i have a 400 wiith a dmc dual system k&n filter with outerwear and a air box lid with holes and i was originally running a 170 main jet yesterday i took the lid totally off and after looking at the plug with i got home i realized the bike was running lean. the electrode was brownish tword the base however it ended with a grayish tip i was thinking about running a 172 main to balance out the extra air. by the way its like night and day with the lid with holes and with it totally off, when i took it off i got tons more power i was soo suprised it pulls harder bottom end all the way to the redline