Here's the rest of the story...
Jon, I tried your suggestion to no avail, so I knew it was time to get the carb back apart. I removed the air horn part, leaving the rest on the intake.
Took this to the bench and I noticed a slight drag on the power piston, so I took the main well support off and threw everything attached to it back in carburetor cleaner.
Took it all out and washed the parts carefully and it was then I noticed the problem. At the bottom of the main well support, there was a small piece of old gasket material I had not cleaned off. It wasn't much, but it was enough that when I had initially reassembled the carb, the support was tipped back far enough that the top of the power piston rod was scraping slightly against the sides of the support and not cleanly contacting the power valve.
RECLEANED everything for good luck, put it all back together, reassembled the carburetor, and it now works perfectly (OK, Carbking, at least well enough for being a Rochester B). And obsa, you might have your son check the power piston assy in the 50 for some similar problem.
Thanks again everyone. That's why I hang around here. You guys are the best.
