Beware of BK mod on '02 426

35 replies to this topic
  • motoman393

Posted October 24, 2001 - 11:25 AM


I agree with forloop...I enjoy working on my bike almost as much as I do riding it! I think it depends how picky you are about your bike! Most of my friends dont care how their bikes run and it shows whenever I pull the holeshot and run away with the moto (and they wonder why my bikes never breaks down and runs great)! I just like to tinker with things (whether it be dirtbikes, computers, lifting trucks, etc).

Oh yeah my bike ran perfect too! I had absolutely NO bog and it started great and no fouling plugs. But hey if I can get more snap for <$5 than I am going to do it!

***If you are afraid of a wrench then dont do this mod and dont even bother emailing me. You wouldnt believe how many emails I get saying "how do you do the BK mod?" I mean come on people, I have a detailed description and pics on my site plus unlimited amount of info right on this board (just use the search button) If any TTalk newbies are reading use the search link, all the answers are right there! Ok enough venting! LOL Later,


I get my kicks on a 426!
Motoman393's MX Site

  • neWRiver

Posted October 24, 2001 - 12:50 PM


Originally posted by kfrosty:
Not exactly rocket science here

What?? uh... this isn't third period, Rocket Science 101??? Posted Image

Jeez! I'm in the wrong class room.

Sorry, nevermind...

Hey, just poking fun... (actually I like to think of my bike as a "rocket") Posted Image

Seriously though, kfrosty, you have made a very important point and I now have a better understanding of what you are trying to say. Sorry for the flash reaction there. Truth is, as a top 5 B rider there is probably alot more that you could teach me than the other way around when it comes to dirt bikes (track or trail). I do like to tinker though. And I agree that casual readers should be wary about any quick advice they get here (or on the track).

Also, I think alot of people are interested in improvements made on newer models. We would all hope that the factory could do better on this throttle response issue. You seem to be indicating that they may have got it whipped for '02 and we are all taking note of your evaluation.


  • YZThump

Posted October 24, 2001 - 02:02 PM


OK ,so don't do a plug reading. Take your pipe off and wipe out the end of your header or exhaust pipe with your finger,BLACK? Exactly,its way too rich. If jetting is on and its still too rich then what? then I tried the "BK MOD" and the Power was Awesome the Bog was Worse,Starting was too easy,So I'm kinda in between--Haul ass and easy starting or less bog ,hard starting,and not so fast. Just my take on it!!!


  • MX_Tuner

Posted October 24, 2001 - 02:58 PM


Frosty, no flames intended. And I don't think you've offended anyone here so far. My perspective is similar to yours. I want to do a mod that works without much screwing around. I get paid to work on bikes, make them better than stock. When I heard about the mod, I was anxious to try it just so I'd have an idea about any possible benefits. I've done it on a few bikes (250s and 426s) and they have all responded quite favorably. I did it on the WR 426 I built for the Vegas to Reno race. All the riders couldn't believe how well it ran for a stock motor (except for YZ cam timing, BK mod, grey wire cut and WB E-Series pipe- no internal mods). They say the '02 bike has updated carb specs. So has every other year YZ and WR. I have noticed some bikes do better than others with the BK mod.

Now, the plug thing. I'm much better with two strokes than with four strokes. Even with two strokes, I never used plug color as a jetting guide. For one thing, exactly which circuit was in effect when the plug got the color you're looking at? Unless you do a wide open plug chop, you can't be sure. Teh polit circuit has a huge affect on throttle response and there is no way a plug reading is going to so much as hint at the pilot circuit mixture. When it comes to a foru stroke, they are even harder to correctly read to determine fuel/air mixture. Dont' even try. Your best method of jetting is plain ol' trial and error and educating yourself as to what feels like what. It can take a lot of time, yes. There are a few guys who have a very good handle on four stroke jetting here. Maybe they can help. Carb basics are similar as a two stroke carb but once you get into the finer aspects, all of what you know goes out the window (pilot air jets, air correction jets.... SAY WHAT?!?!? It confuses the hell out of me sometimes.... no, all the time).

