[decooney]

My cam is 252 @ .050 and I get 12lbs of vacuum all day long on my 428 CJ with only 12 degrees of initial timing. I've tried every discharge nozzle and .031s are fine on most FEs with these cam specs. 7 lbs of vacuum is too low if you ask me, and Idle at 550rpms? just seems too low. A simple (quick test) is to crank up the idle to 900 or so and fatten up the idle mixture screws as others have suggested to see if something in transition circuit is not keeping up. If nothing changes, 1) check for vacuum leaks on intake/carb. 2) Clean the entire carb, replace all gaskets if you have not. 3) If you have a Power Valve in this carb, check-replace if needed. If you are not running at least an 850 on this motor, I've seen problems like this before with lean transition circuit Holley 750HPs on large FE 428+ and even SB Chevy 400s. Just had the same problem on a SBC 400 build. Ended up having to richen up the transition circuit in the primary and secondary metering blocks to cure the problem. Carb was too small to begin with. If we started with an 850, would have never been a problem to begin with.

Still have a few questions though -

Exactly what model Holley carb is this (xxxx-x) ?

What size jets do you have in it?

What size Power Valve is in it?

Are the butterflies drilled, if so, front/rear, how big?

are the secondary butterflies cracked open some, if so, how much?

Are you certain all 8 cylinders are firing clean?

What is your initial timing set at?