PMFJIH-There are a lot of bad theories here- even the 'two stroke guru' is dead wrong. Modern small engines use an internal flyball governor. It does NOT "shut down the throttle when the revs pick up" - it OPENS the throttle when the engine load increases and it does so by working against a spring. The OP's Yahmaha is not overvoltage because it is running too fast- frequency is related to shaft RPM, not voltage. It sounds like the Yahmaha has multiple problems 1) engine performance and 2) electrical issues. The engine performance could be from carbon in the muffler (needs to be cleaned on a regular basis) or leaking crankshaft seals or both. A good small engine shop can tackle those. As for the electrical problems, well that is what electrical shops are for.