A 13-6 is pretty lightweight for a .90 FS. My .70 OS turns an APC 13-7 at a comfortable 10,300 or so before I back it down a bit.

You should be using a 14-6 (or so) on that engine. Less than that and you're simply converting glow fuel into noise. The Spacewalker is not a fast plane under any circumstances, so a high pitch prop is performing with very poor efficiency. The flatter pitch (ie: 6") will allow for greater static thrust and I feel, a better delivery of useful power inside the speed range of the spacewalker.

Good luck!