Am I running lean? '01 yz 426

5 replies to this topic
  • honyam83

Posted June 28, 2009 - 04:41 PM


I have had a problem with what seems like a "flat" spot in the throttle. my bike seems to lose power at a certain spot, less that or about 1/4 throttle. This only happens at higher altitudes. I live @ 5000' but ride in the hills 8,000'- 10,000'. I have tried adjusting the fuel screw to no avail. I have checked the jetting data base but have not found exactly what I'm looking for, here are my jet numbers.

main 155

pilot 42

starter 72

needle 3rd notch down

Any feed back will be very appreciated. Thank you.

  • ajeb

Posted June 28, 2009 - 07:42 PM


Provided you have checked all aspects of your pilot circuit, I would check the float height aswell.

You will find the measurement and instructions in your service manual.

  • davcon

Posted June 29, 2009 - 06:46 PM


Has the outside temp risen since you last rode? If so its probably a bit rich. If it were me i'd lower the needle.

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  • honyam83

Posted July 04, 2009 - 05:02 AM


Could the accelerator pump cause the temporary loss of power I explained?

  • grayracer513

Posted July 04, 2009 - 06:41 AM


If it's happening at higher elevations, but not lower, that indicates rich, not lean. Your main jet size should be close to right, but your pilot is still stock. You should probably be using something on the order of a 40 at such high altitudes.

On your next ride in the hills, try this:

If a smaller pilot doesn't do it, you may need to change the needle to one with a larger diameter straight section.

  • honyam83

Posted July 05, 2009 - 05:40 AM


Thanks for the replies, Gray I appreciate all the knowledge you share on this forum. I will try what Eddie suggests next time I am out. When I get it tuned right I'll post it in the data base. Thank again to all.

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