Just my 2 cents worth. I've seen the governor on the inside of the crankcase a few times. It's usually a PADDLE! The oil splashing off the crank is directed at the paddle. The higher the rpm, the harder the oil splashes against the paddle. So the way they are set up, if the rpm decreases due to load, the spring on the outside will open the throttle, providing more power. So if you start going up a hill on your tractor, the engine rpm goes down, the oil splashes with less force, and the spring opens the throttle for you. That's the balance that can be hard to get.
All my life I've heard small engines going up and down in rpm like rrr RRR rrr RRR rrr RRR. I've always wanted to be able to adjust this fluctuation out, sometimes with no luck at all!
Sometimes the little shaft coming out of the crankcase is "D" shaped to accept the lever that clamps down onto it. That way, usually the adjustment is on the lever.
Sometimes low or too much oil or wrong viscosity can have something to do with it.
Also, some gas tanks have a brass or nylon mesh filter in the bottom of the tank. If the tank is allowed to sit and all the gas evaporates out, that mesh filter can become completely gummed up with gas "varnish".
I once boiled an MG gas tank on top of a barbeque grill, I filled it with POR-15 Marine Engine Clean & water, which I think is basically a lye solution. It cleaned that old tank very well.