My bike had a dead spot and back fired before installing the fmf jet kit. After installing according to the directions for my altitude, my bike now revs clean with no back firing. Very simple and easy to install. Do wish the kit had came with a new pilot jet and fuel/air mixture screw and not as many extra main jets that will probably never get used.
Great instructions included with the kit... haven't had the opportunity to actually test this jet kit, but i'm not worried because FMF has a great name because of the quality and reliability of their products... VERY EASY TO INSTALL
This kit really dialed in my 04 YZ450F. The stock jetting was never right. Always ran rich despite the needle adjustment. FMF Needle is much smaller and the jets are smaller as well. I'm running the 148 main which is much better for humid or high elevation conditions. The stock main was waaay too big (162).