Mariner 20c 2 stroke overheats…eventually…

[Oardacity]

Hi all, I’m having a problem with my old 2 smoker and hope someone could shed light on the problem.
Mariner 20hp 2 stroke late 80’s model I believe (20c) ran great for about 4 hours of slow cruising the other day before over heating and cutting out. Left for 15 mins to cool and started up again, ran fine for the 20min trip back home.

Had another head and gasket so changed both (old gasket looked totally uncompromised) Suspected thermostat wasn’t working so removed and ran the next day with no thermostat - ran fine for 1.5 hours before overheating and cutting out.
Tele tale is looking good, water pumping well up to the thermostat housing, impeller has probably done 25 hours at the most since being replaced last summer. Engine used on river so rarely does many revs. Any suggestions would be much appreciated, thank you in advance

 

[BruceK]

The water passageways may be getting clogged with silt and debris from the river, timing too advanced, carb running lean, or it's sucking in air after the carb. (intake mainifold or crankcase gasket). You'll have to chase it down I'm afraid.

 

[Aussie farmer]

When it's getting hot is the tell tale water hot or not really ?
If its not hot you probably have a blocked gallery somewhere.

 

[Oardacity]

The water passageways may be getting clogged with silt and debris from the river, timing too advanced, carb running lean, or it's sucking in air after the carb. (intake mainifold or crankcase gasket). You'll have to chase it down I'm afraid.

Okay thanks you, that gives me a few things to look at.

 

[Oardacity]

When it's getting hot is the tell tale water hot or not really ?
If its not hot you probably have a blocked gallery somewhere.

It’s difficult to say. As it doesn’t give much warning, it runs fine, keep checking things, etc, then after at least an hour it gets ho, stops and won’t start for 10mins. Then, it’s fine and the tell tale is cool.

Maybe I’m wrong but if it were a blockage I was thinking it wouldn’t take so long for it to overheat. Or maybe because it is only ever runnning full pottering mode, and without a thermostat currently it takes a while to get hot (or even up to temperature)

 

[Oardacity]

Took the leg of last night to start inspection - thought best to go back to basics first. Looks like I’m impeller has got very hot. Also note the scoring on the shaft. I have my own theories and possibilities of what might be going on but would love the thoughts of more experienced folk
Thank you in advance