01 WR426 jetting, Can someone dial me in

I am new to thumpertalk and assume this question has been asked many times in the past. I have a stock 01 WR426, open air cleaner, throttle stop mod, and baffle removed. I foul a fair amount of plugs, so thought I would be rich . If the weather gets around 50 or less the bike doesn't want to idle.

What would suggest?


The mods you listed should have little or no effect on the stock jetting. Take your time and clean the carb really good. If you still have a problem, turn your fuel screw in until the idle changes and back out a half turn or change your needle clip position. Do some run tests and adjust the fuel screw as needed. To prevent from going crazy, CHANGE ONE THING AT A TIME. Get your pilot circuit under control first, then move to the needle, then the main.

We could help you better if you explained how the bike runs at different throttle positions. If your interested in changing to different jetting, I suggest getting the OBELN, OBEKN from Yamaha or the OCEMM (Sudco) or try the jetting in my signature below.

All of these needles add extra punch to your Mid range.

Thanks for the reply, The bike does cough sometimes when coming off idle. THis is increased in the cold weather.

I am not sure what elevation you are at, however Baja Designs out of California can send you a jet kit and jetting specifications for your bike with mods, elevation and trail type for around 20 bucks. I checked with the local Yamaha dealership that gave me the specs for my local riding area, and then I went to Baja Designs for a set up in Utah, about 5000 ft less than my local riding area. If you’re at 4000 ft or 9000ft I would be happy to give you my specs. Hope it helps.

Just off closed throttle is the pilot / needle.

Have you done the BK Mod to shorten the duration of the accelerator pump squirt? Each time you wack the throttle, you are probably injecting a 3 - 5 second squirt of gas. The Bk mod is very easy to do and will give you the ability to adjust this squirt duration.

The mods you listed should have little or no effect on the stock jetting.


Okay, what differences would account for the jetting difference twixt YZ and WR? Same cam profile (albeit diff timing), same carb size and model, same head, same displacement…

…you know where I’m going with this. Motor is the same. What is different? Exhaust is plugged up with a pinky-sized insert, and air box is sealed (kick stand and headlight won’t affect jetting). Undo those two things and you now have a YZ motor, so why not make the transition complete and use YZ jetting? One part number away (okay three, but the needle is 90% of the diff.). I say this so often I’m even boring myself now. (Yawn)

We need a “Yawn” emoticon for future Hick WR Jetting posts (and YZ 450 4 speed posts).

Thank you.

"What, another Hick jetting post?"




what is your suggested solution? I just want the bike to run better off idle and not foul plugs.


What is the "BK" mod.

I'm interested.


I only meant the mods he made would not cause a RICH condition in his jetting, fouling plugs. The WR's come all clogged up and jetted fairly rich. By removing the air box lid and the Exhaust plug, thus creating a free breathing engine, his stock jetting would now probably be close to on, if he chooses to keep the very tame DRR needle. If you read my post a little better, you'll see I do suggest changing to a different needle.

But hey, I'm glad to be corrected by the likes of yourself. Much respect for your jetting knowledge. :)

YAAAAAWN.. :D Sorry I bored You.

The BK or GB mod can be seen all over this board.

Try a search for BK or better yet, Go to the MOTOMAN393 web site, which has a link on the main TT home page.

Yeah I see your point HaremScarem, and I did notice you recommended an E needle, I really should stay away from WR jetting posts since I own a YZ. I've ridden and jetted 3 WRs for riding buddies though. I just jetted them like YZs :).


The BK mod is a very simple modification you can make to your carb. It involves drilling and tapping the body of the carb. You still here?

All kidding aside I think you are cheating yourself if you don't try this. It is very simple, the area you will be drilling is not important to the function of the carb in any way, in fact there is a tab cast into the carb body, and the only function it could possibly serve is to act as a holder for an adjusting screw to act as a stop on the accelerator pump actuating arm.

The best instructions and visual aids you could possibly want are on Motoman's site.

Click on "Tech Articles," then on "BK mod."

Your plug fouling problem is a bit weird, but also common at the same time. It may not even be carb related, there have been a few members with fouling problems that persisted until they replaced their CDI. :D

But, like HaremScarem, I don't see how you could be too rich. You are almost certainly too lean, probably everywhere, but most obviously on the idle circuit. I also think everybody should put a YZ needle in their WR.

Have you tried the fuel/idle mixture screw? It is accesible from underneath the carb, recessed into the bowl, pointing almost straight down but angled a bit to the front of the bike. I use a small slot-driver bit to turn it. Anyway turn this screw all in and count the turns to find the current setting. If it is stock it will be 1.75 turns out. Go back to that setting, get the bike warmed up, and go out (out is richer) some more until the idle speed improves. If you end up too far beyond 2 turns out you should try a larger pilot.

At your elevation, which I'm assuming is around 500 ft. or so (???), at colder temps, you are almost sure to be too lean.


Try the OBELN needle on clip 3 or 4. You can get the needle from your local Yamaha shop. It should be listed under the YZ250F parts list as an option. Again thats from a YZ250F :D:) (Just Having fun, HICK)

This needle and your stock fuel jets are a pretty good setup. You may want to pick up a 168 or 170 main jet also and give them a spin. You should have a 42 pilot and 75 pilot air already in the bike. This combo is very good unless you want to subscribe to the TAFFY style of jetting.

definately do the bk mod first, It sounds scary drilling your carb but it's really hard to mess it up. motoman393's pictures make it really clear and easy to understand.

It will make it a lot easier to jet the bike as you wont have tons of unmetered fuel pouring in there from the accelerator pump like it does in stock trim.

