Heavy nuts on the crank can affect the responsiveness of the motor. The best way is to move existing mass around. If you already have your battery pack as far forward as possible, consider moving you motor forward. Usin a longer mount or making up a ply "donut" ring to move the existing mount forward might do the trick. I know it will look a bit strange, but less weight is always better. Low wing load = longer glides.

Safe Flying!