holley 1920 carb question

Best guess is that the regular idle mixture feed hole is full of dirt and needs serious cleaning. Putting your hand over the carb increas the vacuum signal and that sucks more fuel mixture out. Opening the idle speed screw is likley moving the idle slightly into the idle progression slot and fixing it that way.

So I would pull it and work on the idle mixture hole under the throttle plate, work also on cleaning out the idle air bleed at the top of the throat, and when done, do a wet float check.

Check here for a good idle circuit diagram: http://www.slantsix.org/forum/viewtopic.php?t=24736