Tomm,

Isn't 18 degrees the amount when the chain is off by one tooth?

Usually if the chain is off by one tooth, the valves hit the pistons. So Eskimo may be right about the timing mark being off. This would mean that the vibration damper has slipped, possibly the woodruf key has sheared.

Its not too much work to pull the damper to check the woodruf key, but do any easy checks first to determine if the damper has slipped.

P E H