# 1966 GTO Carburetor 4034S Question



## Mastergreenbean (Oct 14, 2016)

Hey, I am restoring a 66 GTO for my dad. It is a 389 4bbl Automatic with a date correct engine. When I purchased the car the carburetor and intake manifold were sitting on the engine and not bolted down. This raised questions for me as there were over 30 gtos in the barn that I bought mine from, so I was worried it wasn't correct. While investigating my carburetor it had a pink tag on it reading “CO PC 4248s” which I was told was incorrect for a 389 and it was for a 326. I was told I needed a 4034S. I ordered a rebuilt 4034S and when it arrived it looks identical to the carburetor I had. I am looking to find out what the difference is between the 4248s and 4034s, and what carburetors I have in front of me? Attached are photos of the carb that came with the car and the one I just received, along with the tag and photo of the car. Thanks in advance for all the help.


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## PontiacJim (Dec 29, 2012)

4248S shows up as a 1967 285HP 326/automatic on the Firebird & Tempest/GTO having A.I.R.(Air Injection passages for the exhaust - Smog equipped engines).

4034S shows up as a 1966 335HP, Block Code YS, 389/automatic Tempest/GTO, non A.I.R. engine. Also used on 1966 B-Body 389/automatic cars.

That said, the carbs are engine/trans specific, with and without A.I.R. passages, and most likely fitted with application specific internals such as jets, step-up springs/metering rods, and idle-air bleeds.


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## Mastergreenbean (Oct 14, 2016)

PontiacJim said:


> 4248S shows up as a 1967 285HP 326/automatic on the Firebird & Tempest/GTO having A.I.R.(Air Injection passages for the exhaust - Smog equipped engines).
> 
> 4034S shows up as a 1966 335HP, Block Code YS, 389/automatic Tempest/GTO, non A.I.R. engine. Also used on 1966 B-Body 389/automatic cars.
> 
> That said, the carbs are engine/trans specific, with and without A.I.R. passages, and most likely fitted with application specific internals such as jets, step-up springs/metering rods, and idle-air bleeds.


Hey thanks for the reply, are you saying that the 4034s is correct and it is ok that they look the same and the difference between them is internal?


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## Ron B (May 9, 2021)

PontiacJim said:


> 4248S shows up as a 1967 285HP 326/automatic on the Firebird & Tempest/GTO having A.I.R.(Air Injection passages for the exhaust - Smog equipped engines).
> 
> 4034S shows up as a 1966 335HP, Block Code YS, 389/automatic Tempest/GTO, non A.I.R. engine. Also used on 1966 B-Body 389/automatic cars.
> 
> That said, the carbs are engine/trans specific, with and without A.I.R. passages, and most likely fitted with application specific internals such as jets, step-up springs/metering rods, and idle-air bleeds.


Jim, my questions is: Do you know what the CFM was on a 4034S? My 66 GTO has a BS carb on it and I want to replace it, but don't know what CFM size. I plan on upgrading the exhaust manifolds to RA and going to a HEI system. Any help would be greatly appreciated.


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## PontiacJim (Dec 29, 2012)

Ron B said:


> Jim, my questions is: Do you know what the CFM was on a 4034S? My 66 GTO has a BS carb on it and I want to replace it, but don't know what CFM size. I plan on upgrading the exhaust manifolds to RA and going to a HEI system. Any help would be greatly appreciated.


To the best of my knowledge, they were arounf 525-550 CFM. They also will have a smaller base bolt pattern and is why you will find the aftermarket AFB's as well as Holley I believe, that will have 2 bolt patterns. The later and larger AFB's have the bigger base. I don't know when they went to the larger CFM carbs, and it may have been in '66 as that was basically the last year for the AFB on the GTO and then the Q-jet was used which is 750CFM.

With that said, You might not be able to bolt the bigger/later AFB on the earlier manifold without a spacer/adapter. You will want to verify this before you get the carb. Others may be able to chime in here and help.

I would, if going with the AFB, might look at the AVS2 in the 600 CFM (*Army* has the 800CFM on his '67 and you may want to get his opinion/advice on installation/use). I would go larger, but if your engine is stock, no need. If a typical AFB, I would use the 625CFM if you don't plan on wailing on it and winding it up a lot, but if you're going to use the engine and have a good cam, I like the 750CFM. These carbs use a vacuum secondary operation which I prefer for a street car and only use what they need so it is kinda hard to go too big.

If you are looking at Holley, I can't help much, but you may also need the spacer/adapter.


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