Beware of BK mod on '02 426

I know I'll probably get some negative responses but I wanted to share my experience on this to hopefully save somebody from going the the same hassle as I did. (Plus save them from potential spill)

If you aren't having a plug fouling problem or a noticeable bog with your '02 426 I would recommend you not wasting your time with this mod. At least for the '02's My bike ran better than I could expected but I bought into this mod when I should have heeded some wise words. "If it ain't broke, don't fix it!!!!"

Anyway, I ran into some riders that had done the mod to their '01's and I asked one to ride mine to see if they noticed the symptons that they had in their '01's that prompted them to make the mod. After riding they indicated they noticed a slight bog but for the life of me I didn't. So I thought what the hell, what could it hurt. (I am a top 5 B class rider and know how a bike should run in case there's any doubt about my experience.)

I followed the procedures to a T and took my squirt from 2.5 secs to .3. Went riding, and about went over the handle bars on over a triple as for getting on the gas before the landing as I did before the mod about killed my bike. Went to another track and was seat hopping a double and with the nice little bog the BK mod presented, faced the landing nicely. Well now I'm thinking it's running lean but the plug looks fairly decent. So I start changing the jetting around and it gets worse and worse and this is after the plug indicates I had tweaked the bike to where it was running a tad rich.

After a lot of frustration and wasted time today at the track I just tore into my bike and put everything back as close to stock as I could and now my bike is performing awesome.

Again, the purpose of this post isn't to try and figure out why the mod failed because I'm not touching it anymore because stock is awesome and it got me a holeshot in a 25 + A and B race. With the mod I just wonder if the bike would make it to the first turn. (Also, with 93 octane pump gas to boot.)

I just really want to save anybody with the new bikes from a lot of hassle.

The BK mod is not permanent. If it is that bad just back the screw out. I performed the mod on my 2000, and the bike runs much crisper.

Thats too bad for you! I know there are about 200+ people who have done this mod and love it, including Team Factory Yamaha (and Im sure if it didnt work Factory Yamaha wouldnt do it), and maybe 5 people who didnt like the mod! Did you make sure the stream doesnt hit the slide? Mine was hitting the slide (after the BK mod) and ran like your was/is and I adjusted the timing screw and now their is no comparision to a stock bike (same with the other 2 bikes I have done the BK mod to)! I had to switch to a 168 main a 1 3/4 turns out on the fuel screw w/42 pilot! I too run 93 pump gas.

Just so that the BK mod doesnt discourage anyone...if for some reason you do the mod and dont like it, simply remove the screw and you are back at stock (it takes 30 secs to remove it)! All in all I think 98% of the people who have done the BK mod are satisfied. Hope this helps and sorry about your problem,



I get my kicks on a 426!

Motoman393's MX Site

Correct you are. I backed my screw out. The only thing I was concerned about was the pump timing screw which I haven't adjusted back but I'm not going to touch it as long as I don't have problems.

I just didn't get much feedback from anybody doing this on an '02. I'm just wondering if they corrected the problem that was present in '00 and '01. Whether they did or not, like I said, mine back to stock squirts 2.5 seconds. But it runs as crisp as any bike I've heard and I don't have problems starting it.

I'm just trying to save people the headache of doing for no other reason than to just give it a try.

Thanks motoman393, I'm not totally trying to discourage anybody but the bikes you have done it on, what models are they? Have you done this on an '02? All I know is stock this bike rocks, I was pulling holeshots and got up to speed to win my first moto on the bike in +B class. However, I too jumped on the bandwagon because 98% of people did it. I'm just warning people that if you don't have a problem like me, don't waste the time doing it just because everybody else did it.

If you are having a bogging problem or are fouling plugs, this in theory would be a terrific first step to eliminate the problem. I'm just curious as to how many people have made this change to an '02 and can they honestly tell a difference if they could get it to perform as well as it does stock.

Like I said, I've learned my lesson with it. If I develop the bog, my bike starts fouling plugs etc, then I will go back to possibly tinkering with it. But now stock is awesome so I'm not wasting another minute on it. smile.gif

I have an 01 and the bike ran perfect stock with no hic-cup like to 00's had.

I too was hesitant to mess with a perfectly good running bike, but since the mod it's more responsive off the bottom. Did you bother to up the main jet? Not sure what the difference between an 01 & 02 but I thought I remember a different ignition mapping?

Same carb?


Moto to Live

Live to Moto


I went to a 168 first but that was too rich, I backed back down to a 165 and still just ran horrible. All the time, I turned the air screw 1.5 turns and dropped the clip to 3.

I'm not sure what's different on the 02's. This is my first 4-stroke. Maybe I just ride different. I ride it like a 2-stroke with my RPM's up most of the time so maybe I don't notice the small bog everybody else does at the very low end. I do know with the mods, if I went from dead throttle to 1/4 or 1/2, it would sometimes kill the engine. I've tried to lug like it is stock and to detect a bog or what everybody else claiming to detect. I just don't notice it. Also, I hopped on an 01 last weekend of someone tauting how crisp their bike was after the mod. Didn't hold a candle to mine. (Not that the 01 was any less of a bike. I think the difference I noticed was in the compression.) But it definitely wasn't any crisper in throttle response.

Oh well, I really appreciate the feedback but like I said, I'm not wasting time with this as for my bike hauls stock and has no starting problems. However, if somebody has messed with some '02's it would be nice to see a post for those contemplating the mod for a new their new bikes.

Thanks everyone!

This gadget has created its own monster...

