I have an 08 YZ450F with JD kit, MRD exhaust, Vortex ignition and CP 13:1 piston installed. Bike used to run perfectly, but now it has started to act strange every now and then. It seems to sputter when I am in the air and giving it some more throttle (not fast really) and sometimes it doesn`t seem to rev up as cleanly as it should. It is actually very hard to even make bog. I really have to sap the throttle from idle to max as fast as I can to make it bog. I tried using fresh fuel, took carb appart and cleaned everything with compressed air, but still nothing. Though I have to note that there were some dirt particles on the bottom of the bowl. Probably because the boot wasn`t tightened enough. At the moment I can only think of it has something to do with dirt getting inside. The boot was poorly tightened for about 4 hours. I really hope it turns out to be something more simple and cheap then worn internals.