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I have a 2006 B&W 150 with 700 original miles. It did sit (inside) in a tropical environment for several years unused. It ran fine for a few months until I left it with no treated fuel for a few months. Once again it did fine after I cleaned the carb and have treated the fuel since.
Recently it runs fine every morning when cold and every time after 10 minutes it suddenly loses power (can’t make it up the hill) and heats up to red line! After I let it cool it’s okay for another 10 minutes.
The cooling system seems fine in that I can feel it hot on both sides of the pump and in the reservoir. I have burped out any air with the reservoir cap off. I have removed the thermostat (which tested okay in hot water). I have not flushed the system as it looks good and the radiator grill is new and clean, fluid and level looks good.
I took the carb to a dealer and had the jets and carb cleaned thoroughly and use fresh gasoline. I did not drain the tank thoroughly. I did check the fuel filter. I don’t see how it could be fuel supply if it runs fine when cool. I’m a bit suspicious of the “auto bystarter” which I believe is some kind of electric choke. Another thought is a bad CDI (capacitive discharge ignition)?
I hope it’s not a warped cylinder or rings or valves from overheating and it seems like that would be constant power problem even when cool. I don’t know which comes first: overheating or power loss as they are simultaneous. Perhaps I should have the valve clearance checked?
It seems like I read a forum with similar (KYMCO) problem and he replace everything without improvement!  I’m at the point of replacing the carb or selling the bike!
I don’t have access to a repair shop here on the island. Any ideas?
Jon