Small Engines (Lawn Mowers, etc.): Kohler CV15S engine run away, cap screws, john deer

Question
I have a John Deer 150 mower with the CV15S engine.  If I use the throttle at fast speeds it sounds like it is running too far over a max rpm.  I can lower the throttle and it runs ok.  I was told the governor was broken, but if it holds its speed while mowing it seems the governor is working.  I tried to adjust the throttle mechanism, but the cap screws that hold the base throttle mechanism have stipped heads.  Can you tell me what broken governor would perform like?  Does the engine have to be removed from the mower to replace the governor mechanism?  

Answer
It may be running too lean.  If the spark plug is gray and ashy, then this would be the case.  If the internal governor were broken, the engine would run very erratic or not at all.  The throttle can only open so far and even at wide open throttle the engine can't overspeed unless there's a problem with the carburetor.  Usually it's an overlean condition causing overspeeds.  You'd need a tachometer for small engines to monitor the actual engine speed and determine for sure if it's turning too fast