My friend's YZ450F has 63hours running, piston changed at 50hours. Suddenly, the bike cannot iddle. We open it up and found out that the Exh valve's top was grinding and the cotter was out of its place. Other valves' clearance are still in specs. Quick solution is grinding the top evenly and insert a thicker shim to ensure the standard clearance while waiting for new valve. The bike runs normal now. We are still wondering what could possibly be the cause of this? Does anyone have the same problem?