Sorry to post this but, I cant seem to figure out whats going on. I just Picked up a 06YZ450f 2 weeks ago. The thing would normally start pretty easy. I checked the valves last week and all were within specs. I was getting water out of the weep hole so i ordered the parts, Shaft, Seals, bearings. It started up before i did this work to it and i ran it for about 10 mins. Heres what i have done to it. 1.Replaced all waterpump parts and oil seal on water pump. 2.Installed i think its a MSR fuel screw 3.Installed 48 pilot jet - 42 was in there before. 4.installed 170 main jet. 4.Installed oring on ap linkage - oring mod 5.put in new oil and coolant. Removed the Cam chain tensioner to check it and reinstalled, I didnt move any thing when it was out. I even rechecked my valve timing. cam gear indents at 12 & 3 oclock position lined up level with top of head. Timing marks on flywheel just past the 3rd line when conuting lines from left to right, so i dont think i slipped any teeth on the cam chain. Fuel screw i adjusted from 1 turn out to 4 while attempting start, nothing seems to work. It back fired a few times and seemed like it wanted to start. Ive tried with gas on-off - choke off- on. ive pulled the plug and changed it out with some other good used ones, im getting spark. Im going to pick up some new ones at lunch today. Plug is wet oil is just above the dipstick full line crosshatching. The thing is giving me a good work out - I miss my DRZ electric start. I was wondering if the little oring on the fuel screw tip, if it got distorted when installing it, would that affect it or if it was distorted would it make it that hard to start? Im lost i hope someone has some input.