"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. "

The key here is "strength of the magnetic field" which is varied by the voltage regulator. The shaft speed HAS to be a constant 3600 RPM to generate 60 hz (or 3000 RPM for 50 hz).

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

Cars and bikes are usually 4 stroke engines- the muffler/screen and ports on all two-cylce engines need to be cleaned- usually at 100-150 hours of operation. Failure to do so will rsult in poor performance and, in some cases, the hard carbon deposits will become trapped between ring and cylinder wall, scoring the latter.

Don't ask questions if you don't like the answers.