Needle check!

Getting ready to install my jets & needle. '01 WR426, YZ pipe (yes!), dewired, delidded, destopped. No BK mod.

168 Main

48 Pilot

ELN needle

100 PAJ

Altitude <1000', Temps 60's F.

1. Start at clip #4?

2. Do I remove the MAJ completely or leave in the stock?

3. Approximate # of turns on pilot screw?

4. Do I dare do the BK mod at the same time or wait until I have it running good first?

BTW, that YZ pipe is awesome. Actually a quieter (or at least less annoying) exhaust note at idle and slightly above. So neighborhood rides are back....short shifting of course. Thanks.

1)Yes, Clip #4

2)Keep the MAJ with a #168. Get a #172 also to compare with it.

3)1 turn (3/4-1 1/4)

4)Go ahead and do the mod. It's easy and gives more flexibility. When you shorten the accel pump stroke go to clip #5.

Have Fun!


JD...Can you confirm some settings for me please?




Pilot screw @ 1.5 turns

WR Timed, Lid on but opened up(3 1" holes down center of lid)Grey wire cut, Baffle out and finally did BK last week. Rode this weekend at Victor Falls(Bonney Lake) and it ran terrible? Had to back fuel screw down to .25 turn to lose the 0-1/4 throttle bog from hell and the bike idled fine with screw completely closed? Any suggestions? Bike was running phenomenal at Naches and Auburn West Hill by Hinshaws before I did the BK and I'm pretty sure the AP is adjusted correctly(squirt starts just after slide raises, does not hit slide at all, and lasts approx. .3-.5 secs.). Seems like I would need to richen instead of leaning out at Victor Falls? Now I'm really confused.gif? Any help would be greatly appreciated.


Y2K WR400FM,Answer top tree w/Accu-Trax clamp,Pro-Tapers(CR-Hi w/ EE handguards),Devol Rad. & frame guards, Guts tall soft foam w/ ballistic cover, 14/52 w/DID VM X-Ring, Kouba-T,Michelin M-12's ft.& rr.,12 oz. flywheel weight, Zip-Ty Racing Link & Shark Fin(plus everythhing else cool that Ty offers), Cycra front rotor guard and T.M. Designworks Chain guide, buffer & chain wheels. Street legal except for the non-"DOT" tires.

Yes, richen it up-

#45 pilot and ~2 turns (out to richen/in to lean)


#175 main

Make sure the carb boot is tight and sealing. All the other bikes using Dxx needles have a #175 main, then there's the WR which comes with the restrictors that get removed.

So far so good! Got the carb out, all the parts in and the BK mod done too. Don't hesitate on doing this guys, there's nothing you can hurt or ruin by drilling that tab. The carb is still out (no time before work to complete it) and want to play around with the BK setting a little more before I put it back in tonight. FYI, the stock squirt was 2.5 sec, did not hit the slide. Man, that shoots a lot of fuel though!! I had no idea. No wonder it sort of hangs on the rev slightly. Can't wait to try it out. Will advise.

Can anyone try to describe the "squirting" process of the carb to me? After reading all of the BK mod posts, I'm definitely sold on doing the mod myself, but not having any time to take apart the carb for a week or so to see it myself, I'm having a hard time picturing it in my head.

Apparently, you can see "the squirt" by looking thru the airbox opening on the carb? I'm assuming that the jet needle raises and lowers (based on clip position) into the tube that attaches to the main jet...with fuel flowing through the main jet/tube past the needle jet...but after that, where does the fuel go? What exactly is this "slide" that people are referring to that the timing screw on the accelerator pump adjusts?


Clueless in Colorado smile.gif

Larry, you gotta take the carb out of the bike to get the idea of what everyones talking about. I wouldn't suggest looking in there when you raise the slide (basically the center portion of the carb); it squirts out about two feet. It's not on the airbox side, it's the opposite side. Once you have the carb out, you'll understand what i'm and everyone else is referring to.


You are not totally clueless, after all, you know where to go for answers grin.gif

Your explanation of the main jet and needle interaction is pretty good IMO, after the fuel flows through the main and past the needle it is inside the carb venturi and on its way into the motor.

But this has nothing to do with the accelerator pump, which is what the BK mod involves.

The slide is simply the carb slide, and also doesn’t really relate to the accelerator pump except that opening the slide (twisting the throttle) is what starts the accelerator pump via a mechanism on the slide pulley (part where the throttle cables attach).

So, twist the throttle, you lift the slide and allow the accelerator pump to begin its operation. On ’00 and newer YZ/WR thumpers there is a screw on this mechanism where you can adjust the operation or “timing” of the accelerator pump relative to the slide. This matters for obvious reasons, not the least of which is that the jet or orifice that delivers the fuel from the pump into the carb is behind the slide, so that if the fuel is spraying too soon it will just hit the slide and make a puddle, doing you no good.

The BK mod allows you to limit the stroke of the accelerator pump, or the duration of the squirt and amount of fuel delivered every time the throttle is opened and the pump operates. Since the nozzle or jet that the pump squirts fuel through is behind the slide, looking into the carb from the air box side (with the air box off, of course) is a good way to see “the squirt.”

