is there a way to polish or clean the piston head without changing too much of its performance or maybe wearing it out? I know I cant see into the engine, but I opened up my OS91FX recently to have a look, cause I saw my plug was being burnt black which generally means im running way too rich... so now the piston is also burnt black on top... I want to remove it if possible or doesn't it really matter? I have tuned leaner now... I was about 4turns out... im at 3now... manual suggests 1.5 to 2 turns but thats too lean... the engine died at top end so I richened until this didn't happen anymore and I am at 3turns currently...