i have a 07 yz 450, fresh top end last summer. it has a serious hesitation on the bottom end, right as your letting the clutch out. in mid range the bike does OK. on the top end it will cut out some times. iv been through the carb at least 5 times. reason being is if i pull the carb, tear it down clean all the jets an put it back together the bike will run fine for maybe 15 min then its back to the same problem. iv gone through an check timing, checked valve clearance, tested the tps sensor. even i believe the ecm (the black box behind the number plate). had a guy at the track let me take the one off his 07, still didn't fix the problem. im guessing by now my only other option is to check the intake valve seats but i wanted to post here first just in case i have over looked anything. if any one has some other ideas it would be greatly appreciated! thanks!