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Expert Needed: Lambda/AFR Question

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Hello everyone,

Two Quick questions about Lambda/AFR here. First the particulars:

Stock configuration YZ250F. Throttle positions to be measured:

Idle: Main Jet usage, throttle at idle only

Part Throttle: Needle usage, throttle open to ~half position

Full Throttle: Pilot jet usage, Wide open throttle

Are there any other throttle positions I should be checking?

Also, according to Eddie Sisneros (maybe second only to God himself when it comes to jetting) I should be looking for the following measurements of the AF mixture:

idle should be 13.5 to 14.0

steady part throttle 13.0 to 13.5

wide open throttle 12.8 to 13.2

Assuming perfect Lambda is 1.0 or in AFR terms for non-leaded Gasoline motors it would be 14.7. Now, I have been told that optimal performance for a motor is slightly to the rich side of 1.0 so aiming for around .8-.9 lambda is best. It would seem however that 12.8 is a quite rich ratio even for Wide open throttle. Should I really be aiming for a number halfway between these numbers or should I be aiming for more toward the rich or lean side of these numbers? I believe the numbers are correct, but which side of the numbers would be most likely to improve overall engine performance?

Ok that's three questions actually but I need some help here! Thanks!

-- Rootuser

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the numbers i gave you are correct.14.7 is only theoretical.

your back ward on jetting.

pilot jet/fuel screw/pilot air jet = idle and just above.

needle taper and clip position =1/4 to 3/4 throttle

needle straight diameter = just off idle to 1/4

main jet =3/4 to full throttle.

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ACK you're absolutely correct, Main jet is what I meant to say. I'm really curious to find out if all this and a proper leak jet will improve the low-end bog without all the mods, but more than that, I hope it gives me better power predictability.

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