Yamaha 2-stroke 4hp overheat problem

[Skarpsynt]

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 unknown for me, I got it with a boat, but it should be said that there are cracks in the swivel brackets. They are stabilized with a hose clamp, I don't think it will cause this problem. The motor has probably not been used in salt water


Any suggestion what to do?

[Fenlander]

Hi and welcome to the forum.

Have a look at post 2 in this thread where I mention what the issue was with my Mariner version of the same engine.

https://www.rib.net/forum/f36/marine...ale-71312.html

I know you say it's probably not been in salt water but with a 20+ year old motor unless owned from new you never truly know.

[Tommy c]

https://www.rib.net/forum/f36/hot-tell-tale-87270.html
....and this was mine from an immaculate looking 5hp Yamaha

[Skarpsynt]

Thanks for your welcome and the tips!

I will remove the exhaust side plate ASAP. I didn't know about the side plate and thought a powerhead removal was needed for that kind of cleaning.

[Skarpsynt]

The channels was cleaner than I expected, but the thermostat was very clogged.

Then I tested it in a saucepan with water and measured temperature. The specification says it should be closed at around 48-52°C and minimum 3mm open above 60°C. It actually worked at first try but probably because I stirred with a spoon all the time to get a correct temperature, it helped the debris to fall of.

After that I added citric acid and boiled the termostat 20min to get it clean.


Closed-open video
https://youtu.be/6-GhUpI2Vwo

I will reassemble tomorrow and hope the problem is solved.