[chiphead]

I looked up the carb. It's a garden variety 4150 double pumper, mechanical choke, non-adjustable airbleeds. The new ones are listed as 28 squirters in the front, 31s in back. A 428 is likely going to want a 31-34 squirter in front with a fast pump cam. Squirt speed depends on how tight the converter/gearing is.

I don't see mention of the transmission or rear gears. A double pumper is going to want a loose converter and/or deep gears to respond properly. Otherwise it's going to have a lazy response until the engine builds RPM.

I would suggest re-reading the thread I linked to in my first response. It's a detailed tuning thread for my QF double pumper on my 400. It'll save me from re-typing it here.

The idle circuit on a holley feeds the motor until 2500 RPM. So start there. I see where you mention that the car won't idle unless the mixture screws are 1.5 turns out. That's WAY too much for a 4-corner carb. A 4-corner carb will typically end up at 1/2-1 turn out in most applications. The QF on my 400 is at 5/8 of a turn from seated on all 4 screws.

I assume this means the transition slot is too exposed on the primaries, in addition to the secondaries being set improperly. This usually results in the idle screws being turned out too far, a high, unstable idle, blackening the plugs quickly, and a big hesitation when opening the throttle. The transition slot/idle throttle plate opening sets the table for the entire rest of the carb tuning.