I have a 1998 yz400 that was converted to a supermoto. My vavle cover was leaking and my buddy put alot of form-a-gasket on it to stop the leak and it ended up blocking a oil port that feeds a couple of the intake valves and they got bent and stuck, i had all new valves put in and the bike was running cherry for 20 minutes then it died. its sounds like something is rubbing in the bottom end and makes it die unless u give it a quarter throttle. it might be a main bearing or the needle bearings on the crank shaft. i dont know what it is or how to check it. if any one knows could u please give me some advice. i realy appreceate it. thank you.