I've been discussing the low compression problem with a teen-age nephew of my wife. This kid has been fascinated by small engines since he was old enough to hold a wrench and has rebuilt a bunch of them over the years.

His opinion (from a distance) is that a combination of short runs (typically only a minute or so) and moderate engine speeds needed to move the airplane with care has prevented the rings from seating.

He's suggested that every time I use it for a while, I run it at top speed for several minutes to see if we can get the rings seated.