holley 1920 carb question

I have a holley 1920 on a slant six and it runs beautiful, starts beautiful hot and cold, good acceleration etc...but the damn thing won't idle worth a crap.....So I purchased a factory rebuilt 1920 and put it on so I could rebuild the original and still drive the car....and the new onw idles great and accelerates great, runs good, starts good cold, but hot it has to crank about 20 revolutions or so until it starts and then it stumbles for awhile into it clears out..... I did the rerouting of the fuel line over the valve cover to help with heat soak but it still won't start hot worth a damn....So, I figured well when I get my old one rebuilt I will just put that one on and be all set....Well , I got it totally rebuilt and it does the same thing as it was originally doing....runs beautiful, starts beautiful hot and cold, good acceleration etc...but the damn thing won't idle worth a crap....When idling bad, if I hold my hand over the throat the idle picks up and smooths out.....If I turn the idle up with the screw it also idles clean and clear, then when I put my hand over the carb it wants to die out......So It is something in the carb.....because the other carb idles great....I tried backing out the idle air mixture screw and that helped a little but then same thing....I took screw out and blew air back in the idle mixture screw and was OK for awhile then started acing up again.....Is there any other reason why it wouldn't idle? The brass seat of the screw was marked up a little bit so I have another one on order in case that is the problem....suggestions? thanks