[carbking]

If you have a friend who will loan you a different carb for a test, I think you may find your problem.

When Pontiac was still using square-bore carbs (the Carter AFB), they used a 575 on the V-8 engines up to and including the 421. Only after converting to the spread-bore design did Pontiac start using 750's on their V-8 engines.

Square bore 750:

Primary - 375 CFM
Secondary - 375 CFM

Spread bore 750:

Primary - 150 CFM
Secondary - 600 CFM

No, a 750 is not too large for a Pontiac 350 (I have an 800 spread-bore on mine); but a square-bore 750 is (my opinion) much too large for a street 350, especially with an automatic transmission. You flood the engine with AIR; thus you run much too lean. Also, primary venturi air velocity is much too low.

Jon.

[lust4speed]

Originally Posted by carbking

...When Pontiac was still using square-bore carbs (the Carter AFB), they used a 575 on the V-8 engines up to and including the 421...Jon.

Jon, I've heard several different sizes quoted for the AFB's used on engines like the '65 421. Were they 575 cfm's?

[biagionim]

Ok so can anyone tell me what size mine is? It's a Carter comp series, the number stamped on the front is 9755s any info would be cool thanks

[Kenth]

Originally Posted by biagionim

Ok so can anyone tell me what size mine is? It's a Carter comp series, the number stamped on the front is 9755s any info would be cool thanks

Your 9755S is a 750cfm unit.

[Kenth]

Originally Posted by lust4speed

Jon, I've heard several different sizes quoted for the AFB's used on engines like the '65 421. Were they 575 cfm's?

From Jon´s well of information:

http://www.thecarburetorshop.com/PontiacAFB.htm

[carbking]

Originally Posted by lust4speed

Jon, I've heard several different sizes quoted for the AFB's used on engines like the '65 421. Were they 575 cfm's?

They were actually 610 CFM!

HOWEVER

Pontiac paid Carter to place a restrictor between the secondaries to reduce the CFM to 575 on WOT. It would have been uncool for the 4-barrel engine to outrun the more expensive tripower; thus the the "tripower insurance policy".

Pictures:

http://www.thecarburetorshop.com/Hotidle3.jpg

The hot idle compensator on the left is unrestricted.

The hot idle compensator in the middle is restricted, and may be replaced with one like the one on the left if hot idle compensation is necessary.

The restriction on the right is just that, a restriction to allow the tripower engines to outrun the 4-barrel engines.

Very easy to make these back into the 610 CFM by removing the restrictor, or replacing the restricted HIC with the non-restricted one.

To the original poster: As mentioned by Kenth, your carb is a 750 CFM, and calibrated for a Chevrolet. I believe that if you could find a friend to just loan you a "test" original Pontiac carb, you would see the difference.

Jon.

[biagionim]

ok so if i do get a new carb then i should make sure its for a pontiac. did holley do the same stuff with the calibration for different motors as carter did or will i be good with any holley?

[carbking]

Originally Posted by biagionim

ok so if i do get a new carb then i should make sure its for a pontiac. did holley do the same stuff with the calibration for different motors as carter did or will i be good with any holley?

Virtually ALL aftermarket carbs, regardless of brand, have a basic Chevrolet calibration.

If you have sufficient ability, parts, and patience; you can generally make most makes of carbs run on a given engine IF THE CARB SIZE IS REASONABLE FOR THE ENGINE.

(Opinion) by far the easiest method for you with your current configuration would be an original Pontiac Carter. A used one from a swap meet, or one of the members here, a rebuilding kit, some hot water, some Dawn dishwater soap, and a little elbow grease would be required.

(Opinion) second easiest method would be acquire an original Pontiac cast iron spread-bore intake for the 350. Then an original Pontiac Q-jet and the other items mentioned in the paragraph above. And much as I like the original Carter AFB, the spread-bore Q-Jet is a better street carb in most instances.

Either of the above will be less expensive, and give you more pleasure than any "out of the box" new aftermarket carburetor you can buy (without doing lots of calibration changes to the out of the box carb).

My guess would be that if you find a loaner carb and prove what I am suggesting about the original carbs; that you would be able to find someone that would swap you an original Pontiac core, and probably pay you "boot" as well.

Jon.

[biagionim]

I will say that I do have the ability to make the required changes if needed. What size carb do you think I should run? All I want here is my engine to run and run well plain and simple! Is there any changes to my current carb that would make it perform better? Maybe down sizing the jets or upsizing for that matter!