Is the throttle cable jamming? I suggest that you disconnect the wire throttle cable at the carb end and then start the engine and operate the throttle from the carb itself. If the engine then runs ok, you most likely have a seized throttle cable. If you can get access to it you could try WD40 at both ends to see if that will free it. Failing that, you may have to remove the cable and then either free it off or replace it