Holley 4150 setup (PLEASE HELP)

[mhouston]

I can't for the life of me get my 345 to run correctly. Idle is fine, but theres a bad stumble when you first start to open the trottle, and then a bad flat spot at about 1/3 throttle. I know its an issue with the carb, so I need help for how to set this thing up right once and for all. It's a aftermarket Holley 4150, 650 cfm, DUAL FEED, electric choke, vacuum secondaries. I hear this is a difficult carb to set up on a SII because of the dual fuel inlets, but its what I've got and I can't afford to change it. Some other stuff:

Compression 140 lb on all 8
vacuum 18 inches Hg at idle
the whole ignition system is new, including a NOS prestolite POINTS distributor
timing set at 8* btdc, vacuum advance works perfectly

I'm tired of fooling around with this Holley and not being able to get it tuned right, hope ya'll can help. Been driving it like this for about a year...it'd be nice if it ran right.

EDIT: The carb is a 570 cfm Holley street avenger P/N 80570.

[Mike Huebner]

Check fuel pump pressure - Adjust floats and set air fuel mixture. you may also have a jet issue

[Bill Worden]

The stumble off idle may be due to a temporary lean-out condition before the carb can adjust to the increased demand. There is a cam operated acceleration pump on the left front bottom corner of the carb. The cam that operates the arm must be set(it is adjustable) to activate the pump immediately the throttle advances from idle. You can tell if the plump functions at all by looking into the carb (engine off) and opening the throttle by hand-you should see raw gas injected into the front venturi's.

[mhouston]

The main jets on this carb are #54's, is this too small for an IH SV? I'm gonna get the float and everything set right this weekend.

[Doc Stewart]

The 54s probably account for the flat spot at 1/3 throttle. 61s might be better.
Have you looked at Holly's website?