I am redoing my entire bike. its an 1985 CR250, i powdercoating the frame and new plastic and everything eles. But the airbox looks like ****. Will i lose performance if i remove the airboxe and just run a K&N pod filter off the back of the carb? I always thought if it could breath better then it will make more power. if it maters it is being ported and bored and the head has been shaved. And i also hava a pro circut pipe.
i doubt that a K+N could cope with the demands of off road riding, i clean my regular foam filter on every ride and its amazing how much dirt it stops going into the engine.
As a dune bike I'll assume you'll be jumping? The weight of the filter bolted onto the back of the carb is gonna hammer the carb boot on landings. If the carb hits the cases from this (very possible) it can make the bike stutter and/or stall on landings. Not good.
I'd clean up and repaint the airbox if it's aesthetics you're worried about.
If the stock airbox is in bad shape, a K&N clamp on will be fine. Make sure that you support it well though, as luvtolean states. I have found that they filter fine if you keep them properly oiled and cleaned, get a prefilter too (very important).
for the air box why not get some of that carbon fiber stickers and put it over i got a really big sheat of it for like 5 bucks and i did my air box, and and gonna do my side pannels and my front number plate and it looks preatty good i have a 85cr250 also and i would think that a bad looking air box will still look better than a big hole threw the bike just my thoughts tho.