I had a VERY dirty carb on my Recon from running a K&N. I don't think the filter passed dirt per se, but what happened was that it sucked oil from the crankcase vent, and the oil brought a lot of dirt with it as it passed through the wimpy, thin K&N! Honda uses this method of venting the crankcase on its ute models and the 250EX, ie. the vent is in the airbox and the opening is covered by the air filter. I know on my Ranchers, Recon, and 250EX, the outerwears on the K&N was frequently SATURATED in oil.

I have since gone back to UNI foam filters! With foam, if you suck some oil from the vent, it is just going to be absorbed by the foam and cause no harm. If you have a machine that has the vent in the airbox, you may want to think twice about not running a foam filter!