Hi, I have a polaris scrambler 400 4x4 2 stroke and it wont start. The other day I turned it over and it started for a second then quit then I tried starting again and it wouldn't do anything for a couple seconds then boom an explosion coming from the exhaust. I took the head off and cleaned the top of the piston and the head up and put it back on and it doesn't even have that explosion now. I'm getting fuel, compression, air, and spark and I don't know why it's not firing. Can someone please help!?