Adrian, the process I use to begin an investigation of governor issues is much the same for any mower. It is carried out with the engine stopped. First, set the speed control to idle and verify that the bell-crank on the throttle butterfly is resting against the idle stop (the idle speed adjustment screw). Second, set the speed control to maximum speed and verify that the throttle butterfly is wide open and against its stop. If one or other of these conditions is not met, trace the mechanism to find out why. The problem is just about always one of two things:
1. Stretched governor spring. Governor springs are normally close-coiled tension springs. If they are not close-coiled for their entire lengths, they can't work.
2. Bent or incorrectly installed spring, linkage or governor base-plate jamming the mechanism or limiting its movement so it will not move all the way from throttle closed to throttle open by moving the speed control lever.
Your governor sounds as if it is suffering from one of these ills. I suggest you follow the process of moving the speed control lever and seeing what happens to the throttle butterfly. If it does not move through the full range, find out why. Is the spring OK? Is the linkage free? If the answer is yes to both, it sounds as if it is probably incorrectly assembled or adjusted.
These governors are very simple. At idle, there is no tension on the governor spring and the mechanism is designed so that it mechanically closes the throttle under these conditions. At maximum speed there is maximum tension on the governor spring, and the spring pulls the throttle open. The throttle opening is limited, when the engine is running, by either a centrifugal mechanism in the crankcase or an air vane near the flywheel fan. Whichever of these the engine has, pushes the throttle toward closed, and is resisted by the governor spring. The governor's mechanical element tries to close the throttle and the spring tries to open it. They find an equilibrium at a speed that depends on how far the governor spring is stretched, which simply depends on the setting of the speed control.
If you go through this process and still can't work out what is wrong, post a series of pictures of the mechanism with the speed control set at both maximum and minimum. Hopefully someone will spot what is wrong - but it is much easier for you to do it, because you have the mechanism in front of you, and you can see what moves when.
