Yamaha 80 overheat on 4000rpm

one eye · 18 June 2007, 20:28

84 yam 80 overheat at 4000 rpm,

had new block, impella and insert.

new gear oil.

new temp sensor.

Timing and and carbs set by (pro).

Runs at 3750 all day long but at 4000 overheat warning and rev limiter cuts in,water stops coming out of tell tale hole, put on idle and within 1 minute will run at 3750 again with no problems.

New block was from 85hp mine is an 80hp, mine had no thermostat but the 85 block did, removed the stat but no diference apart from slow warm up.

any ideas guys?

Nos4r2 · 18 June 2007, 20:57

Whip the cowl off when the alarm's going off. If there's smoke in there then the powerhead/leg gasket hasn't sealed properly when you changed the block and the exhaust gas is pressurising the water system.

one eye · 19 June 2007, 11:58

Thanks for that

Not what i wanted to hear, but such is life.

I was warned about this b4 installing it, so i took great care to clean both surfaces with 400 wet & dry and a flat metal block, and was also told not to use any gasket sealant.

Would you advise the use of anything with the gasket?

Tims Osprey · 19 June 2007, 12:10

You may have to have it professionally re skimmed to get it perfectly flat before refitting it again, to make sure it seals properly this time?

Nos4r2 · 19 June 2007, 12:33

This stuff is very good. It can be applied with a modelling paintbrush. It's heat resistant to 195 degrees C and will fill a big enough gap that you shouldn't need to skim anything unless it's badly warped. (saying that, if it works then I'd whip the powerhead off and get the face skimmed anyway)
It doesn't set either.

That's assuming that it's what the problem is.

one eye · 19 June 2007, 13:28

Thanks tim, may well have to look at that option.

Nos you said this stuff is realy good! What is it?

one eye · 19 June 2007, 13:29

Sorry ignor that last one just realised it was a link.

Thanks

one eye · 21 June 2007, 14:38

Iv got a couple of other thoughts, may be worth investigating before i have to strip the powerhead off again.

one is- the tube conector on top of the pump housing, should that be water tight to the tube on the engine?.

The other is the prop size, it has a quicksilver 48 16988 a5 18p, I presume that is an 48x18p.

Would that be arround the correct size? If its over proped that could give me an overheat problem may be?.

I tried mono ski with it last weekend and it did not seem to have enough to pull me out fast enough.

No problem on two skis.

one eye · 21 June 2007, 17:39

That would be a very large prop, Cant be 48x18.

Just mesured the diamiter, its 13".

So must be 13"x18p.

I know a bit about model plane props ( but air's thinner than water) and that seems to be alot of pitch to me.

Nos what have you got on yours?

Just trying to eliminate evrything b4 i have to get the tools out and refit the powerhead.

Pete7 · 21 June 2007, 18:25

Quote:

Originally Posted by one eye

Just trying to eliminate evrything b4 i have to get the tools out and refit the powerhead.

I would do a compression test before stripping it down.

Pete

Nos4r2 · 21 June 2007, 19:20

It won't be the prop-nothing to do with propping will affect the pee stream unless the exhaust is severely restricted by the quicksilver prop.
I'm running a 17" pitch and probably need to go up to 19" btw but that's more about speed than pulling a skier out.

I'd do a compression test on it too but the only way low compression could tally with the pee stream stopping would be a cracked liner or leaking head gasket. Might be worth popping the head off, giving it a thorough examination and maybe doing a crude leakdown test on it.

If you want to have a chat to a proper Yamaha marine engineer it'd be worth going over to the iboats.com forums and asking the question in their dedicated Yamaha repair forum.