Join Date: Sep 2000
Location: Washington, Michigan USA
8 psi is too much - Holley needle/seats are overpowered over 7psi - you need a pressure regulator (mounted on inner fender or closer to carb) set at 6 to 6-1/2 psi. As indicated above, turn your idle mixture screws lightly in to the seats, back out 1-1/2 turns, and start from there. Set idle rpm (screw on driver's side of linkage), connect vacuum gauge to manifold vacuum source (NOT ported vacuum, as there is none at idle), and adjust idle mixture screws 1/4-turn at a time to get highest steady vacuum reading; work back and forth on each side, make finer adjustments (1/8-turn at a time) once it smooths out, and touch-up idle rpm screw as required to maintain correct idle rpm. When you get highest steady vacuum reading, turn each mixture screw IN 1/8-turn and you're done. All of this assumes you have a relatively mild cam - if you have a radical cam, you will have other problems and the process is different.
You didn't say what engine combo or mods you have, if any; if it's basically stock, the 750 is way too much carb for it and you'll have poor off-idle driveability and fuel economy due to low air velocity and poor vacuum signal for metering. A 600 or 650 would be a much better choice for all-around driveability and performance; "Bigger is Better" works fine for horseshoes and hand grenades, but NOT for carburetors. 90% of the street driveability problems I get asked to look at are a result of way too much carburetion and/or too much fuel pressure - race cars are one thing, street cars are another. Speed shops LOVE to sell big carburetors and electric fuel pumps, but they don't work worth a hoot on basically stock street engines.
'69 Z28 Fathom Green