderful carb that gets a bad rep because of its complexity and that you really have to know them inside out to get the best out of them.

The 'Computer Command Control' system, with its fussy electronic Quadra-Jet carb, is bad news. Don't throw away money trying to fix a system that was unreliable when new. If you're having issues with the 'CCC', replace the carb with a pre-electronic Quadra-Jet, or an Edelbrock,..along with an earlier Chevy V8 distributor with its advance controlled by engine vacuum. You will never look back!

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As a side note i have my original q jet carb for sale out of my 69 with a spare for parts if ever needed. I rebuilt mine a year ago.. ran great! But when i switched manifolds i went to a edelbrock because of height clearance issues with the hood... so i wasted a couple hundred dollars rebuilding it and getting it tuned.. oh well :)

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My recently acquired 71 4 speed has a Holley. It was taken off the road in the 90's with just over 17,100 miles. Not sure why it would need a new carb at that mileage but I was surprised not to find a QJet.

Removing shims and lowering body to frame to Studebaker spec so like Drat, height might become an issue, not sure if edelbrock/carter is lower than the Holley, will have to measure my 63 to see.

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RQB3263 i recently replaces the "electronic" quadrajet on my 81/305....I traced all the wires back and removed the "computer" and all other associated stuff...(not a big job).....and replaced it with a standard reman Quadrajet from here i Jacksonville fl..less than $200....the TV cable from my 700r4 trans hooked right up no special brackets to buy, only one e wire to hook up......BUT...did require a new HEI vacuum advance distributor...not a big deal either...got back to basics...works great !!!...BILL

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I recently had my nephew build a Quadrajet for me, and we removed the useless Holley spreadbore that someone had put on before me. What a difference! Much better driveability and no more hot start problems.

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