CRH, before you switch props.... I use a tried and true method when I try different propellers. There is a propeller load factor that can be computed for each prop. It's simple the diameter of the prop squared times the pitch. The higher the load factor, well the higher the load on your engine and less RPM's. As an example, a 13x7 prop has a load factor of 1183 while a 15x4 has a load factor of 900. Your engine will turn higher RPM's with the 15x4 than the 13x7.

i've attached the excel spread sheet that lists many different combinations. I had to save it as a text file to upload, so I hope it's formatted properly