Re: New Quadrajet, idling problems
SMI delivered a 750 DP Holley to me with no jets (at all!) in either metering block, and the blocks reversed (primary on sec side, sec on primary side). Garbage.
Check the top casting for mismatch with the main body. These should normally be paired for life, since Q-Jets take a "set" over time when the front bolts are tightened. They often look as though the top casting droops when viewed from the front and not attached to the main body yet. Castings that are alien to each other have a great chance to leak fuel, vacuum, etc., even when "pillowed" by the thick gasket. It will run, but otherwise perform strangely.
Is your Q-Jet a Carter-built unit? It will say so right on the side of the carb: "Quadrajet by Carter". These do not stand up to time very well. The problem is inferior metal chemistry compared to Rochester versions, and internal corrosion affecting the drilled passages. Again, it will generally run, but exhibit strange tendencies.
I have built hundreds of Quadrajets over the years and found them to be generally excellent street carburetors, and among these are the lessons I've learned about the particular foibles possible with the type.
Hope this helps.
Inside every GTO is a Chevelle trying to come out
1966 El Camino with a 1964 GTO body.
Blueprinted street/strip 572, sequential injection, TKO-600, Dana 60, cryo'd 1350s, Global West w/ coil-overs, 12" 4WDB, intense mods to the Elco frame, no music, hardcore 'Nam-era 1971 style Two Lane Blacktop car. Lions/OCIR veteran.