[Frank Messina]

Mark,
I know you've switched carbs back and forth trying to isolate this problem so this may or may not be relevant but I wanted to throw it out there anyway. When I first started my FRPP 351 crate motor I had a terrible time getting it to idle right. It was smoking like a diesel. What I did to zero in on it was to temporarily replace both power valves with plugs to take that fuel out of the equation. Then, even though they were new, ( brand new HP750 out of the box) I changed both needle and seat assemblies. You can clean them 'till you're blue in the face but sometimes they just stick.

From there I was able to get a clean idle by increasing the size of the idle air bleeds which really helped a bunch. Then I and added the power valves back one at a time but at a much lower opening point until I was sure everything was working right. Then I put the correct PVs back in. HTH.
Frank