There have been a few cases where the engine will start and run fine, but after several minutes it goes crappy, sounds way rich, won't respond to needle adjustments, and will in general drive you nuts because you will try everything, rebuild the carb, change the fuel system, new this and new that. The cure is to get a new ignition, because the old one apparently has some component that goes ratty when it gets hot.

Try the ignition from one of your other engines and see if this fixes it. If not, well, you haven't lost anything but time and you have eliminated one possibility.