BTW, I was just talking to a buddy of mine who lives in Denver, and apparently Excel Yamaha there has changed the pump diaphragm in a few guys’ carbs. That is just a different, unadjustable way of doing what the BK mod does.

Well, got to test it tonight...and it runs terrible! I started at 1 turn out, started first kick, idles fine. Pretty much flat all through the mid-range, no hard pull at all. The idle was hanging, so I kept going out on the screw, up to 2 turns out. Runs better off the bottom but mid range is horrible and not much on top for that matter. Where do I go?? I can't tell if it's lean or rich, seems like I should be fairly close. Most have had great results with this. Can ONE clip position make a huge difference? I mean I feel like I'm light years from where I was stock. Also, what's the best way to change the needle without taking the carb off?? (I did all the jets & mods with the carb off the bike.)

YZ pipe

ELN #5

170 main

48 pilot

100 paj

BK mod .4-.5 sec.

Dudes, I'm bummed. I just went through all this crap with my Husaberg...don't know if I can take it with this one!!

[This message has been edited by Scott in KC (edited October 17, 2001).]

Please tell me you did NOT take the slide plate out to look at it.. please... (It can get put back upside down!)

This is really odd. The pilot screw should not need 2 turns out, sounds way too rich. Is there an air leak? TPS connected? Hot start closed?

You have YZ426 jetting basically other than a bigger pilot jet. It should run like a YZ??

No, I did not take the slide out. Everything went back in like it came out. TPS connected, fit tightly into intake & boot, hot start closed. I'm stumped...not to mention bummed. The mid-range is just dead. I'll give it another look just to confirm, and will have a longer test period tomorrow.

James, at this setting, even if I'm off a clip position, or 2 numbers on a jet, is that enough to be causing something like this?? I mean could I be too rich on the needle & main, rich enough to cause this?


The idle speed should not hang at all with a #48 and 1 turn out, 1 1/4 max. This is usually an indication of a lean problem. Clip position #4 would be a possible step, should also work very well. Something is not right. Tank full? pinched fuel line? Was the stock pilot a 42 and how far out was the pilot screw before?


[This message has been edited by James Dean (edited October 17, 2001).]

Yes, stock was 42, 1.5 turns out, 75 paj. We are talking about clip positions from the top correct. Just checking. I just went out and rechecked everything, too late to run it. The tank is pretty low, but on should be getting plenty of gas, but I will fill it tomorrow just the same. I'll also check for any pinched lines or hoses.

Whats the best way to get to the needle without totally removing the carb again? Take the tank off?


your buddies on their wr426's are all running jetting below the 400's i would think due to the greater volume of air that the engine pulls through.

they're running EMP clip 2 which is 2.5 drop of the needle. THAT'S A HUGE DIFFERENCE.

they're running 40 or 42 PJ's and most definately not 48's.

i would suggest you drop the needle one clip and if you note an improvement (ie less misfiring) then i'm sure you'll be only too pleased to do the same again and then a third time if your doing well!

as far as your pilot circuit goes i would drop straight from 48 to 42, stay with the 100PAJ and put the PS out to 1.5 turns.

do little snap wheelie tests with the PS going + & - .5 turn. this takes minutes with the correct screw driver.

quickest way to work on carb?

no fanny flaps on the rad

one bolt on the tank

one bolt on the seat

both engine plates on the left


twist it having undone the front and back screwa so you can open the top. to grab the needle, snap the throttle open hard and catch it with your fingers! (quite funny to watch).


pull carb outside the frame. BTW, i have no APJ housing cover at all. so it's a doddle.


[This message has been edited by Taffy (edited October 18, 2001).]

Wayneo426, same bike, same mods, same elevation & temps....but he doesn't run the YZ pipe...his bike was vastly improved with this exact jetting, in fact he's even 2 clips leaner than me. Does my YZ pipe flow THAT much more air to cause such a big difference?

The weird thing is, that it does feel lean in the mid-range to my untrained wrist (I had gone through some similar jetting issues on my previous Husaberg), which is harder to believe considering how rich it is now.

Thanks for everyones help here, I know jetting is not easy, just hoping for a miracle out of the box.


woody is on an EMP-c2 with 42PJ

slashman is the same (45PJ)

stefe's needle is up two and he's unhappy as well!(42PJ)



True, Scott. Same settings, and Im now on clip position #3. Major diff from #4. Sputter is GONE, and Im kissing the front fender w/ ease. Almost looped it on the trail last night.

How did Burns put it... "check the float height or just go jump off a cliff now"?

Thanks Hick...that was a very helpful description. I'm sure the remaining unanswered questions will be answered upon getting time to play with the carb.

Now I just need to find a way to convince my wife that working on my bike in the garage this weekend is a good idea! smile.gif

I'm at 1000ft and ELN #5 works great. It seams like my bike wants one setting richer than everyone elses for the same conditions for every needle. 1 1/4 on the PAJ is fine. Is the ELN now suggested over the EKP? I like the mid hit of the ELN but would buy the EKP only if there was a little more.

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