Dismantled the carb to look for phantoms. Discovered a gasket leak on the left side by shooting carb cleaner in all ports. There is a small cover plate there. Inside the chamber is "valve" that apparently opens when the engine is warm. I was so stoked when all that carb cleaner shot out and hit me in the face that I forgot to take a picture.

Anyway, this valve resembles a bi-metal strip with a (potted) screw adjustment and o-ring that seals off an internal vacuum port. I suspect that it existed for some automotive application, like vacuum-actuated opening of the flapper on the air filter inlet, or something like that. It could explain why it was OK cold and then allowed the vacuum leak at the gasket only after warming up. The vacuum line that connects to the chamber is actually a loop that goes to another vacuum port elsewhere on the carb, effectively capping it. While I was at it, I replaced the carb base / intake manifold gasket.

Runs great (again). Big test coming up during my 4 day Thanksgiving desert trip.

Hope this wasn't too boring for you guys, but it made me crazy for awhile so I thought I'd share it.

Glad you got it sorted out. Thanks for posting what you found out. It may help some of us out later on.
mick

ditto,
good information is never boring

Chris