1989 outboard overheating

[chads]

I have a 1989 90 hp yamaha engine that gets used quite frequently. Last year I had to water pump and impeller replaced. the boat is still putting out a really good stream but my overheating alarm will sound if i run it around 4200 rpms for an extended period of time. Anybody have any ideas as to what could be causeing this problem

[jb123]

water pumps, thermostats, should be changed yearly as preventative maintenance. prv valves should be checked and also extremely cheap enuf to change.

[slim]

usually any overheat instance around 4500 or higher is caused by corrosion in the heads and covers. if it is corroded, your gonna have to remove the heads and covers, sand or bead blast them to remove all the crud and repaint with zinc chromate primer. install with new gaskets and torque all bolts and you should be good to go.

[RJF]

Do what slim says I had the same problem I cleaned the heads and head covers replaced the gasgets and all runs great I even picked up 100rpms Thanks slim