I might have found the answer to the warm start problem. Removed air cleaner and noticed that the top of primary throttle valve was wet with fuel (from auxilary venturi).

The folks at Pierce Manifold and Jam Engineering said I should be watching out for that symptom, which could possibly indicate a problem with either the float level or a needle seat in float chamber. Flooding condition perhaps? I didn't see any fuel soaked spark plugs, so I assumed I wasn't having any flooding issues.

I know I readjusted the float levels when I rebuilt the carb, to get it closer to specs. It worked fine before I messed with it, no dribbling fuel. I have a feeling that by readjusting, I created this new problem. Should have left well enough alone.