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john.hilton@cape.k12.de.u

Quick question for Jetting Gurus

8 posts in this topic

What throttle position do the different Jets control?

For example, I thought the main Jet controled from about 3/4 to full throttle. Now I'm not sure that I'm correct.

What about the pilot, clip position, and air/fuel screw?

Thanks for your time.

John

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There *is* overlap between the circuits, but as a general rule:

pilot= 0-1/8

needle=1/8-3/4

main=3/4-WFO

Merf

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John,

There's also a great carb circuit profile in the tuning section of the manual. This shows when each circuit is in use. Be sure to check it out.

DaveJ

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The pilot jet, fuel screw and pilot air jet (there is also a main "air" jet) all play a role in the starting, idle to 1/4 throttle, and the "snap" right off the bottom.

The needle is, IMO, the single most important factor in the power your bike makes. You YZ guys already have a good stock needle (EJP) and the difference would be minor, but my WR came with a MUCH leaner (DRR) and the difference was huge when I switched mine to an (ELN)! Clip position also makes an enormous difference as well. In my case the needle was totally new and I started right in the middle at clip #4...it ran horrible and I was bummed. I then got my composure, got some more advice from James Dean (the real jetting guru) and moved the needle down (up to clip #3) and it was a totally different bike. Huge hit, awesome power, everything.

As suggested, read your manual and hang around here long enough and you'll learn a lot. Some great guys giving advice!!

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Originally posted by john.hilton@cape.k12.de.u:

Thanks Merf:

Any thoughts on the air/fuel screw on the bottom of the carb.

John

Think of the fuel screw as a fine tuning adjuster for the pilot.

And, I can't speak for your bike, but mine's finicky as

can be on that adjustment.

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Thank you all very much for your help. I was just getting good at jetting my KX 250 when I sold it and got the 426.

I thought I knew which jets effected what throttle position. But after talking to a friend of mine this summer, I wasn't sure.

However, it turns out that I was more or less correct. That air/fuel screw on the bottom of the carb. is new to me however.

Thanks again,

John

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Originally posted by john.hilton@cape.k12.de.u:

Thank you all very much for your help. I was just getting good at jetting my KX 250 when I sold it and got the 426.

I thought I knew which jets effected what throttle position. But after talking to a friend of mine this summer, I wasn't sure.

However, it turns out that I was more or less correct. That air/fuel screw on the bottom of the carb. is new to me however.

Thanks again,

John

John, just to clarify, it's a fuel screw.

If the screw is between the carb and motor, it's a fuel screw.

If the screw is between the carb and airbox, it's an air screw.

On fuel screw, out is richer, in is leaner.

On air screw, out is leaner, in is richer.

Merf

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