The governor functions as a balance of forces. Spring force, varied by the accelerator pedal counters the centrifugal force of the governor, dictated by the speed of the engine.
The balance of this force determines the position of the control collar. Since the lower end of the IP is lubricated by engine oil (multifuel) while the top is fuel, the two are sealed. And the single control for the fueling rate passes though this barrier. As such, it has very small clearances which are highly prone to fouling.
Take the engine stop solenoid off. Then cut the safety wire and remove the two screws securing the fuel control lever. Remove the hitch pin connecting the control lever to the governor. Now soak the control lever in Berryman B12 carb cleaning solvent for a few hours.
Reassemble making sure the notch in the lever's pawl match that in the collar around the hydraulic head's plunger. Grease in the pawl's socket will help hold the pawl in correct position.
Viton o-ring on the lever mechanism in case the existing o-ring is shot. New safety wire, then coat the gasket in Permatex Ultra Black before reinstalling.