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Broken valve stem

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What could cause an exhaust valve stem from my 2017 KXF450 to break except for bad luck? 

Can it have something to do with rich mapping, (flushing away lubrication, is this even possible or too far sighted?) oil type, (I use Motul 10W40, change every 4 hrs) or anything else?

Note; this bike appearently suffered from detonation causing (minor) piston damage about 8 hrs ago. Head/valves seemed fine at the time.

According my dealer OEM Kawi mapping is too lean so they adjusted the mapping. Also I went from 95 to 98 octane fuel. I even tried 102 but bike did not run as well as with the 98.

Your input/opinion is appreciated.

 

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chances are, because the engine (cylinder head) overheated extremely, that the exhaust valve kinda started seizing or was seized in the guide and nobody noticed..would be nice to have a look at the other valve and guide and compare it with the damaged side

or just extreme heat before contributed to the failure... I don't think its bad luck, cos there would be others with the same prob

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