I have, often on auto and tractor engines. The open valve face tends to pick up some oxidation, which allows for some leakage. I have never pulled a Rotax head but would suspect the same there. Try running a compression check on a Lycoming that has sat for a time, same effect. Compression comes up after even a short run.
Originally Posted by EXflyer
Why would the valves not seat? Have never seen that on any auto engines that I have worked on.