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.
__________________
"Good carburetion is fuelish hot air". 
"The most expensive carburetor is the wrong one given to you by your neighbor".
If you truly believe that "one size fits all" try walking a mile in your spouse's shoes!
Owner of The Carburetor Shop, LLC (of Missouri).
Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.
|