UNRESPONSIVE IDLE MIXTURE SCREWS ARE THE KEY.....

And point to a vacuum leak someplace.
Somewhere, someplace extra air is getting in to the engine causing that Holley to think your engine is running higher RPM that it really is therefore allowing more raw fuel to be dumped into the engine.
Idle mixture screws should have an IMMEDIATE response within 1/4 turn. By Immediate response, I mean when the engine is hooked to a GOOD RPM gauge and a GOOD VACUUM gauge (everybody is using those even before they touch the idle mixture screws RIGHT!!!!) You will see those gauges register SOMETHING with a change as minor as 1/4 turn. On a good engine with a fresh correct carb even a change as small as 1/8 should influence both gauges.

If it doesn't you have one (or more) of the following problems:
Gauges not hooked up properly (hey, I have done it)

Carburetor to dirty and needs a rebuild (I recommend daily drivers get rebuild every 2-3 years, carbs that sit for more than 3-4 months need rebuilding afterwards)(put a pint of gas in a glass jar for a month and see what happens)

Vacuum leaks (from the carb, carb gaskets, baseplate, spacer, but don't forget anywhere on the engine or even elsewhere) (had a 65 Thunderbird that could never get tuned and running right, brand new engine no vacuum leaks but ran like crap, traced it down to the vacuum assisted door locks in the doors and the vacuum assisted air vent in the rear seat, all that original vacuum tubing had leaks thru-out)

Engine in poor shape (remember your engine is basically an air pump and MUST be able to pull or draw air into it thru the carburetor for the carburetor to funtion properly)

Carburetor to big or improperly adjusted (see above, if the engine can't pull enough air across the venturis the carburetor will not funtion properly) (Or if you have the bench setting so far out of whack for what you are doing (perhaps you have read to many hot rod magazines and decided on your own to "hop" up the settings to your "own" specification)


I would start there, and see what you come up with,
Hope that helps,

Bill White
White Automotive