I've got the same problem with a blown motor runing 2 Edelbrock 600's, the idle fuel screws do absolutely nothing. I run them all the way in and the engine keeps on idling like I did nothing. Also the plugs are very black. I talked with an engine builder, he says these carbs are jetted/needled fat. I would look at the plugs after a run down the freeway/ track, hee hee, to see how they look. You might be to darn rich for the idle circuit to do anything. Just food for thought.