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2nd piston/cyl damage in 2 weeks

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i have a 2006 kx250,i changed the top end a few weeks ago-the throttle was stuck wide open on start up with new top end-i shut it off played with the throttle,started up again still wide open throttle so i let it run while i tried to get the rpms down,well lost compression/and died the result was a broken piston and damage to the cyl just above the power valve.i sent the cyl and head to power seal,while the i was waiting for it to be repaired i cleaned out eninge,i did not find any metal in engine so i didnt split the crank case and do any work to the lower end,crank does not have any up/down freeplay.I got the cyl back today so i put in a new piston and rings,took a few kicks but it started and ran at normal rpm for a cold engine,i let it run for a few minutes,in the choke and then when i was warm i turned the choke off to let it idle,then it died but would start back up but died when i gave it throttle.lost compression again.same damage as before but not as bad.what am i missing guys?????

I have not taken the cyl back off yet so i don't know what the piston or cyl look like yet

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Well don't know why I didn't notice as I've done several top ends since I've had the bike but I put the piston in backwards and the rings were hitting the power valve opening.

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