Back to idle restrictions with twin carbs. The idle circuit is set by a fixed jet and is a rough setting. Fine tuning is possible by the adj screws. With twin carbs, it seems that even with the adj screws all the way in, the idle can be too rich. Carbs designed for 2x4 operation should have this taken care of, but if you purchase two carbs, each designed for single oepration, you can have a problem.
The answer is to have a carb shop replace the idle jets with smaller ones, or to insert a fine "V"-shaped wire into each of the jets to effectively reduce it.
Here's a bad pic from Dave Emanuel's book on Holleys:
I found stock carbs did run OK, but with the screws all the way in. I replaced them eventually with units from Pony Carburators and they must have done something to the jets as I can now run with all screws about 1/4 to 1/2 turn out.