I have a 2008 400EX. It ran perfect before. I just installed an HMF performance series slip on, uni filter, K&N air box cover, 165 main jet and (?) which ever pilot jet that was supplied with the HMF kit. It idles ok (when it's warmed up) and full throttle feel great. At slow speeds (1/8 throttle to 1/2 throttle) it feels like the engine putts. I was thinking I have to adjust the needle but I was told on the 08 that the needle is not adjustable. What can it be?