Hi Bob;
I have to admit I can see the problem as you have solved it too. I guess what lead me to my solution is that he mentions that it idles smoothly during the warm-up and gets rougher as it makes full operating temp. (if I understand him properly) I think he is using the fast idle to smooth it out, without dropping the jets to enrichen the setting. That's why I advised leaning it a bit.
For sure if I was tuning it myself, I would end up going lean and rich just to get a feel for it.
Either of us could be right. I just hope he writes back to state what kind of carbs he has. Describing how to do a piston lift to determine mixture is a bit tough over the 'net, but that might be next, and hopefully we get some feedback from him.
Rhys