When to replace a dirt bike's piston?

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tolla t

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Hi I have a 2000 model Yamaha YZ 250, I bought the bike a few months ago, since then it lost allot of power, and there is allot of black 2-stroke oil coming out of the exhaust. The bike is also fouling plugs on a regular basis and I sometimes get flat-spots. I mix 40:1.

Is this an Indication that I need to replace the piston and rings
 
Hi look, it's prob not the piston at all,if it is leaking oil into the combustion chamber it's prob leaking through the piston rings maybe even a valve seal neads replacing oh wait it's two stroke yeah look at the rings there prob burnt out.this could def explain the lack of power and whitish/grey smoke.
You could waste so much time and money pulling it down yourself or buying new parts straight up.
Best beat is to get a quick pressure test at your local bike machanic it needs to be seen first hand.
You may have burnt them out by having the wrong air fuel mixture as far as the ratio you mix too, ask at your local bike shop if thats to high or not just check.Anyways best of luck mate not a bad bike
 
Needs new rings by the sound of it! You may as well replace the piston and wrist pin while your at it. Pistom kit and top end gaskets should come in under $200. Do it yourself, basic tools only needed, plenty of info on the web!
 
Try a compression test first this will tell you if you need to replace the piston and rings. A little oil will come out of the exhaust on a two stroke but not alot. Make sure you are mixing the gas with the correct 2 stroke oil. More oil is better than burning up a motor. Hope this helps.
 
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