Hi Flicka: I think that an O.S. .90 four-stroke would be marginal power at best for a 15 pound bipe, probably requiring full throttle most of the time. Would suggest trying a lot more power. If your airplane has a rather high wing loading, you just have to accept higher takeoff and landing speeds, or give up and get a different model, I think. I like bipes with rather low wing loadings - much more relaxing to fly, and dislike hot little bipes that fly on the engine and glide like bricks. I have a friend with a 44" span Sig Miniplane, that he had crashed several times. I flew it for him, and found it to be one nasty handling little beast. Weighing over 7 pounds, it is actually considerably underpowered, even with its piped YS45. I would consider 5 pounds to be the upper weight limit for this particular bipe. We taped on a trial cardboard rudder trailing edge extension, and it became far friendlier. He flew it about 20 times, removed the extension, and stalled and crashed it on his next landing approach. It needs either a very steep landing glide, or a lot of power on approach. With the crude rudder extension, it could be flown much slower with good control.