How do I adjust the governor on mountfield SP474 SV150-M?

The engine hunts when warm and you can see the governor arm osilate slightly when on causing the throttle to open and close and the engine to hunt. The governor on this model is inside the crankcase. Suspect the governor or spring from throttle is faulty. Can the governor be adjusted? I have already checked air filter, carb (float and jets) which all appear in order.