First, I am not convinced this is a carburetor issue.
My standard checklist in this situation is:
(1) compression test
IF compression is good then
(2) complete ignition test
IF both compression and ignition are good then
(3) fuel pressure test
IF the fuel pressure is below the allowable limit then time to look at the carb.
A 500 CFM is too big for a 283. Too large a carburetor NORMALLY leads to a LEAN mixture, as the air velocity is too small to create the necessary negative pressure in the fuel wells. The engines with two four barrels used carburetors that were approximately 385 CFM, and they were run progressively. That is, under all but wide open throttle application, the engine was running on ONLY the two front barrels of the rear carburetor, which were significantly smaller than the two front barrels on a 500 CFM AFB.
Also, you did not state which 500 CFM AFB you are using. The Chevrolet fuel curve is 180 degrees different than the fuel curve for a Cadillac, Chrysler, Dodge, Ford, Pontiac, etc., etc., etc. If the carb was not designed for Chevrolet, have you changed out the jets, rods, idle tubes, airbleeds, and especially the secondary airvalve?
Jon.
