03 YZ 450. Is my timing off?

12 replies to this topic
  • Crzy Joe Devola

Posted June 25, 2010 - 03:09 PM


TDC has been checked and double checked. Got spark and the only thing I can think of is the timing is off. Anyone think it is off?


  • clevernickname

Posted June 25, 2010 - 07:23 PM


Timing looks ok. if your getting late spark check your flywheel key, you have to remove the flywheel and there should be a crescent shaped key between it and the crank at tdc, or the cdi.

I just spent about 2k chasing problems with my 06yz450 and it turned out to be the cdi.

  • grayracer513

Posted June 25, 2010 - 11:05 PM


If the key was bad, then the timing would be off. :)

You don't have to pull it to check. Compare the mark for TDC to actual TDC found by probing for the piston through the plug hole.

  • Crzy Joe Devola

Posted June 26, 2010 - 10:27 AM


Still cant get it started. Fresh plug getting spark, fresh gas. It does however let out a HUGE backfire when I am kicking it over. Figured it was flooded, so removed tank, plug and header, kicked over maybe 30 times, nothing really came out, reassembled and no start and backfire still persists.

Any thoughts?

  • grayracer513

Posted June 26, 2010 - 11:20 AM


It does however let out a HUGE backfire when I am kicking it over.

From the pipe or carb? How frequently?

  • Crzy Joe Devola

Posted June 26, 2010 - 11:23 AM


Comes out the pipe. Loud as hell. Maybe after 12-15 kicks.

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  • clevernickname

Posted June 26, 2010 - 04:07 PM


Hows your valve clearance?

  • grayracer513

Posted June 26, 2010 - 04:28 PM


It's doing that because it isn't firing in the cylinder. If the timing is correct and the compression is good, the likely causes are weak ignition (low spark voltage, fouled plug), or improper idle mixture.

Try cracking the throttle open just a little while you start it.

  • Crzy Joe Devola

Posted June 26, 2010 - 04:57 PM


Thanks for all the help guys. Finally got it going. Rechecked the valves, a little on the tight side, reshimmed and my buddy kicked the hell out of it and it finally fired up. He did give it a little crack of the throttle to finally get it fired. Took for a short ride, let it cool down and started right up again.

One question I have is before I replaced the jets, 3rd and 4th gear would intermittently cut out at anything above qtr throttle and backfire on decel was common.

Now, 1-3 gear is clean and no backfire on decel, but 4th gear completely cuts out every 3 or so seconds. All my jets are stock sizes and my fuel screw is out 2 and a qtr. Any thoughts?

  • grayracer513

Posted June 26, 2010 - 06:53 PM


You mean that the engine actually misfires/stops running monetarily, or that the engine continues to run but you feel a harsh jerk?

  • Crzy Joe Devola

Posted June 26, 2010 - 07:44 PM


Continues to run, it never actually dies. But yeah, is very herky jerky.

  • grayracer513

Posted June 26, 2010 - 08:52 PM


That's a problem with the transmission. Second, third, and 4th gear pinions (the gears on the main shaft) are worn and need replacement, along with the shift fork that moves the third pinion.

  • Crzy Joe Devola

Posted June 27, 2010 - 06:00 PM


Thanks for the help Grey, I will look into the tranny.

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