i own 2003 400 EX. I just modified my airbox lid by adding some prefilter plugs. and i also took of my exhaust baffle. My question is (is it bad to run it without the baffle?) It's a totally stock quad... and is there any other little stuff i can do to boost power?

 

removing the spark arrester and modding your airbox lid will require rejetting. you won't gain performance without jetting. the bike will run lean and hot if you don't, which takes life away from your engine. a cam and piston will really wake the bike up from stock.

 

on ebay you can usualy find a end cap that gets rid of the spark arrestor and makes your exhoust look good for like $30. you should take a look. cals400ex is right you wont get anymore power without re-jetting it.