Bob, sorry to hear that your friends YS is acting up. I just did the maiden flight on my UCD w/ YS .91FZ last week and, with the help of our local YS "expert" (thanks Rich!), am VERY impressed with this combination.

While the YS engines are a tad more complicated than other 4-strokes, I think you will find that it is worth it in the end. I'm sure that somebody over in the Engines forum could help you with your (or your friends) problems.