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FCR Main Air Jet Advice


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Hi,

I own a KLX300 and have recently put in a FCR37 from a 2005 CRF250X. I've got a wideband O2 meter which I'm using to dial it in.

Patrick Burns wrote in his FCR tuning guide:

"Main air jet:

If you find that the engine runs perfectly at WOT near the torque peak, but becomes lean toward redline, select smaller main air jets. Conversely, if the engine runs perfectly near the torque peak but richens toward redline, select larger main air jets."

My engine is definitely getting richer towards redline. The graph shows the air-to-fuel ratio I have measured at 100% throttle:

Gettingricherwhilerevsincreaseat100pcthrottle.gif

Now the FCR I have has the plain casting (which is equivalent to a 200 MAJ) in place of the removable main air jet. My question is, based on the wideband data shown, should I drill out the MAJ passage or modify the carb body to accept a removable main air jet or is it not enough of an issue to worry about?

Kind Regards,

Matt.

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You are way rich. You need to go down probably two ore sizes in main jet.

But... you want to be slightly on the rich side (12.5~12.8) to aid throttle response and deal with extreme loading (like hill climbing) You only want to hit that magic 12.8~13 if you were running a drag bike and the load was a constant, even force.

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Yeah I only tested the 145MJ today and thought it would be ok but as you can see, it's still too rich. I did test a 130MJ the other day and it was heaps lean but my logger didn't save the data properly.

If I had a removable main air jet, I'd change it straight away. It's just that I'll be doing irreversible mods to the carby (drilling a passage is not a major deal though), but I want to check if people think it will be worth doing before I take the plunge.

Cheers,

Matt.

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Not worth doing from the little bit of info you provided . On that motor the redline is high but power really falls off fast after the hp peaks around 8500 (with stock head and cams). I'd concentrate on the 7k torque peak to 8500 hp range and more importantly run quality.

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