If they're standard idle screws, it sounds like it's rich at idle. Have you considered opening up the idle air bleeds a couple thousandths at a time? I know that is a step some might not like to take, but it's an option. I guess you could drill and tap for adjustable air bleeds if your Holley doesn't have them. Are the secondaries adjustable on that carb? Could you open them a little to get a little more air into the mix?
Additionally, if it's still running when you close off the mixture screws, it's getting fuel from somewhere...are the nozzles dripping?
