I remember installing a jet kit, Supertrapp muffler and a K&N air filter on my XR650L. The air filter didn't make much of a difference, but when I went on a test ride without the side cover which covered the air box, the bike went like stink! The intake to the airbox was so small that it didn't matter what was inside, the flow was extremely restricted. So I drilled lots of holes into the side cover, put it back on and everything was fine. Upon hard acceleration you could hear the angry beaver inside the box!