Don, several years ago I installed a K & N filter in my K3 box with a few added 1/4-inch holes in the bottom, upped the mains to 120s, installed a 4-1 header, and almost had as much power as my stock K0 from 0 to about 90..........then the cam showed its weakness.
My brother-in-law put an 836-kit in his K4 along with a performance cam and Barnett clutch. Otherwise the bike is stock, including pipes and air-box. It ran poorly until I put a K& N in it, reduced the jets, drilled the holes, and installed iridium plugs. The air-boxes with the F on the side may breath a little better, but the extra holes seem to help. I like how the K0 box looks, but it does not breath as well.
