Got an 07 450er and it had a foam filter on it when I purchased last year and did'nt have any problems at all with bike but filter was old so i bought a new K&N as well as an outerwears filter cover for it. It has stock header with HMF silencer running with air box lid off and forget the main jet up larger than stock. After installing the new filter there is no performance issues accept it will stall often when slowing down from a hard run like stopping at the top of the dune or at the end of a mile race. Then it doesn't want to start, almost need to choke it to start it even when warm? Could it be running too lean with the new filer? Wouldn't have thought the change would make that big of difference.