Jetting a big bore

Paul Mac

Paul Mac

PR Addict
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?
 
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.
 
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.
 
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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