WR426 Carb Tuning Question

10 replies to this topic
  • 1slowDR

Posted July 27, 2015 - 12:00 PM


Hey there guys, I just bought a 2001 WR426f the other day, it has been converted to street legal and came with a motorcycle title which was a huge plus for me. The previous owner is a member on here, he bought the bike in non running condition and got it going. I bought it and rode it 15 or so miles home, when I got off the bike I noticed the header was glowing bright red, I assumed that the carburetor is running a bit lean causing this, but I'm unclear because everyone seems to act like it's a completely normal occurrence. I'm not wanting to burn my exhaust valves out though so I'm a bit concerned about it. The bike is stock with the exception of the "grey wire" being cut and the spark arrestor/baffle thing broke and is no longer in the factory muffler. As far as I know the timing hasn't been changed or anything and I don't believe the carburetor has been re-jetted but I really don't know. So am I just being overly cautious or is it a legitimate concern? Thanks in advance!

  • Krannie McKranface

Posted July 27, 2015 - 01:34 PM


The header absolutely should not be glowing when moving, period


You need to pull the carb and service it, and get the right jets in there 


Do it ALL not just parts accesible from the bottom.


READ THE FAQ section to get to know your bike and copy someone elses jetting

  • 1slowDR

Posted July 27, 2015 - 01:59 PM


Yeah that's what I was thinking as well, thanks for the response

  • 1slowDR

Posted July 28, 2015 - 08:32 AM


I am tearing the carb down right now, the main jet is 172, pilot jet is 42 and what I believe is the starter jet appears to be 65 but it's pretty mangled up. From the little research I have attempted the main jet isn't stock and the rest is. I also just got the needle out and it says OBDR R and the clip is on the clip is on the 5th slot going down from the top of the needle if that makes sense

Edited by 1slowDR, July 28, 2015 - 08:52 AM.

  • Krannie McKranface

Posted July 28, 2015 - 09:50 AM


165 MAIN







Change the apump system to 2008 specs using these parts:

*change the apump linkage spring to a stiffer version  http://tokyomods.com...arburetion.html

*replace the pump diagragm with a 2008 version/upgrade kit  http://www.crfsonly....roducts_id/3932


Bike must be uncorked intake and exhaust with this jetting.......



- clean the hot start plunger and cavity and polish is corroded: use light antiseize in the cavity and on the cable

- replace the slide plate seal

- install an R&D remote fuel screw and read up on how to adjust it (highest idle speed)


Now you have a NEW carb that will work BETTER then FI

Edited by KRANNIE, July 28, 2015 - 09:50 AM.

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  • 1slowDR

Posted July 28, 2015 - 09:58 AM


Thank you very much for the information, I will do that, I guess now would be a good time to do the BK mod as well? Or does the spring for the accelerator pump you posted take care of that problem, also the accelerator pump kit seems to be for a honda, I assume that it works fine on the WRs carb as well? Another thing I am wondering is if the bike is running lean now, wouldn't going from 172 to 165 make the lean condition worse? I've never had a very good understanding of carburetor so growing up in the fuel injection age haha, I'm happy to be learning more about it and appreciate your patience in helping me.

Edited by 1slowDR, July 28, 2015 - 10:54 AM.

  • Krannie McKranface

Posted July 28, 2015 - 03:32 PM


All real dirt bikes from 2003 to the present use the exact same carb.


Do not do the BK mod.


Your bike is NOT running lean at wide open throttle (main jet), it is very rich, unless you have a YZ pipe and lighter flywheel and a YZ head.


Pilot is for idle and 1/8 throttle only.

Needle is everything else, and the most important.

  • 1slowDR

Posted July 28, 2015 - 03:52 PM


That makes sense, thanks again for your help I appreciate it

  • bobpara

Posted July 29, 2015 - 07:30 AM


This thing seems to be running REALLY lean

I'd also check for an air leak at the boot between the carb and the cylinder

Best way is to spray the area with WD40 and see if it sucks it in

If it does, the idle will drop


Your jetting seems to be in the ballpark (unless you live in the Mojave desert)


My gut tells me something is WAY off leaning it out real bad

  • 1slowDR

Posted July 29, 2015 - 01:14 PM


Well the only things I found that were out of the ordinary is the pilot air jet was completely clogged, the main air jet was a bit dirty, and the end of the needle had a bit of a burr on it. I cleaned everything up, and carefully sanded the burr down on the needle, making sure not to change the profile of it. I also adjusted the fuel screw exactly 2 1/4 turns out so I know where it is now. In a bit I'm going to put it back in and see what happens. Hopefully that will help and get me by until I can do the mods KRANNIE told me about.

  • 1slowDR

Posted July 31, 2015 - 12:59 PM


I put the carb back on Wednesday evening and started it up, it took longer for it to glow at idle, if say closer to a minute before it started, before it would happen 10 seconds or so after starting. Then I took it out on the road for maybe a 1/2 mile, when I got back it was red only around where the header bolts to the head, so at least cleaning it made a little progress. Hopefully some of my parts arrive pretty soon and I can get it completely taken care of


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