After playing with the larger 42 pilot/168 main area with an eln needle for the first six months I have since subscribed to the "TAFFY" style of jetting.(see my signature) I have to say the crisper throttle response and the way the bike feels clean and smooth and torquey from idle all the way up to that eyewatering top end is much improved.

I always need the choke to start it (first kick normally)when its cold but after that , even after crashing on hills etc. She fires up first kick 99% of the time without using the hot start and the spark plugs come out spotless with a light tan colout to the earth electrode, I have also gone to a colder 9 heat range NGK iridium plug and have had zero fouling since the rejet. They used to come out black and sooty with stock jetting and an un-"BK modded" carb.I thought it was awesome back then but I would never ride one jetted that 'fat' ever again.

Hook into it, its not that hard.

Waynewr, my 2002 wr 426 also has the same mods as you do, and it to looked like it was running rich because the plug was black every time I looked at it, and when the weather changed (colder) it ran like hell (lean symptons, but still black plug). I haven't done anything yet because bike is up north and will not ride until early spring, but I am getting ideas on this forum on how to rejet it. I ordered the obeln needle and some other jets. I think the bk mod or putting on the p-38 lightning to reduce the gas squirt makes sense. I am still a little confused about which way to go with the mj when I put my obeln in. I have read that you should drop the mj because it is a richer needle, however Harem and some others I have seen went up? I think the 42/75 will be close with this needle from other posts.

I was wondering what the relationship is with the clip position and the main jet. If I start on clip 4 and lower the mj to 160, would that be the same as starting on clip 3 with 165. I think I have a handle on how the pilot jet works and when you need to go up or down. Can't wait until spring.


If I start on clip 4 and lower the mj to 160, would that be the same as starting on clip 3 with 165.

Hmmm. No, because the two changes aren't really comparable. Changing the main jet size does affect jetting in a lot more places than just WOT, but changing the clip position is much, much more noticeable at around 1/4 to 3/4 throttle. Other than changing the taper changing the clip is the most abrupt change you can make I think. Your best bet IMO is to jet for the main with a few WOT runs. Forget about the clip, find the best main, then you can find the best clip position next.

Changing the needle taper from D to E is a big change, and affects nearly 100% of throttle positions. If you were jetted perfect on the main with a DRR needle, and went to an E taper needle, you should end up with a smaller main than you had before. This is because the more abrupt taper opens up the main more quickly.

When I jetted my friend's 520 I went to an E taper and went down three or four sizes on the main right off the bat. On my friend's WRs though I only went down one size, but we are at 4,000 ft. At lower elevations I suspect (don't have much seat time at lower alt.) that the WR/YZ 426 could benefit from richer than stock jetting on the main jet, especially in moderate temp.s

Hope this helps.

Hick, could you explain what you mean by at a lower elevation (I ride at 1000), the wr could benefit from a richer than stock setting. I understand at higher altitudes, the leaner you can go because your bike would be running richer at stock than I would. Just trying to make sense of all this. Thanks

Hick, could you explain what you mean by at a lower elevation (I ride at 1000), the wr could benefit from a richer than stock setting. I understand at higher altitudes, the leaner you can go because your bike would be running richer at stock than I would. Just trying to make sense of all this. Thanks

Mr reasoning here is that I'm only one size leaner in most aspects vs. stock and I ride at 4,000 ft. + in hot temps. Over the winter you could probably get by with stock or nearly stock jetting here on a 426 (this is a low humidity environment).

Based on my jetting experience with other bikes at this altitude that is relatively lean stock jetting. I almost ran out of clip positions on my last two stroke, and was three sizes leaner on the pilot. Both of my riding buddies 520s, which have (almost) the same carb, also ran richer here than my 426, but it was pretty close I guess...

Anyway another guess I would make is that at colder temps and 1,000 ft. your bike may like richer than stock jetting depending on the weather, gas, type of exhaust etc. :)

At 1,000 ft. in Mi. I really doubt stock jetting would be too rich.

I have ridden my 426 at lower elevations on one occasion, four days at Glamis a couple of years ago. That is sea level (and warm). I went richer than stock before I rode it ten minutes, and it ran great the rest of the trip.

I too have a 01 WR 426 with no Airbox lid and uncorked stock Exhaust. I have done the BK, Grey Wire Mod, and YZ timing. I rejetted as per James Dean's advice to Wayneo426.

165 to 168 main

48 pilot jet-3/4 turn out

100 main air jet

OBELN needle jet #3

I ride ground to 1000ft

No stutter at low end, just smooth seemless accel.!

YZ Tank & Seat

Still out there Wayneo426?

Ride Safe



My .02 is to find owners with the same setup as your bike and try their setup! Not to dish anyone, but I see too many people offering advice who have aftermarket exhausts and ride at higher elevations than you do.

Good Luck!

Hope I didn't piss on anyone's shoes :)



I also have a '01 and have very similar settings to Bruce Senff however I run a Vortip in my exhaust to comply with the DNR. I ride at ground level to max of 1000 ft (whatever it is here in Michigan) runs really well at 60deg or so.

I tried the clip position on #4 and #3 and like #3 way better, more snap and better starting. The 42 PJ worked better than the 45 PJ for me. I have yet to foul a plug now that I have these settings. I may try the 165MJ just for fun but its not important now.

Also look up the light blue wire mod. It disables the neutral detect circuit and gives a stronger spark while in neutral. This helps in starting a ton which in turn keeps me from flooding the bike etc. The rest of the settings are in my signature.

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