6 pak run bad cold

The one idle screw not responding, and the erratic idle speed = problem.

I would tear down the center carb, go down and get some spray carb cleaner with a "snorkel" tube. Pull any plastic/ rubber out of the carb--needle and seat, o rings, etc, and blow through ALL passages you can see and get to. Unscrew the idle screws and blow through them from the screw side, from the transfer slots, and the air bleeds.

I still think you have a vacuum leak