Won't return to slow idle

5 replies to this topic
  • BubZ

Posted March 02, 2010 - 03:43 AM


New 09 YZ 450. Bike will idle fine. but after the throttle has been rotated many times it will not return to the proper idle speed. Letting clutch out slightly when bike is in gear will return idle to normal.

Any ideas?


  • grayracer513

Posted March 02, 2010 - 11:10 AM


You have a lean condition, and the problem you describe is called a "hanging idle". First, try dialing in the pilot mixture screw, then check for external air leaks between the carb and the engine. After that, check your valve clearances.

  • dmoney1074

Posted March 02, 2010 - 11:14 AM


Blow ur jets out while ur at it...

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  • BubZ

Posted March 03, 2010 - 03:10 AM


Thanks for the tips! I will look into it. I never thought it would be the jetting since it runs so good, but I felt sure it wasn't the cable adjustment because the throttle snaps back and it will idle fine once you load the motor temporarily.

I noticed it started happening after I set the idle speed up. It wouldn't run off idle with the low idle speed set at the shop, should have been clued into the jetting at that point!

Thanks again.

  • Aaron_YZ450F

Posted March 03, 2010 - 07:27 AM


I had the exact same problem. The mixture screw is what fixed it for me in combination with cleaning the carb and replacing all the jets. I bought a tusk fuel mixture dial, it's a screw with a numbered dial on the end that extends past the carb so you can make the adjustments and remember your settings much more easily. Made it very easy for me and only cost $14, and I am fairly sure you can find them for cheaper.:thumbsup:

  • BubZ

Posted March 04, 2010 - 04:22 PM


Thanks Aaron, I plan to have the carb in hand tomorrow evening. I have the bike stripped to the frame for the once over and grease routine all new bikes need. I just need to remove the shock and out comes the carb.

I have a JD jetting kit and already had the R&D remote fuel screw and power bowl before I started reading all the opinions on them. I figure I will install all of the above as it can't hurt and set the jetting conservatively.


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