Jetting needed: 06 YZ450 in Utah

3 replies to this topic
  • natehawes

Posted December 26, 2009 - 11:56 AM


I've searched the jetting database and can't find anything that lines up with my situation. I have a stock 06 YZ450 that has the stock jetting (165 mj and 42 pj). My altitude ranges from about 4,300 feet to 8,000, with most of the time being spent toward the lower end of the range. I mainly ride in temps from 70-90 degrees. The humidity here is very low.

Can anyone suggest a good jetting setup for my bike?

  • grayracer513

Posted December 26, 2009 - 12:44 PM


Assuming 80 degrees and 5500-6000 feet, a 160/42 or 158/42 is a good start. The 42 pilot may act lean in warmer weather and/or at lower altitudes, but don't go too fat with it.

I don't know why you say these wouldn't have helped:


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  • natehawes

Posted December 26, 2009 - 03:25 PM


The first one you reference is a guy who kept changing his I didn't put too much confidence in his recomendations.

The second and third ones are 2007....can I use their settings?

Just pulled the jets. I had assumed they were all stock, since the rest of the bike is. Come to find out, the main jet is a 150! No wonder why I've been having bad popping on decel and bog off bottom end.

  • grayracer513

Posted December 26, 2009 - 09:57 PM


'07's are slightly different, but close. Remember, you're only going to get a ball park starting point out of it.

Lean main jets do not have much to do with decel backfire, because the main circuit isn't active during decel. It can contribute to off-idle problems, though, as that is largely dependent on the major diameter of the needle, assuming the main jet is in the correct size range. If the main is way too small, everything that runs off the main circuit will be affected.

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