08 yz450 timing question after valve shim

2 replies to this topic
  • yzrider487

Posted September 18, 2015 - 01:40 PM


I just shimmed the valves on my 2008 YZ450F for the first time. The bike started and ran good after finishing but it was backfiring bad and seems to be running hot. I removed the 45 pilot and installed a 48 from the jd jet kit with the screw open 2 turns. Still backfiring very pronounced on decelleration. I removed the spark arrestor and found what appeared to be fiberglass packing. After running without spark arrestor it almost seems to be slightly backfiring at idle?? I can't imagine the bike would be running lean/backfiring with the 48.


-I checked TDC with screwdriver thru spark plug hole and it was true with the I mark after H mark thru inspection port. I got my exhaust cam mark as close to 9 oclock and intake cam mark as to 3oclock as I thought possible. Pin count was 13 between the two top marks on intake and exhaust cams. I lubed cams with bel ray assembly lube. Pics are below, sorry the pic quality is pitiful but it's all I've got.


-Is my timing off or are there any other beginner mistakes I could've made to cause this? Thanks



IMG_05511.JPG IMG_05522.JPG

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  • grayracer513

Posted September 18, 2015 - 03:39 PM


Be sure the exhaust is sealed up well.  Could be n air leak there.  Also be sure that the pilot screw O-ring is in place and intact.  If you had the slide out of the carb, you may have installed the vacuum release plate upside down.  The square end with the hole near the edge goes down.  

  • yzrider487

Posted September 18, 2015 - 06:17 PM


Thanks for the help, I'll check those points out!

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