Outboard overheating

[GART]

I have a Mariner 8hp 2-Stroke I serviced it over the winter, and replaced the impeller. Tested in tank all seemed to work ok, with a strong stream of water coming out of both tell tails. I launched the boat Saturday; the engine started first pull and ran perfectly for 15 – 20 minuets, then cut out. After about 5 minuets it restarted and ran for another 10 minuets and cut again, I left it to cool down and it restarted again, the water coming out of the tell tails was scolding hot. And it stopped coming out after about five minuets but there seemed to be steam coming out before it cut out again.
I have not found a thermostat, oil mixture is correct or even too much oil if anything, it is possible the ignition timing is slightly out on the advance control rod, but there is no misfire either at tick over or full throttle and no hesitation.
ANY IDEAS???

 

[Freeway]

Hi Gart,
I had exactly the same problem with my Mariner 8, it took 2 seasons to eventually find the cause of the problem but, according to the people who service the motor, some sort of grommet that sits in the water intake (just above prop) had come lose and twisted in the waterway and constricted the flow. The funny thing is that the engine would run fine whilst cold but after about 20mins the tell tale would gradually fail and steam would start pouring out instead. The only way to get the flow back was to run the engine at high revs but after a while even that did not work. I initially thought that it was due to the thermostat failing but removing it gave no improvement. The fault finally got so bad that I had no tell tale at all even when cold; I came to the conclusion that the head gasket must have gone and that the pressure was impeding the water flow so I was expecting a huge repair bill. I couldn't believe it when they told me it was all down to a poxy grommet.

 

[Lakesailor]

The same cause as Johnson overheating on their small twins. Sounds like yours is at the bottom of the coolant pipe though. On the Johnsons it's the top, under the power head. Worth investiagting and speaking to Mariner mechanics (is it the Yamaha engine, re-badged?) If there are more grommets it's worth doing them all at once.

 

[GART]

Yes it is a re badged Yamaha unit, I will investigate the bottom end.
I also have found out where the thermostat is, I just cannot undo one of the six screws to get at it at present. But I could not see why the thermostat would cause the problem unless closing when getting hot rather than opening.

Thanks for your help everybody.

 

[Lakesailor]

Or being jammed closed or partly open.

 

[Boathook]

My Yamaha Malta had the same problems a few years back. Found the thermosat blocked with sand/salt. Cleaned out, new thermosat and worked ok. So worth checking thermosat.