When I put the K & N on my 400 (which was running rich at the time) I also added a modded airbox, that i bought from Modquad. With the stock airbox on the 400 when you punch it the hole for the air tends to close some and actually keep it from getting all the air that it needs(from what I can tell). With the K & N and the airbox i noticed quite a difference with throttle responce. The modded airbox has an aluminum tube comming out of it now so it dosent close under acceleration.

 

i didnt have to rejet untill i modded the air box. before that i was fine. just get the hpd air box mod, it comes with the jet kit and works great. i love the throttle response now.