I have a 140, run only a few hours since 1991, but enough to keep up the battery and run out the fuel before it gets too stale. Maybe use couple of gallons per year. The tractor is really worn. Needs total rebuild. But time and energy dictates taking care of just a few things at this time. Water pump, radiator and steering, for sure in the immediate future. While doing those, the governor needs attention. Often when the throttle lever is pushed forward, the engine speed increases slowly, taking some time to go to full governed speed. But sometimes, it accelerates very quickly, going well past the governed speed and then the governor closes the throttle quickly, then resuming correct speed. While working on the front of the tractor, I might as well do the governor, too. Familiar words?
From the description, is there likely to be a problem within the governor? Worn? Rebuildable at home? Is this governor involved in the timing of the ignition as with a Cub?