outboard looses power at full throtle

hope someone can give me any ideas,
i have a 1989 yamaha 60hp engine, most of the time runs fine at 3/4 speed
but if i open it up to full speed it can run ok but sometimes it just drops in power right down to 1/4 speed and i have to chug home at that speed,
the next day it seems to be ok,
I have changed to plugs and the plug leads in case one is breaking down.

Is there something which cuts in to stop the engine overheating or am i not supposed to have it at full speed for more than 5 minutes at a time.

as a novice at this ,is there something im doing wrong,

hope someone can point me in the right direction.

[jonp]

Pete,
Yes, That engine does have a RPM reduction mode if it begins to over heat or get to very low on oil. Basically it will cut a cylinder from firing and will run a bit rough...

Does the block seem very hot to the touch when this happens? You should be able to touch it. But if is beginning to over heat it will very hot...

Have you replaced your water pump lately? A worn impeller will pump water but not enough to reach to top of the block. Also check all you fuel lines and connections for leaks/restrictions. First things I would do...

Good luck

thanks

thank you for your reply ,
this will be the thing i will be looking at.

pete.