Cooling water flow Where does it go Yamaha 15hp

"A friend of mine has a Yamaha

"A friend of mine has a Yamaha 15 that overheats when it is run flat out for more than a few minutes. The paint around one of the spark plugs has been discoloured by the heat, leading me to think that the head isn't being supplied with water. The water pump has been replaced recently, and the tell-tale pushes out a good stream.

What I am trying to work out (before I start looking at the engine) is Where does the water flow?

When the thermostat is open, it obviously flow from the pump, through the block and heads, via the thermostat, and into the exhaust. But what happens when the thermostat is closed? Water must fill the water passages in the block and head. Is there a by-pass for when the engine is closed?

I have searched the list archives, and the only clue is that the head gasket could be leaking, causing the water to be blown out of the water jacket. I will chech the gasket by doing a leak-down test.

Any description of the coolant flow, or a link to a diagram would be appreciated.

Feel free to email me direct at:
[email protected]

Thanks
Smiffy"

"Smiffy,

Many of the Yamaha

"Smiffy,

Many of the Yamahas have an anode in the water jacket under the cylinder head. As it dissolves it tends to clog the water jacket. I don't know which model you have, but most of the 15HP models have this anode.

A bad thermostat will cause the problem you mentioned also. There is a by-pass which allows some water to circulate when the thermostat is closed.

Water is pumped up from the water pump, and passes through the exhaust (on the port side of the block) first. It circulates through the block and down the starboard side even when the thermostat is closed. Some shoots out through the indicator hole, but most is discharged through the exhaust tube and out the gearcase. When the thermostat opens water circulates through the cylinder head also.

Tony"

" We took the head off this ev

" We took the head off this evening, and there was alot of crud around. The return from the thermostat was almost completely blocked.
The anode was also crumbling.
Hopefully, cleaning it all out will resolve the problem.
I was surprised that there is no water jacket over the top (the back realy) of the head. This accounts for the discouloration of the paint around the plugs.
Thanks for your help Tony.

Smiffy "