Ok heres the deal, my parents have a 99 Scrambler 400 that will not run. I am told that they were casually riding about six months ago when it lost power and started backfiring, then steam came from the engine area. They could not get it started so it was towed home were it sat for the next six months. Here I am now trying to fix it. I tried to start it today but the battery is bad so I'v been pull starting it. All I can get it to do is backfire every now and then(never run ). At first glance it looks like it has a blown base and head gasket due to the accumulation of oil in those areas. So I start tearing it down and when I removed the expansion chamber it had a lot if raw fuel and oil in the pipe. This leads me to believe that it is backfiring and blowing the oil on the engine making it look like the head and base gaskets are blown.(btw compression feels ok, testing tomarrow) I also found that the wire clip that attaches to the thermal sensor in the head is broke or the sensor is broke. Could this one sensor have something to do with this? Any help will be appreciated.

 

It sounds like the motor overheated. The sensor usually is what tells you if the motor is getting hot.

 

Sounds like your on the right track OLDBULL. I would replace both the head and base gaskets and check the piston and rings while you are there. If it is scratchy, put a new one in, if it looks good, slap it back together and go have some fun. I would also check the reeds to see how they look.