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HELP!! 2003 kx 125 losses compression and then 2 days later gains it back again

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hi i have recently bought a 2003 kx 125. i bought it with a brand new engine rebuild but the first few times i rode it, it would foul a plug every 10 mins we eventually got round to chainging jet sizes and cleaning the reeds and power valve and it runs great. a week later i took it to my friends and rode it for about 3-4 hours straight. it was not rode to hard but it wasnt just poodled around the field. we then took it pined across a field for about 10-15 seconds and then took it over a jump and it cut out in the air. it then started to smoke for about 30 seconds really bad out of the exhaust. we let it cool down for a while after that. obviously at first we thought it was the spark plug so we changed that but it still wouldnt go. so we tried kicking it and it had no compression. we then tried bump starting it and it wouldnt go. so we rolled down a massive hill and then bumped it. it started and rode fine until i shut the throttle off, then it cut out again and wouldnt start so i took it home and left it in the shed for 2 days i have just been down there now and all the compression has come back i have not tried starting it yet because i dont want to do it any more damage than i allready have.

do you guys have any sugestions on what it could be

I REALLY NEED HELP!!!

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It sounds like somthing is opening up when it gets hot. The sudden cutout,smoke, fouled plugs, no compression. then when cooled down it seems better. Sounds like there may be a crack in the cylinder or head. Possably the crack closes up when cool and opens when hot. Two stroke easy to work on. Pull top end and check. Could also be a head guasket.

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I'm gonna go with lowmass, but I'g lean more towards the cylinder. Possibly a crack in the bore or along the exhaust bridge. Otherwise, you've blown a ring or destroted the piston. I suggest getting it apart most ricky-tick.

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