Problem with Holley 4 bbl

Has it ever run right? Does the engine die when you seat the idle mixture screws? The power valve could be OUT and it would not affect idle since no signal is at the boosters to pull fuel in. So you can rule the PV out even if it's buster all to hell. If it doesn't die with the air screws seated, it's definitely too rich. Probably needs leaning out on the idle circuit. Does it have adjustable air bleeds and fuel restrictors?