My 08 450 XC-F was lean from the factory. I backed out the A/F screw 1 1/2 turns out and that solved my problem. I would try adjusting the A/F screw first before you buy the JD Jet Kit. On the other hand, my 09 525 XC had throttle lag and adjusting the A/F screw didn't help any, so I installed the jet kit. While installing the kit, I noticed that the accelerator pump wasn't squirting (FCR carb). A thorough cleaning solved the issue. The 525 XC was brand new off the showroom floor with a clogged accelerator pump. The 450 XC-F was brand new also, but it didn't have the FCR carb with accelerator pump.