03 YZ450 with 13.5 Jetting Issues

4 replies to this topic
  • OUCR250R

Posted April 09, 2008 - 04:47 AM


I just got done doing a complete rebuild on my bike. I replaced the crank, cylinder, head, valves, springs, retainers, and cams (yamaha oem). I also installed a wiseco 13.5:1 piston in search of more power. The carb has never been apart and is completely stock save for a msr fuel screw. My question is does anyone have any good jetting setup tips or a good baseline to start from? My bike seems to be running incredibly lean. I haven't ridden it yet, only started it a few times to heat cycle it. Any help would be greatly appreciated! Thanks guys.


  • OUCR250R

Posted April 09, 2008 - 06:32 AM


Even anyone with a setup similar to mine would be helpful. Or at least a hint as to what changes I will need to make. I am assuming I will need to richen up the main, but I am not sure about the pilot. Any help would be appreciated.

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  • grayracer513

Posted April 09, 2008 - 07:48 AM


Why do you say it's lean?

  • OUCR250R

Posted April 09, 2008 - 07:56 AM


Because it is backfiring really hard when I let off the throttle, and typically shoots a 3-4" flame out the exhaust when it does this. I have tried adjusting my fuel screw, but have had no luck getting the backfire to go away.

  • grayracer513

Posted April 09, 2008 - 08:17 AM


Check the exhaust system for air leaks first. If you are convinced that there are none, then go up a step on the pilot jet.

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