jetting help requested

5 replies to this topic
  • Paul_in_Wa

Posted February 18, 2002 - 07:15 AM


Tried searching for an answer to this but couldn't find one:

I'm helping a buddy jet his '00 YZ426. It runs well, but pops on decel when the throttle is closed. I thought this would be caused by the pilot screw/jet. We tried turning the pilot screw out. By the time we made it to 3 turns out, the bike didn't idle well at all and the popping on decel. still happened. His current jetting is bone stock, he's got an after market pipe on it (sorry, don't know the brand of the top of my head) Would going to a 45 pilot jet (I assume 42 is stock) be the likely answer, or should I raise the needle or go to a different needle?

Also, what's the consensus on the best needle to go with on the '00 YZ426? Changing the needle on my 250F made a huge difference, I'm wondering if the YZ426 might still have some untapped gains in that area.

Again, it's an '00 YZ426F, bone stock jetting, aftermarket pipe (I can find out the brand if necessary)


  • therapture

Posted February 18, 2002 - 08:34 AM


I just finished rejetting my '00 426, along with the BK mod.

Even before the mod though, I was lean on decel, which causes popping...I trashed the stock 42 pilot and went with a 48 on recommendation of several other people. Why such a large jump? Because I was fixing to do the BK mod and for my sea level altitudes this was again recommended.
For your buddy, I would get both a 45 and a 48, you could very well need the larger due to the aftermarket exhaust. Also, if you are going to be doing the rejetting anyway, might as well do the BK mod, the 00's seem to the the biggest benefactor of this modification, it is easy as pie to do, and makes very substantial ridability gains. Check my other thread--
here for some more information...

[ February 18, 2002: Message edited by: therapture ]

  • skthom2320

Posted February 18, 2002 - 09:17 AM


Yep - aftermarket pipe = free flow/more air = leaner. Therefore require richer jetting.

Try what Therapture said. He is spot-on. After you get the pilot set (which affects the idle or deceleration condition), you may need a richer main and needle clip position as well.



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  • MikeOK

Posted February 18, 2002 - 10:28 PM


45 pilot, re-adjust the air screw. Go in 'til it idles rough, go out 'til it idles rough, split the diff. Us a mirror, change it through the drain bolt hole in the float bowl. No need for anything but a mirror, a small screw driver, and some patience.

[ February 18, 2002: Message edited by: MikeOK ]

  • Hick

Posted February 18, 2002 - 03:53 PM


Originally posted by Paul in Wa:
Also, what's the consensus on the best needle to go with on the '00 YZ426? Changing the needle on my 250F made a huge difference, I'm wondering if the YZ426 might still have some untapped gains in that area.


The stock 42 pilot may be one or even two sizes lean at sea level, and the lean condition that creates the popping happens with most aftermarket exhausts relative to the stock YZ system.

But easily the largest “untapped gain” on the whole bike is swapping the stock ’01 426 needle or this optional ’00 needle which is just a half clip leaner: 5JG-14916-EP.

On either one you would probably want to try clip # 4 first.

BTW, stock ’00 needle is EKR, which is much too lean off the bottom at sea level. The ’01 (EJP) and the optional PN I gave above (EKP) are two sizes richer just off idle. If they don’t have the standard ’01 YZ 426 needle in stock ask for the PN I gave above, but beware that it may be listed on their parts fiche as an EVP and not EKP. Ignore that, that PN will in fact give you an EKP.

Also, a general rule of thumb is that if you end up more than two turns out on your idle mix you may need a larger pilot.

Here is the jetting in my ’00, but keep in mind I am at almost a mile of elevation:

EKQ # 3
40 pilot
160 main
~ 85 on pilot air jet screw (only way I could get mine to quit surging/popping)
Main Air Jet (size 200) removed
“BK” accelerator pump mod, ~1/3 of stock pump stroke, ¼ turn additional delay.

Hope this helps.

  • Paul_in_Wa

Posted February 19, 2002 - 07:49 AM


Thanks Hick. I appreciate the help. We'll try the bigger pilot tonigh, and the needle as soon as we can get it.

Thanks again,

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