Q'jet idle elusive - need insight

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As done many times, cleaned and re-set this Q'jet on a 1970 SB. Went through all of the steps and settings that usually result in nice idle and great running. Replaced gasket between the top and middle section and used a slightly less thick gasket to manifold.

Started and ran great at higher RPM but, at first, would not idle AT ALL without setting manual choke closed a bit. Looked for air leaks and plugged the vac openings, and once it is warmed up am able to get it to faster idle, but rough. Idle mixture screw adjustment has no effect. Spraying carb cleaner around outside fittings/gasket area does not show much effect.

Is this problem possibly a gasket issue? Have now tried everything I know to fix the idle (including reading a lot of past posts on here) and will appreciate any insight on possible cure before I take it off and start over.