Sudden loss of compression

FWIW, I've had low "hand" compression on one cylinder ever since I got my plane 6 years and almost 500 hours ago. Doesn't show up on the diff compression either hot or cold. Nothing special seen with a borescope; oil filter, magnetic plug, or spark plugs and oil analysis show nothing amiss. Beats me why, but seems somehow normal for this particular 2005 vintage engine.

If a valve was leaking a bit, is there an easy way to lap it without removing the head? 

What is the procedure? 

Mike Busch of Saavy Aviation has a method for lapping a valve in place. Go to his website and you will find info on it there. 

I've not tried my Vividia 400 "stick" bore scope on Rotax (yet) but before attempting anything like a Saavy valve LIP procedure...

Try to get a look at the valve seat if that is possible. Very easy to remove the valve cover to 'see' when the valve is fully opened so I would probably not hesitate to watch that  without my scope in the bore yet. Then when valve is open, try to view (and snap/post) pix of what you see. We love pictures...

On lyc/conti's a valve can hang open by (presumably) carbon foul on the valve stem/guide though I've not experienced that in my limited Rotax experience (yet).

Do the Rotax have a valve rotator mechanism assembly component to 'turn' the valves in engine running operation? 

Mark, you said 625 hours, how many years and how often do you get her out for 'exercise'?

Mark, thank you for keeping active in this discussion. Please do us a favor and when you figure it out, please return and give us a final (and any follow up) report so that we learn from your experience, what you find, and how you solve it.

Cheers