09YZ450F Jetting question

3 replies to this topic
  • husky390or

Posted November 22, 2010 - 09:57 PM


I checked the jetting posts and none of them worked for me so here is what I have. Stock jetting, air temp is 25-50 degrees, altitude of 5,200', FMF Q4, Delkevic header, 06 CDI. The bike backfires when letting off from 3/4, 1/2, and 1/4 throttle. Do I need to raise the needle or change the pilot jet or both? It seems to smooth out a little in the 1/2 to idle range when I have the air screw turned out 2 1/2 to 3 turns but that's a little much for my liking. Thank you.

  • locomike

Posted November 23, 2010 - 07:06 AM


Go to a 48 Pilot and see if that solves it. After that, I would try raising the needle (dropping the clip) on position.

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  • grayracer513

Posted November 23, 2010 - 08:08 AM


Start by checking over the exhaust to be sure there are no leaks in it at either end of the header. After that, baseline your pilot jet adjustment according to the manual, then evaluate the setting based on this:


Go richer if it fails the above test. By the info you give regarding your pilot screw setting, it looks like it wants a bigger jet.

The needle has little or nothing to do with the backfiring on decel, and frankly, if it's jetted perfectly, it WILL do that *a little bit*. A few pops on decel is normal, a constant loud popping is not. Some people at lower altitudes like to go with a needle with the next step smaller upper diameter to get better throttle response off idle, but this should not be necessary for yours at 5000' once the pilot is correct.

  • husky390or

Posted November 23, 2010 - 09:32 PM


Thanks guy's. According to that link and what you said gray, it seems to want a larger pilot jet. I threw one in tonight and will see how it runs on Friday.

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