Not running a filter can result in bugs, dust or sand to get into the crankcase as the piston goes up and down - creating pressure and vacuum.
I've got a 660 with no airbox lid and been wondering what crankcase filter I need... would that filter on your 700 fit my 660?
2004 YZ 250
Pro circuit platinum pipe, R304. Wiseco piston, boyesen reeds
2002 Raptor 660
Curtis Sparks X6, Foam Pro Flow Kit,
Alba Needles, Airbox lid removed, 160L-165R mikuni mains,
25 pilots, fuel screws 2 1/8 out, flipped rear link, clear tube mod