yamaha 80 overheat at 4000rpm

[oneeye]

Yam 80 (1984)

Starts and runs great until i try to go 4000 and above, then overheat warning comes on and rev limiter cuts in, leave to idle for a minute then all is fine until 4000 again.

has had a new block and gasket/impeller and sleeve and gasket/temp sensor,timing has been set top and bottom end.

I removed the stat on the last run to see if that was at fault but still the same.

[boats.net]

That is normally a water pump problem, you recheck water pump housing and impeller.

[oneeye]

Have removed pump and upper/ lower housing all is fine, no sign of a blockage in the system either.

[boats.net]

My next thought would be the water tube grommets in water pump housing and at top of water tube at base of engine, may be restricting water flow.

[oneeye]

Being that it runs fine with good flow until i hit 4000, could possibly be a slight resriction.

The poewhead was replaced and was warned to get a good clean mating between the two surfaces and not to use gasket seal, so i used a flat metal with 400 wet&dry.

Is it possible that if this has not sealed perfect it could give me this fault?

[oneeye]

One other thing!

Looking at the chances of it being over proped.

Got boat and engine second hand so no idea if its the correct prop, its a quicksilver 48 16988 A5 18P, pitch must be the 18 bit but not sure on diamiter?

If it is very over proped i guess this could cause an over heat also?

[jb123]

you SHOULD change out pump assembly yearly, along with stats.
prop depends on your max rpms.

[oneeye]

Worked out the prop size its a 13x18.

How do i get to the top of the water tube?