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I'm not 100% sure, but probably sand and little bit salt that reacted with the copper in thermostat. I will look closer tomorrow, there are still some of this sludge behind the thermostat. The other channels looked pretty good IMO.
I'm not allowed to upload pics here (?!) so please look at the...
You were spot on! the thermostat was clogged. I hope that will solve the problem.
I needed to get the motor running ASAP so I posted the issue at another forum as well. Here comes a link if someone have the same problem and want to read more...
I have a Yamaha 2-stroke 4hp (4AC). At startup there are a nice water flow through tell tale, but after driving a few minutes I noticed the motor was slowing down and steam gets out of tell tale, so I stopped it. I replaced the impeller but it made no difference.
The history of the motor is...
