A friend of mine was riding my Dirt Bug with a modded Predator, not pushing it too hard (4600 rpm) when the was a pop then it locked up and had no compression. After tearing it down, the culprit was the intake valve had failed. Beware of this issue, i was running stock valves with 18lb springs and 1.3:1 rocker arms on a stock cam. Here is the damage: