Holley 1920 problem

You'll probably have to find another carb to get that piece: I think it is a later model metering block. Or, fix the lever with epoxy if you can. You can get a complete rebuild from Mike's Carburator in WA state for $159; 'd trust that more than a parts store unit.
http://www.carburetor-parts.com/Holley-1920-Remanufactured-Carburetor_p_2934.html

It sounds like you have checked for vacuum leaks but that still sounds like your problem. Did you put a vacuum guage on it?