The mystery persists. Vacuum at idle was registering a little low at 13 (measured at the carburetor). I drove the truck about 20 miles on country roads at speeds of up to 65mph, and everything seemed to run fair. It is running a little hotter than usual, and it’s hard to remember the power before the head gasket, but I managed fine with 5 adult passengers.

I’m still having trouble understanding why the vacuum advance changes so much at idle compared to static.

Just took the carburetor apart and it is quite clean with no noticeable clogs. Still planning the rebuild when the parts arrive.

Everything seems to run well, it just won’t stop with the black smoke.

When removing the carb I noticed some smoke/steam emitting from the manifold vacuum for the break booster. Not sure if that’s relevant.

Thanks again for the replies.