There does not appear to be any leaks and once started the engine runs fine.

I'll check the electric choke and fast-idle cam. During winter (even here) it does get cold enough to need the choke, but on 100+ degree days lately, as you said, the choke may be activating.

I also wondered if it could be vapor-lock or timing-related. Is 8 to 10-degrees initial advance too much?

The engine in question, is an AMC 304 V8 in my CJ-5 Jeep.