Paul;
I can go along with your diagnosis, but why does it run "like a sewing machine" for a period of time before it runs like crap? If it is a vacuum leak I would think the leak would be present at all times. The thing that I can't figure out is that it has had two carburetors {one old and rebuilt and one brand new) and the symptoms turn out the same. Almost sounds like some sort of heat related vacuum leak not associated with the carburetor itself. Let's see what the OP finds after going through your checklist.
SimS
