Something else to check, while you're running it on Seafoam. Look at the throttle linkage from the governor while the engine is running. I've seen WAY more than a few Cubs and Lo-boys that the throttle linkage was bound up on the block, or rubbing something else nearby, and preventing the governor from actually controlling the carburetor correctly. If those little arms get bent a little too much the wrong direction, they'll hang up or rub against things. The tractor will run, and all will seem to be working fine, but that little bit of rubbing is actually enough to make them surge and hunt, preventing the governor from working quick enough.
1951 Farmall Cub, Cub Cadets 102, 104, 1811, 1864, Simplicity Legacy XL 4x4 Diesel with FEL, 60" mower, 50" Tiller