Have you measured the actual maximum operating speed r.p.m.? Have you referenced a manual to find specified maximum operating r.p.m.? If you have not, I recommend you do so. You can buy an inexpensive but accurate electronic tachometer that operates by wrapping a wire around the spark plug lead. It also doubles as an hour meter. Here's a link to a good quality unit (model 51) for $35.00. www.redingtoncounters.com.
If you confirm that r.p.m. are too high, you'll have to figure out what type of governor the engine uses. Some are simple throttle set screws others are vane/spring actuated. Once you establish the governing method, you can determine what is causing it to malfunction.
This message was modified Nov 26, 2007 by borat