Strange cooling problem Yamaha 5 hp 2stroke

[juup]

Hi
Today I tried to fix my son in law's outboard engine.
It runs fine when the waterpump is out of the water (not too long because of overheating).
when the waterpump is in the water the engine stalls after 2 minutes. Then there is water inside the combustionarea and the sparklug is wet with water. When blown dry with the aircompressor the engine runs again for two minutes.
When we loosely unsrewed the cover over the cylinder where the coolingwater flows abling coolingwater to bleed there (thus reducing the waterpressure) the engine kept on running fine.
So how does the cooling water enter the combustionarea. I had the feeling it came in via the exhaustopening at the cilinder but that not likely as the exhaustgasses escape with some force.
All the gaskets that have to do with cooling waterregulation are fine and the thermostat workes fine too.
There is no seperate cylinder head (and gasket) that could be blown as the cylinder is made out of one piece and the engine can be split (vertically) aroud the crankshaft.
Could a leaking gasket around the propellorshaft cause cooling water too enter under the piston? and so spoil the process?
regards from Holland
Juup

 

[robert graham]

Re: Strange cooling problem Yamaha 5 hp 2stroke

there may be a pin hole in the aluminum casting of the cylinder or head. could be a pin hole particularly around the spark plug hole threads. corrosion caused by water left in the motor can cause this. If you can locate this hole you may be able to have it welded up. Good Luck!

 

[juup]

Re: Strange cooling problem Yamaha 5 hp 2stroke

Hi Robert
Thank you for the reply.
I was thinking, is it not so that above the piston (combustion area) in a two stroke engine, therre is always some pressure ( explosion, exhaust gasses, compressing fuelmixture)? Under the piston there is at one stage a vacuum.
Now, how could the waterpump make so much pressure to enter the combustionarea and spoil the runningprocess of the engine.
Whereas it would be easy if the (cooling)water enters somewhere under the piston during the suctionphase of the piston and then press it together with the fuelmixture through the sidevent (dont know the english word) to the combustionarea above the piston.
Is that more likely?
regards
Juup