I’m sure we all appreciate your observations kfrosty, at least I do, but it is what it is. What is accomplished with the BK thing is very simple in concept and can’t be blamed nor credited with how your bike runs because there are too many other interdependent factors.

You mention you are new to four strokes. Complaints about jetting mannerisms of YZF & WRs in particular from thumper neophytes, myself included, are as old as this board. Actually, you might say they are nearly the very foundation of this forum.

On advice gleaned from Thumpertalk, before anybody knew about the BK thing, I disconnected the pump altogether for the purpose of jetting my bike. That was a very revealing exercise, and one I recommend if you are serious about understanding how the accelerator pump affects this bike.

But if you are not so inclined, and are currently happy with how your bike runs, by all means, ride it, don’t waste any of your free time working on it. Ride it.

BTW as far as I have heard and read the carb is unchanged from ’01 to ‘02

i don't uderstand how the stream can be hitting the slide. As soon as the throttle is cracked open, the slide leaves its resting position quicker than the eye can see. Ive messed around with my timing screw and couldn't get it to hit the slide even when i tried.

Frosty, your mistake is jetting according to plug readings.


MX Tuner

MX Tuner,

You may be correct in you observation but I would appreciate some insight to your statement if you don't mind.

The plug I had in the bike was an extremely clean and a light brown. I made the mod with no jetting and the plug went to a light grey or an off white. Which to me would say it's a little lean along with the way it was running. I ended up going up on the main sizes. (162 to 168) The bike had the same symptons but worse so I checked the plug just to see where I was at. It went from a light brown to black. Are you saying that on a 4-stroke you can't get any indication as to whether the bike is lean or not. I was under the impression you could but again this is my first 4-stroke and if I'm under the wrong assumption I would appreciate your input.

Also, I hope I haven't offended anybody and knew I would get a lot of responses but again I wanted to post for those out there not having problems. This mod on my bike won't gain me one position on the track and I'm basis it on riding an '01 that has it and is suppose to be crisper. Everybody is correct in the fact that if you do it, it was very easy to back out of. So if you are the curious type or like to tinker, by all means give it a try. But if you are like me and doubt it would make much difference to start with but feel you should do it because everybody else does, these are the people I have posted this for. I didn't see any post like mine when trying to decide whether to do it or not and wish their was. Because just one would have put my mind at ease and I would have left my bike alone to start with.

However, thanks for all the responses. This forum rocks!

If it ain't broke, don't fix it!!


--Life is too short, work hard--

Sponsored by Yamaha of Cucamonga, Larry Roeseler's Stroker Fourstroke Speed Equipment and Answer Racing,

Just to share my experience. My bike, a 01, run great stock too. It had no bog or hick-up.

To me this mod was just too simple not to do and like it has already been stated, you can adjust from stock to no pump.

I also believe Hick has the correct approuch. Disable the pump and jet the bike. I just have not found that kind of time and if I was going to spend that kind of time, I think I would want to rig a O2 sensor up.

Ok, I do the mod and leave my jetting as before. Stock piliot, third clip, 158 main, and 1.5 on the fuel screw. I went .5 sec duration not .3.

First test ride and there was a awful dead spot. The kind that would put you over the bars. I knew I had only changed two things. pump delay and duration. I assumed the bike was rich before so I adjusted the delay. Well after addding about 1/2 more delay the dead spot was gone. The slide now 1/4 to 1/3 of the way off the bottom.

It has now gotten cooler here in Texas and I am running the stock jetting again. The only issue I have with the jetting now is, if it is idling and I wack the throttle WFO it will die. Now, I never tried that stock so I don't know if it is from the mod or not. I have no problems with it at the track. I am going to play with it more and see.

kfrosty, I could not agree with you more. If your happy with it leave it alone. Me being an engineer I can't help myself.



01 YZ426F #85 Vet C

I have something to say about this. I just changed from a 01 YZ250 to a 02 YZ426. And I can tell you there is nothing wrong with the motor in it's stock form. You could put McGrath on a stock 125 and he would kill everyone one on here. What I am saying is that instead of spending time with the BK thing, maybe we should be praticing. It is not the bike or the motor, they are fine, it all comes down to the rider.

Thanks candlkirk, great point. FYI. I'm in the same boat as you in the fact I switched from a YZ 250 to the 426. My email address is I have some questions for you about having a problem hitting hard spots while trying to kick the bike for which I'm curious to know if you've ran into.


One thing I wanted to point before there is any flaming started. With the '00s and '01's some people had problems with noticeable bogs, and fouling of plugs. So this mod would be a neccessity for those conditions. However, either we've just really been fortunate to get good bikes from the factory. Or a combination of these things fixed the problem for '02. (From the yamaha website)

New for 2002:

Revamped carburetor settings deliver crisper, more linear throttle response for optimal engine output.

Remapped digital CD ignition system delivers more precise spark and optimal timing for faster, stronger response during hard acceleration, and less kickback during starting

What I am saying is that instead of spending time with the BK thing, maybe we should be praticing. It is not the bike or the motor, they are fine, it all comes down to the rider. [/b]

I am pretty sure all of us here know that its all about the rider and mainly the desire of that rider.

I ride for fun not money. With that in mind, I also work on my bike for fun not money. If it makes me happy to have a snappier throttle or a well tuned bike, I am going to have it.

Also, a lot of people find working on a bike and getting it to run its best is just as rewarding as winning a moto.



01 YZ426F #85 Vet C

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

Originally posted by kfrosty:

Not exactly rocket science here

What?? uh... this isn't third period, Rocket Science 101??? confused.gif

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") grin.gif

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.


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!!!


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