15hp Johnson, Only Half power after it’s fully warmed up???

[SibFisherRy]

Long story short, basically the engine runs perfect and starts on first pull every time without fail, it flys and runs like an absolute dream! Drive round for a while, turn it off for a minute and then when I start it back up again starts fine but open her up and it only runs at less than half power! It’s soooooo Annoying! If I let it cool for half hour or so it’s fine again????

Done all the usual, Cleaned carb,
Changed the plugs, changed filter and fuel lines ect ect

15hp Johnson

[ChrisParish]

That is a weird one.
Given that it runs from cold then stops working properly once the engine has been stopped but it hasn’t cooled down then that would indicate to me that it is an issue with something getting too hot once the active cooling from the water pump has stopped.
I would guess that it is probably running too lean / hot and that the cooling water is keeping it just cool enough to run but once the water stops the temperature rises.
You could Try returning to idle for a couple of minutes before shutting the engine down and see if the problem is still present, but honestly I wouldn’t run it again until you have found the problem.

It’s not a small job, but you may need to check the crank case seals and reed valves as you might have a crank case compression problem. This would lead to the engine running lean at high speeds causing overheating and eventual seizure of the engine.

[ChrisParish]

Of course, it could be an issue with cooling water flow. Check the impeller for damage.

[warder54]

Thermostat stuck in closed position?

[Sibfish]

do a compression check when it is hot (run it in a bin for a while until the issue arises, it could be piston/ring/bore wear that is fine when cold but as the heat soaks in to the bores and they expand causing blow by, take the cylinder off and see if the skirt on the pistons is black from unburnt fuel. Just a thought.

[PeterM]

What SIBFISh said - I have had one of these were cold it was an amazing engine , hot it was impossible to start / no power if it was started etc . Turned out it was a fundamental issue with the rings etc - that needed a lot of (costly) work to sort