Guys

Having run the OS91 pumper for about 7 years, the only complaint I've had is that the regulator cover begins to drip fuel after about 6 months of use. They have finally fixed this with a replacement cover that lacks the cute OS logo and has reenforcing ribs instead. Otherwise, the engine is extremely reliable.

13x8 is the standard prop on less than Heli fuel. Otherwise, the larger prop is fine. I put on a 14x10 a few years ago. The plane was very slow and constant speed. Great vertical, but my rates were too slow and I didn't feel like reworking everything, so I went back to 14x8. I've also flown 13x8 3 blades successfully. Several guys have also used 11x9 4 blades.