Hi,
You are starting to have an engine that should be broken in by now. You really need at least 10% nitro and some Castor oil in this engine. I use 15% in both of my 91's as well as my 30 and 52 Magnum 4 strokes. I use 14x6 or 15x4 props which work well for me depending on the plane.
You have a really nice looking plane that demands lots of power......you may find an FAI 91 is the best choice for you?Good luck, keep us posted.