I have a similar engine (OS 46AX) which was somewhat gutless in the midrange area where I prefer to do my 3d flying.

I fly a 128 cms wingspan profile of a Yak 54 (scratch built) that weighs 1.95Kg, wet. My prop is an APC 12.25x3.75

The response out of hover or high alpha situations was not acceptable. The engine never quit but it was taking more than expected to react.

My solution:
1. OS 8 plug
2. Very thorough inspection of fuel lines and 0 air/fuel leaks
3. Tune the HN to just shy of 12k
4. Close the idle needle to just a tad less than 1/8 turn (from original OS setting)
5. With engine running, move the throttle stick (halfway in my case) to were you think the engine shows lack of reaction and use the HN to improve the performance. Play by ear.

By doing that, I optimized the sector where most of my flying takes place. I rarely go WOT with this plane but in the few WOT up lines I did, the engine never gave any indications of leaning out.

I hope it helps. I know it did for me