John, I had a M20 that I repaired and sold awhile back that had a sticky throttle valve. See this post.
http://www.onlycubcadets.net/forum/s...ad.php?t=44023
I never figured out why it did that. I just ran it on to bench, but maybe in a tractor it would have surged. It was like when it was running the shaft had a bind, when stopped it was free as a breeze. Good luck.