Chuck, my advice is to give the engine a bit of running time (at least a gallon's worth) before expecting to see the true potential power that the motor is capable of. My experience with the YS 61 is that a 12 X 12 is the absolute biggest prop it can turn comfortably. I actually prefered a 12 X 11 on those motors.

Not sure how you break your motors in but I always used a slightly smaller prop initially (guessing a 12 X 10) otherwise you run the risk of the motor slugging through the break-in....which might result in the motor not reaching it's potential.....IMHO.

Jeff