My son has a 03 yz450f; He had just got on his bike after cooling off and he was going down a straight away and he shifted down for a turn and the engine locked up and died. We took it to our local Yamaha dealer and the mechnic said it might have droped a valve, and if it did it would cost about $2-$3000. to fix because you would need to replace everything. We left the bike for them to check to see what could be wrong with it before we decide what to do. The bike was running great before this happen, We had changed the oil the night before and we need everybodys help any ideals what we should do.