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  #1  
Old 14-12-2016
clearasmud clearasmud is offline
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KTM125 mystery bag

I've picked up a 98 125sx that "went fine" and just stopped for no reason..

Diagnosis so far. Either the piston has been in there forever and decided enough was enough or it's hit the powervalve or both.. Thoughts ?
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  #2  
Old 15-12-2016
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Rebuild Rebuild is offline
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That's not mechanical damage, that's melted.
Maybe it was ridden without coolant?
Don't think it's detonation, as usually will have lot's of marking on crown too.

Check the bottom end for any movement up/down

How's the cylinder?
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Old 15-12-2016
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ran too lean or held wide open for 30km's, maybe crank seals have gone.

as Rebuild said that's not a mechanical fault that's gotten too hot or too lean and melted the crown of the piston off, they may have got the ignition timing wrong also
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Old 16-12-2016
clearasmud clearasmud is offline
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Thanks for the input. Much appreciated. The cylinder had some black marks above the exhaust port but I've been able to clean most of it up so far with a honing wheel and a scour pad. No movement in the bottom end
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Old 16-12-2016
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would have gone well just before it stopped though
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Old 16-12-2016
clearasmud clearasmud is offline
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To add some context here's the rest of it pre-tear down
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