Most of the posts here about loosing tune, or changes to the engine once in the air then having to retune, involve the "walking needle" problem.

We've have had a number of PTS P-51's and other planes with Evolution engines at our field that exhibit this.

Our resident senior engine "expert" (he has written several books on R/C engines) learned this the hard way helping me to get my plane flying.

Now he advises these changes with all Evolution engines.