Who to believe. Who can you believe when it comes to free advice over the internet? I try to give people the benefits of my experience working on bikes for the last 25 years. I don't have all the answers, that's for sure. But if I post something, you can count on it being somethign I'm fairly certain of. Not saying anything bad about your friend, but believing a rider just because he is a sucessful A class racer is a poor way of selecting a source of good and accurate info. Your guy may very well be an excellent source of good info but I know WAY too many A class riders who are clueless.

Doing the BK mod is only half the job. There is jetting to go along with it.

MX Tuner

  • motoman393

Posted October 24, 2001 - 03:21 PM


Very well said MX Tuner! The BK mod is only half of the mod. If all you do is drill and tap a screw w/o adjusting your jetting to compensate for the loss of fuel at the acc. pump you are only doing half the steps. Go back and read the original post that Brian Kinney wrote and he explains it there.

--> Hey admins - is a FAQ "in the works"? it sure would be nice and would benefit all the TTalk users...if not I just may "splurge" and make my own and put it on my site, if anyone has something to contribute to an FAQ then let me know <--

I get my kicks on a 426!
Motoman393's MX Site

  • kfrosty

Posted October 24, 2001 - 03:27 PM


Ok, I lied, this is my last post. Posted Image MX Tuner, thanks. Could I have your email I'd like to ask you a couple of questions on the jetting topic. It won't be too painful or to long I promise. I would post here but hopefully this thread will end as is. FYI. I emailed tech support and asked if they could change the title of my post. After looking at it I did poorly title this topic and I've asked them to change it to just "Read this before making the mod on the '02's" or something to that effect. I can see how the title could set people on the defensive and miss my point.

Thanks. BTW. My email is

  • Hick

Posted October 24, 2001 - 03:49 PM



This turned into a really good thread, I don’t think there is anything wrong with the title.

I think you should make another post about your bike's jetting. For one thing, I'm curious as to what has changed for '02, and I'm sure I ain't the only one. Secondly, how you described your bike wanting to toss you over the bars sounds like a lean condition somewhere, maybe clip pos. And finally, it seems just about everybody at sea level or near it around here has gone to richer jetting on all model YZFs.

IMO the jetting on these bikes is much harder to sort out than any two stroke I've ever owned, and I've owned two KX 500s. Yuck.

I believe that a great deal of this difficulty is due to the presence of an acc pump. I personally don’t know how you are supposed to accurately determine the best clip position with the thing turned on. I had a very hard time deciding, and turned out to be wrong, because every time you roll on the throttle to try and feel where you are coming onto the needle the pump is squirting.

I dunno, that is just my own experience, I don’t have the jetting experience of some of the other guys around here.

  • Taffy

Posted October 28, 2001 - 01:51 PM


i like hick would ask you to put up your jetting specs for '02. please look for the following; main jet, main air jet, needle and it's clip position, pilot air jet, pilot jet and finally the slide cutaway height which has always been 1.5mm.

hope you can do this by checking your owners hand book. hick have you given up on actually running the APJ?

both hick and i have had success at lowering the pilot system as a "unit" and the "bwoooaaaar" noise when you load up hard in second is now non-existant. i have a silencer (as in a rael one!) so i can hear this difference.

i have been running without the APJ for 8 months now and wouldn't put it back on if you paid me, i've been able to make valid jetting modifications.

if we can have that info maybe we can help others?


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  • kfrosty

Posted October 30, 2001 - 01:46 PM


As requested:

Carburetor on '02 426:

I.D. Mark 4SF1 00
Main Jet 162
Main Air Jet 200
Jet Needle - clip position 4th
Cutaway 1.5
Pilot Jet 42
Pilot air jet 100
Pilot outlet 0.9
Pilot Screw 1 - 3/8
Bypass 1.0
Valve Seat size 3.8
Starter Jet 72
Float Height 8mm
Engine idle speed 1700-1900 rpm

I've heard of some '02's fouling plugs since this post. In order to try and help others out and beings my major series is wrapped up. If I get time I'm going to try and tinker with the mod again. My main reason for doing this is to get a baseline for people interested in doing the mods to the new '02's. No use in anybody reinventing the wheel. If I get it figured out, I will post. If anybody else has done this on the '02 and has any pointers that isn't already listed for the previous models, please let me know. motoman393, when I get this finished I'll try to put in doc form if you want to post it on your website. I know there will be various such as altitude, weather etc. However, what I'm hoping to do is note symptoms of the main jet being too large or small. The Pilot screw need a turn here or there and what it corrects. Hopefully, it will save everybody a lot of time initially. Then the tinkers can go from there. :)

  • Hick

Posted October 30, 2001 - 03:02 PM


That must be the most detailed jetting post I've ever seen. You are da man.

