You have your plane proped for pretty good all around performance. It probably has pretty good vertical.

The pitch of a prop is the ideal distance it will move forward in one revolution. With your 14x8, each revolution moves it forward 8 inches, ideally (there are losses due to slippage). If you want it to go faster, you need a higher pitch prop. When you go to a higher pitch at the same diameter, the engine will slow down and lose rpm's and maybe speed. It, thus, becomes necessary to reduce the diameter to keep the rpm's up. As you reduce the diameter, you lose thrust. Thrust is what gives you vertical. At some point, the thrust becomes lower than the weight of the plane and that causes problems in taking off and maintaining normal flight.

If what you want is speed, a 13x10 will turn about the same rpm and produce more speed at a cost of thrust and vertical.

You need to watch the rpm's and not over rev the engine when your playing with selecting a prop. The "redline" on your engine is about 11000 rpm. That is unloaded, in the air. On the ground, with the additional loading of being held still it will turn about 85% of the rpms it will turn in the air. In other words, if you tach the engine on the ground, do not let it exceed 11000 x 85% or roughly 9300 rpm. This is not perfectly accurate, but a guide line. Error on the low side so as not to over rev the engine in the air.

Also, keep in mind that every plane will have flutter at some speed. If the plane reaches that speed, there is a high probability that it will not come back in one piece.

The 13x10 will probably produce the results you are looking for.