I was doing a later PT the other week (Chinese block) had the exact same problem. Changed the carby body, then no issues. I noted that the cam was easy to turn part of the way, but became harder as it went back to what would be the idle position, then became easy as it went back to the cold start and stop position.
It was as though the cam 'legs' were snagging up on the carby body (maybe the plastic had age expanded), then suddenly letting go. Perhaps with the lifter, and diaphragm with springs, the resistance meant the throttle cable just twisted/moved in the snorkel without changing cam position in these difficult spots.
