It also sounds like you are trying to turn too large a propeller on the 15. We ran several a year ago and found the 7-6 APC gave the best performance on a small light plane. Heavier planes required an 8-4 prop. If the engine is just breaking in it is best not to overload the engine with too much prop. Even a 7-4 works well for break in as the RPM can come up with out being too lean.