The OS 46 SF or newer FX should both scream thier heads off with a 11X5. That is the combo I have used for years. I have that on my Kyosho Cap232 and it flys the pants off of it. They do drink the fuel with such a flat prop in a hurry and that high fuel flow helps keep them cool. If you are certain that you are not too lean, you might consider the bearings. Steal cranks in aluminum cases to tend to rust rather quickly. When you run them after sitting for prolonged periods, the rust breaks loose. It seems to find its way to the bearings and lodge there. Some poor bearing winds up skidding around and gets a flat spot and pooka, and engine that won't run worth a darn. Often the engine will have a sick sound, sort of whinning. And sometimes they sound normal. You have to play with the engine in your hand. Turn the prop back and forth sveral times and see if you can feel the bad bearing catch. It is a very small sensation, so you must pay attention when checking. Don't expect to feel it with each revolution. Bearings are one of the weak spots of these little engines because of the dissimilar metals and oxidation states of the metals. They act almost like a battery.