asmund...if you have or can get one try an APC 12 X 4...it will put less load on the engine than the 12.25 X 3.75...although IMO 13,100 is a good R.P.M. for that prop on that engine.

The Tower muffler will also work better at a higher R.P.M. and my TT .46 Pro will spin a 12 X 4 @ 14,000+ on 10% and O.S. #8 plug.
The highest I ever saw was 12,800 w/ the 12.25 X 3.75 (stock )


loughbd...I haven't noticed and problems with idle or transition as of yet. I think mainly because the mods that I have tried aren't really that extensive.
The highest R.P.M. increase after the work on the outside of the sleeve is about 400 give or take a few. (that's an aircraft engine...however I have done a couple of car engines, and they sound like they are revving up more than that after the mod?)