Last year I bought a 2002 400ex. I was recently have a problem with it not stay on. I would get it started and it would stay on but would cut in the middle of riding. I bought a spark plug and that fixed that problem it fired right up with no problem. I took it for a test ride to check and see if everything was fine. It seemed OK but my cousin who was riding behind me said he felt something coming out of the exhaust. So we took but right before I did it cut off and made a loud gun pop. I'm not sure if it's the air filter or the old gas. I've also been having problems with oil disappearing.