I have a YS 63 on my top cap Arf. I fly it w/ a 14x4 zinger. It will peak at 10,800, I fly it at 10,000-10,200. I am very happy w/ this engine combo. I think w/ a 12.25x 3.75 size prop is way to small, your over rev, your engine.
13x6 APC peaks 11,600 fly it at 10,800

Put you a 13x6 or 14x4zinger or 14x4W Apc on it and fly. You will love it. You bought a $250 motor might as will use it. Instead of worrying about only running less than 3/4 throttle.

My combo will take off in less than 5ft. will hover my topcap arf about 1/4 throttle, and pull out at 1/2, shoots out @ 3/4, and like a gun at full. If you go w/ the 12.25 you will be dissappointed in your motor and your plane.

the 12.25x3.75 works great on OS 46 or 50, TT 46, ST 45, Irvine, 46 or 53.
Not a power house like the YS 63, you need a bigger prop to realize the power of the 63.