Funny, when I was at school the emf generated by a conductor moving in a magnetic field was proportional to velocity and to the strength of the magnetic field. The calculations were done on the number of lines of force cut in unit time.

On the Yamaha, it is the spring that opens the throttle, trust me, I've pushed and pulled the linkage while the engine is running, and the spring opens the throttle, and the governor lever (operated by who knows what inside the crankcase) closes the throttle.

And if you google "Medway Mowers" they are supposed to be the gurus of small garden machinery round here. Experts on small electrical machines seem not to exist, the throwaway culture having rendered them obsoltete.

Silencers do not usually need cleaning. When did you last clean one out on your car or motorbike?   

How does any governor know that the load has increased? Only by a drop in revs.

Since the gadget is now apparently running stably under load and producing the right voltage, I am unlikely to split the crankcase to find out how the governor works as a matter of idle curiosity.

But if anyone has a workshop manual for a Yamaha ET500 I'd be most curious...