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Jetting a big bore

Discussion in 'Fuel' started by Paul Mac, Jan 9, 2014.

  1. Paul Mac

    Paul Mac PR Addict

    Location:
    Columbus OH
    Racing Number:
    5
    Anyone have experience with a crf450 big bore? Did you have to go down 1 or 2 sizes on your main? What about your pilot? Needle clip?
     
  2. adam63

    adam63 PR Addict

    I have no experience with it but my gut tells me based on general engine proficiency, leave the main and needle alone. Assuming all you changed is the bore size, I would go up one on the pilot jet and down 2 on the leak jet. Make double sure the actuation and timing of the accelerator pump is spot on.

    Would be nice to talk to an engine builder who has actually been there and done that.
     
  3. GeorgiePorgie

    GeorgiePorgie PR Founding Father

    Location:
    Ohio the 440
    Racing Number:
    740
    Did you try riding it without changes?
     
  4. Paul Mac

    Paul Mac PR Addict

    Location:
    Columbus OH
    Racing Number:
    5
    Winter project is complete but no time/place to ride yet? The worst assumption people make is bigger engine bigger jets! May work in cars and fuel injected bikes but just changing the bore on a carb bike causes the engine to suck harder. Harder sucking (more are flow through the carb) pulls more fuel. Everyone that has done this went down 1 or 2 on the pilot, but some have been fine with the main and needle and some have had to go there too. Just looking for experience from Honda owners.
     
  5. MX724

    MX724 PR Addict

    I didn't have a honda 450 but I did do a big bore on a 250f. Your conclusion is a good starting point. I ended up bringing the main down one size with good results.
     

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