I am in a very similar boat as you... only with the Y.S. 91. I've tried to read all the threads and it's still hard to come to a conclusion at the end due to there being so many different pilots, flying conditions, etc. Just when I think I'm seeing a pattern... bam, it changes.

My choices were either the Y.S. 91AC which I already have (through a trade) but have not used or a Saito 100 which I would buy new. I decided on the 100 simply due to the fact that if I will be buying, I will buy the 100 over any of the 91s and from what I've seen both the Saitos and O.S. models perform very well in other planes. I persoanally own a Saito .72 and love it. I've owned O.S. 2-strokes and would almost buy nothing else for 2-strokes. I've not owned a 4-stroke O.S. but see many at our field that also perform very well. In fact, the only UCD that I've actually seen fly in person is at our field with an O.S. 91 non-pumped and it seems to perform very well - but that's only observation - not being the pilot. It seems to hover well and pull out well but I've never witnessed any throttle positions. Definitely sounds smooth. That's for sure!

Although my Y.S. is heavier, one reason I'm leaning toward it is that it's pumped. I'm not much interested in always making sure that the low needle is beyond human perfection for inverted running without loading up. I've seen people experience that at the field with various brands and also saw UCD posts that were similar.

Soooo.... I'm still on the fence a little but think I will indeed try the engine I already have. Are you looking for 3D out of that O.S.?

Oh, and my UCD just arrived today and looks great!

-s