But what is the letter-code of the needle? :)

Stock '01 is EJP

I wonder if the "revamped carburetor settings" is just another different acc. pump diaphragm (they also said "revamped settings" for the '01 year), because unless the needle is different your jetting is the same as my '01.

Also, what is your elevation/climate? I think I remember you saying you were in NC? Anyway, anybody that rides in a similar area who is confident in their jetting (w/ or w/out any pump mods) please chime in here, I'm thinking that Kfrosty could benefit from the collective wisdom...

..such as it is.

  • BK14MX

Posted October 30, 2001 - 04:14 PM


Everyone: I have a 00" 426, all stock. Just did the BK mod and am having some trouble with the jetting. I am primarily riding at 1000ft of elavation, and up to 2000ft. I run 93 oct. gas, is there anyone in the Atlanta, GA area that did the mod and can recommend a starting point for the jetting. I have put all settings back to stock, 162m, 42p, stock needle, 3 pos.

What I tried was a 165m,45p, dropped the clip, "raised the needle" one place. ACC. pump at .3-.4 sec, timing was ok, never hitting the slide. I hear that there are a couple of brilliant jetting guroo's around, need some helpful hints!!!

  • kfrosty

Posted October 30, 2001 - 04:25 PM


OBEJP-4 for the Needle Jet. :)

I live in Charlotte, NC and to tell you the truth I haven't even checked what the elevation is here.

On another note, since I've went back to undoing the BK mod, my bike has ran the same. Was thinking the small change I made to the timing screw had something to do with it. Because every since, the bike sounds good but it just seems flat in higher rpms. Meaning some areas I could rev out in third gear before hand, I know have to hit fourth to keep the same speed. If it were a 250 2-stroke I would say it needs a set of rings. However, the bike is only a month old and from what I hear these things only need to be rebuilt once a year or so. One thing I did notice though, after changing the mod back I decided to go ahead and put a fresh plug in and didn't pay attention. I just used what the shop gave me. Now looking I am using a BG9E and it had a BG8E. So I'm hoping this plug is the cause for the loss of power as for it's not as hot.

  • Moto747

Posted October 30, 2001 - 04:47 PM


I just got rid of my 2001 YZ250 and got a 2001 426. I aint gona touch anything (jetting). I used to own a 1998 YZ400. I was a lot faster on it than my 250. My feeling are 10% bike and 90% rider giving 110% makes a good rider or racer.

[ October 30, 2001: Message edited by: Moto747 ]

  • Moto747

Posted October 30, 2001 - 05:09 PM


OK I changed my mind. I am going to ride it this weekend. If I notice any thing I'll try the BK mod. It will give me something to do with during the week.

  • Taffy

Posted October 30, 2001 - 10:42 PM


i wish i could give you great directions but all i can see is that the Pilot Air Jet should go from #100 to #85 for a possible improvement.

as you will already have seen though most blokes get an adjustable PAJ called the PA Screw (PAS).

i run no APJ but i have a 82DB euro/canook pipe so different strokes for different folks.

hick knows what he's doing. Hick, i'll be going to stroker pipe in a fortnight so i hope i can allay some of the gaps in our jetting.

also did you click on that Keihin web site? they think the needle straight comes into play at 1/8-1/4 throttle! would you, like me disagree and go with that needle chart that has the "needle straight" firmly against the left (and therefore "starting and idle").


[ October 31, 2001: Message edited by: Taffy ]

  • Taffy

Posted November 01, 2001 - 11:21 PM



most of us are running (i think!) 9mm float height. infact there is a web page by a bloke named richard burns who says something like "if you haven't set your float height at 9mm, you might as well die!".


[ November 02, 2001: Message edited by: Taffy ]

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