Beware of BK mod on '02 426

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

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

Ok, I lied, this is my last post. smile.gif 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


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.

Not trying to flame, but here's some more fuel to the fire.

I don't think anyone here is saying that these are not great stock bikes. There sure is a huge aftermarket for performance parts, though. One of those being Factory R&D's P-38 which serves the same function as the BK mod. I think it's pretty clear that there is good evidence that a performance improvement can be attained (even if you wouldn't believe my personal testimony). It's also interesting that this mod comes from a factory mechanic and that no factory puts their riders on stock bikes (whether they could beat me or not).

Some people ride stock bikes. Some people buy lots of aftermarket goodies (even when performance gains are hard to quantify). Some people like to tinker with their machines. Some take them to professional tuners. It's all good! And hopefully, all of us practice our riding skills regardless.

Now, here's my beef. First, realize that I am a lowly trailrider on a WR250F, so go ahead and piss all over me if you want for invading your board. But, if you are going to pick up a wrench then be prepared to take responsibilty for what happens to your bike as a result! I know exactly what it is like to approach what seems like a fairly straight forward and simple mechanical project only to find myself feeling over my head half way through. That is some frustration man!

I won't start a project that I'm not pretty darn sure I can get MYSELF through. I won't touch my bike with a tool unless I know that I can get it back to where it was before I started if everything goes to hell on me (or at least have a pro mechanic lined up just in case).

If you were happy with the improved throttle response of the '02 and found this particular mod to be unnecessary, well you could have just said that up front. However, the fact that you have struggled over this leaves me only half convinced that the mod couldn't be applied successfully to an '02 with some benefit.

[This message has been edited by neWRiver (edited October 24, 2001).]


Not going to flame you and I respect everybody's opinion but as I've already stated, this is my first 4-stroke so I didn't have anything to compare to really. I don't think I'm over my head rejetting a bike. Not exactly rocket science here. (No offense.)

I've said from the very beginning I wasn't looking for opinions on what I may or may not have done wrong. Because I've got the bike back stock and it runs awesome. There's no sense in messing with something that's perfectly fine especially if you are trying something that isn't going to make a difference with what you are trying to accomplish which in my case is to win races.

This sight seems to have a lot of tinkers on it that as one previous member stated it makes them just as happy making a slight mod to a bike than how they finish up in a race. That's great and if that's what you are into I understand where you are coming from and as pioneers you are great! As for racing, I don't want to be a pioneer. I just make the modifications to my bike to where it's solid and it's not other bikes that beat me but more so the riders. Another bike is not going to beat me because of this mod.

My post was for those out there like myself who read these posts and have no idea of the skill level or the goals of the persons posting these responses. I looked at it like "well if everybody else is doing it and a factory mechanic is doing well I must buy into it as well." Just like the fuel topic, I heard so many say that you should with out a doubt run racing fuel. However, when I talk to the racers most notably the 25+ A and 30+ A National champion who is a friend and he tells me the bike performs great on 93 octane fuel in the bikes, I go along more with the persons who show the best results. However, with that being said I still like getting ideas from others whether they are trail riders or factory mechanics. I learn a lot from all different types and when I run across something that sounds like it would be a huge advantage I give it a try. I just want to say that this mod for people with my mind set towards riding is going to better your bike noticeably provided that your bike isn't having some sort of problem. That's the only reason I posted this topic.

This is my last reply on this particular topic. I think it's lost it's meaning. If you have comments or would like to discuss it with me offline, please feel free to email me at

Again, thanks to all who have responded and this topic was definitely not to rub anybody wrong or offend them.

I did not mean to start a fire here. All I am saying is that I know alot of people that try to buy speed, which can not be bought. If you just like working on your bike, than that is fine. I know a guy who bought a bike that Steve Lamson raced, he thought he was going to be so fast and it turned out to be right the opposite of what he thought. I dont know about a 00 or 01 426 but a 02 426 has all the power that you can handle.

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?


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. :)

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.

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!!!

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.

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 ]

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.

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 ]


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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