My ST .90 ran so well the first 3 minutes I had it running, that I decided to fly the plane I had it mounted on.

I used an APC 13x6 prop (as recommended). Fired her up, ran it up just shy of peak, and pinched it every 10-15 seconds or so. I brought it down to a very nice idle, jammed the throttle forward a couple times, and this thing just ran great. I've never had an engine run so well right out of the box. I still only have about 20 runs on it, and its my favorite engine.