When to replace a dirt bike's piston?

[tollat]

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

 

[manchew]

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.