I'm looking to buy a new Air Filter. I ride MX/XC/Dunes, pretty much every type of terrain. What is the best type/Brand of air filter? What mods should I do to the air box such as vents. Should I get a "pre filter" too? Do I have to change my jetting? This is for a 400EX.

 

I use a K&N for all of my riding, you must get the prefilter though. I would drill about 8 1"holes in your lid and get a dyno jet kit, so you will have perfect jettings. Also I think uni filters are good too but, do not flow as good as the K&N. If you ride where water could get in your air box, then don't drill holes.

 

for MX and XC go with a foam filter, such as a Uni..I recommend the outerwares airbox cover, it allows airflow like no lid, yet keeps most dirt and water from even seeing the filter.