Yamaha 60 rpm problem
 

My brother has a Yamaha 60HP FETO L (a 6H2 L, serial no 458506) outboard
and it has developed a problem whereby he was out in his boat a while ago
fishing and switched the engine off for a while, about half an hour. When
he restarted it it would not rev above 2000 rpm. It would happily rev to
4600 prior to this. I know nothing about outboard engines but have rebuilt a
few car engines and it has been my hobby for years. He says the buzzer is
not going off (the overheat alarm?) and the oil feeder system is functioning
correctly. He thinks there is a problem with a contriol circuit which
reduces the revs if the engine overheats. It isn't overheating though. One
thing that has come to mind is I believe the overheat sensor in the head
works like a switch and grounds the signal so it could be something as
simple as the sensor going defective and shorting to earth. Any
suggestions?