Jetting 2001 YZF426 w/ Yoshimura pipe

I bought a used 426 last Christmas. I noticed a couple of problems like glowing head pipe, lean on deceleration, slight overheating in lower elevations. Well, I was going to go riding at 8000 and took a look at the jet and noticed it had the stock jet (162). I had assumed that the previous owner had rejetted when they installed the the Yoshi pipe... I am now running a 168 and it feels better at 2200ft, but when I open it up it seems to miss at the top end..... I can't find the jetting formula on yoshi's site or anywhere else.... ya know 426 + yoshi = which jet???????? TIA!

If it starts to miss on the top-end before the rev-limiter, lower the main jet 1 step at a time until the motor revs out fully.

One of the design features of the jet kits (above) is to allow bigger main jets for bottom-end and mid-range while giving clean jetting on the top. You would be able to run a #168 or 170 with the jet kit (part no. JDY001 in this case).


lower the main jet 1 step at a time

Ok, just to clarify.... you mean move the needle clip up a notch


actually change out the main jet itself and go smaller? Doesn't the smaller jet lean the bike out? I am confused!

I apologize.... I am an idiot when it comes to carbs and I am really trying to figure this stuff out.... :cry:

The 17mm plug on the bottom exposes the main jet right? That's at 168 right now...

Thanks again for your help! :cry:

actually change out the main jet itself and go smaller? Doesn't the smaller jet lean the bike out


The 17mm plug on the bottom exposes the main jet right? That's at 168 right now...

Yes, the main jet is accessed thru the float bowl drain plug (17mm). Your main jet meters full throttle jetting. A sputter at full throttle when the motor revs out is either the rev limiter or too rich of a main jet. It's not lean, that would rev smooth with less power up until the rev limiter, then it cuts out at a higher rpm.

The stock main jet with your stock needle is a #162, so try a 165, then a 162 if needed to get peak revs.


Thanks JD, I will give it a try.... I had that sputter WFO with a #162. I have only tried one other size, which is the #168. That's why I thought it to be a lean condition.... that's the only place my buddies CRF450 will get away from me.... he can hold it up top, I let off thinking I am hurting the motor.....

I will tinker with it this weekend. I really appreciate your help!

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