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2009 Honda CRF250R jetting

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Hello fellow MX racers, I just bought a brand new 2009 Honda CRF250R because I think it’s the best CRF250R ever built. I strongly believe I can still challenge some of the new machines on the track racing this bike.

I’ve read many articles on how to twitch the jetting but none addresses my dilema. Along with the new bike, I’ve spent some money buying several aftermarket parts (including exhaust system) to make it race ready so my budget for extras is almost gone. This said I need your expert advice on the following 2 questions:

1) What is the best way to eliminate the annoying bog between these 2 options: 

  • Swap the 40mm Keihin carb for a 2004-05 37mm or
  • Install the R&D Power Bowl 2 to the stock 40mm carb (which MXA magazine gave it a 5 star rating)
  • (Maybe doing both is the best way to go but right now I don’t have the budget to do it)

2) I live and race 5,000 feet above sea level, our average temperature is 71 degrees and 75-80% humidity. Considering these factors, your response to the previous question and the fact that I installed a brand new MXA 5 star rating "Dr.D single side exhaust system" (muffler and header), what would be your suggested proper jetting settings?

I will really appreciate your expert advice.


Thank you

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For the bog issue... Before you go spending money on mods for an issue that may possibly be solved just by working with the carburetor as it is... check and set the accelerator pump timing on the carb. If you find no issue with the timing, then go to a smaller leak jet and check the timing again. Your carb has a #70 leak jet installed, try a #65 or a #60.

1. The timing determines when the accelerator pump starts to shoot fuel. If it's too late, the engine will bog.

2. If the timing is correct, the leak jet determines the volume of fuel that the accelerator pump delivers. It also helps to extend the overall squirt duration time. Eg. going smaller on the leak jet will cause the acc pump to squirt more fuel for a longer period of time. It will also advance the timing (of the acc pump) a bit. You don't want the acc pump to start squirting fuel too soon, though, or it will be hitting the back of the sliding throttle valve.

Edited by techgreg

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