Piston help. Newbie

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Aug 27, 2016
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#1
Hey im new to the two stroke engine so seeking some help on this piston thst came out of my girlfriend yz125. We went riding at the track. She did and hour of hard riding but she let her bike cooled down acouple times before went back out there. So we bring the bikes back home. Next days she wanted to go riding again and then all off a sudden no compression and the bike didnt wanna start. So i opened it up and notice one power valve was stuck close and the pin that holds it to the clip fall out. I reinstall a spring clip in it and slap that pw back on and they both move nice now. Now time to inspect the piston as its only 3 hours old. I notice that the ring on exhaust was stuck inside the piston and cant get the ring off.i also notice a siezed mark where the ring is stuck but the cylinder has no marking or groves. And piston is all blackish/brownish on top and some on the side. It is also blackish under the crown where ur piston pin sits. Im running 40-1 mix in her bike. She also have bent rads but does not leak.is the bike getting to much fuel and causing detnation? Is the bikeit overheating because of the rad
? Her sparkplug always comes out brown.is the colour of the piston normal? Its a prox piston
 

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jaguar

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#2
the piston skirts were blackened due to the blowby past the stuck ring.
make sure the filter is clean and oiled.
usually you can recover a piston like that. just make sure there is a little vertical play for the ring. you can use an exacto knife to scrape the surfaces.
the black under the piston crown means the fuel/air mixture was too lean. enlarge the main jet and raise the needle one notch.
 
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