Compression runs about 125psi so the problem wasn't there.
I say 'wasn't' because on the advice of ZombieChopper I gave carb cleaning a 2nd try. I bought a 3-qt can of Berryman's "chem dip" carb cleaner and put the stripped carb in it overnight (the directions say not longer than four hours). That stuff strips EVERYTHING, even the sealant around the welch plug was removed. Anyway, I reassembled the carb and used some of my daughter's nail polish to reseal the welch plug. I let everything dry then reinstalled the carb and BINGO, the low to high speed transition problem is gone!
I'd read in this forum and others that canned "spray type" carb cleaner doesn't hold a candle to the "dip type" cleaners. I can tell you they are right. Berryman's is highly recommended if you have a carb to clean, especially a really dirty one. The Berryman's was $28 + tax at AutoZone.
Technically there is a chance that the welch plug was leaking air and benefitted from the resealing, but I suspect that ZC nailed it with his suggestion to try a second cleaning. Thanks ZC.
