I have a 1993 honda 300ex bought it less than month ago when i bought it didnt smoke at all ran great with very little valve noise, i put a hi rev cdi box on it so the next day a guy wanted to buy it for 1000 so i said ok stupid of me im only 15 so i didnt thnk he would screw me over and my parents didnt tell me he was a druggy so any way i had to pick it up a couple days ago and didnt have a key so i hot wired it and tried to start nothin so then i tried drift starting it and thats when i found out the engine was locked up i took the spark plug out and the it was shattered then i took off valve cover of and the bracket on the cam was broke but ii coundnt take the head off cause i dont have an alan wrench big enough to fit the bolts wat do u thnk caused this? he ran it with out the silencer so could backpressure caused it?

 

Too lean, and or over rev'ed it. Ever see Days of thunder? That is what the guy did to your quad. Sucks, because it will be expensive to fix.