BK Go-Go ! Like a Pro Stock Dragster . . .

What year is your 426? I have a 02, and am wondering if it need's to be done on the new one's



Flamed --

What jetting are you running? I live in San Diego and I'm thinking about doing the mod. Thought it would be nice to be close on the jetting when I'm ready.

SoCal, if you do it make sure you record your stock settings. I made the mod, changed jetting and all and couldn't get my bike to run half as good. Check the Beware of BK post for all the info. I have ridden '01's in which those have bragged how much difference it makes and mine ran much better off the showroom floor.

However, if you do make the mod and don't like it, easy to back out unless you change the pilots and adjust the air screw. Even then only takes slightly longer but just remember what you have.

Also, please do the everybody a favor and post your findings if you make the mod. Problems you were having before and the difference you noticed afterwards. Also, any jetting changes you've made to help out others should they decide to do it on the new bikes as well.

Good luck.


There is no need to post info about jetting/success stories on the BK mod! Because if you do a search and actually read the topics you will find tons of info that has already been posted. This mod is like a year old so there have been posts about this subject long before you were a member on TTalk! Here is the search link for all the new members who havent used it before http://www.thumpertalk.com/bike/cgi-bin/ultimatebb.cgi?ubb=search&search_forum=6



I disagree. If someone wants to post jetting/success stories they should. I'm interested in reading them.

Isn't this board for discussion? Don't you think pretty much anything anyone would ever want to talk about has already been talked about? If so, should everyone stop posting right now? Did someone else make this same point in the past? If so, did someone else argue? (I didn't search to check.)

Frank B,

I didnt mean it like that...I was trying to get the point across that the search works great and Im sure not everyone uses it! Alot of the newer members probably dont even know about it (I didnt when I first started reading on TTalk and dirtrider.net, it took me a few months to find it) You can find tons of info on the BK mod on both the WR/YZ426 forums (all the jettings gurus hang out on the WR side LOL, like James Dean, mikeolichney, etc)

Here are my specs after the BK mod (I live @ sea level) 168 main, 42 pilot, middle position on needle, fuel screw @ 1 3/4 turns out, accel pump duration at ~.35 secs, and almost hits the slide. My bike has no hiccup and it runs clean all the way through the rev range. Later,



Glad things are working out for you!! I am still amazed at the before and after mod difference. The half pregnant thing is pretty funny too!!!


Forloop - after the jetting change I reset my duration and delay, I ended up setting my delay so the squirt just misses the slide

SoCal - I have the 01 model, do a search on ' BK mod ' I think there are some guys who ran into problems with the mod on 02 models!

Frank B - I ended up with 165 Main, 45 Pilot and 2 turns out on the pilot screw

Changed my jetting tonight and what can I say, don't think even a Pro Stock will be able to stick with me now! I couldn't stop riding up and down the street; she just pulls clean all the way, with no hint of a bog!

You guys were right, doing the BK mod without changing the jetting is like getting a woman 'half pregnant' .LOL :)

Ernie thanx for the jetting tip, guess what, it's spot on :D , can't believe it my self.

I'm now running: BK mod at .3s, 165 main, 45 pilot, needle clip no. 5 and the fuel screw 2 turns out.

Hey Mike, go for it you wouldn’t be sorry!

Thanx to all you guys who helped me to get the most out of this mod!


Congratulations! I feel the same way about this mod. My enjoyment level since the BK mod has been exponential. In fact, one of my riding buddies who has been a dedicated CR500 rider since the 80's rode my bike prior to the mod and admitted that his initial test ride opinion of the bike was marginal. He has now ridden it after the mod and he now wants a 426. He could not believe the difference. This guy is scary fast on his CR500 so this is a monumental comment and testimonial on his part. Thanks to all who contributed on this mod.



Where did you end up with the delay? Right after slide or 1/2 turn past?


I am totally confused here!

I ride the 01' 426.

What is the bog you guys are talking about?

This BK mod sounds like an improvment and from the testimonials here, you guys have me wanting to try it.....lol....to fix what?

Is there anyone else here that feels like me?

My bike runs like a top fuel dragster guys....crisp throttle and easy to start (unless my kids pump the throttle 35 or so times when I'm at work)...lol.. only happened once!

Should I even attemp this mod?



I'm with you, my bike ran great before. But, I did it anyway because I'm a guy and we like to "improve" things. I guess it's alot like installing an aftermarket pipe. The greatest part is I can easily convince myself that there is an improvement whether there is or not.

(I did it last night in an hour or so.... didn't remove the subframe, just twisted the carb. and went to work with my drill.)

If it doesn't help, I'll back the screw out and it will be just like it was. The great thing about it is, it really is a non-destructive, inexpensive mod.

Frank B,

Will you let me know your results?..I just want someone to describe the difference the BK Mod makes to a bike that already runs great.

:D I might just bust off and do it myself this weekend :D

I'd appreciate it. :D

:D I might just bust off and do it myself this weekend :D

Kirtwell@yahoo.com :):D

hmmmmm now I'm starting to wonder.

My BK mod is at around .4 sec

I ONLY put in the 165 main and fuel screw at 1 3/4 turns and runs great, no hesitation.

I have the powerbomb header.

So now I need to move the needle and pilot screw too?

Frank B,

My current jetting after the BK mod are 168 main, 48 pilot air, stock needle #4 clip, and I will need to check the turns on the fuel screw as I am not sure where this ended up on my last trip but I believe it is about 1 1/4 out. I am sure I have not yet determined the best jetting yet. But since my last Desert trip was with 95+ degree weather @ sea level and recently I was in Gorman with 70 degree weather @ 4k-5k, my adjustments have been limited to the fuel screw via the Kouba Tool only. Life keeps me busy and am unable to ride as often as I would like to so I will keep working on it. Others are having luck with larger mains and changes with the clip also.

Good Luck,


Kirtwell and FrankB,

I too have often wondered about the BK mod. I have a 00'426 and it flat out ripps. I couldn't imagine making it faster. I also have a Yamaha FZR sport bike - so I know what fast is. From what I've read, the BK mod eliminates the bog - I don't have any bog. The only time I experience any sign of a bog is if I take a big hit - if I come up short or go too long and flat land. That's my doing-my fault. Other than that, there's no bog and absolutely no hesitation. This is by far the most potent bike I have ever ridden. I did experience a bog in a friend's 01'426 and he came off mine saying mine ran better. Oh well.

My biggest problem right now is deciding which freakin' pipe I want. I thought mine sounded good until my friend installed his WB R4 slip-on. That thing sounds baaaaaaad asssssss! Believe it or not - with just the R4 slip-on and no jetting changes - the bog is completely gone. :) This completely confuses me - I mean it's just a muffler.



On the advice of Rich Rohrich, I've been experimenting with VP MR2 fuel. While this fuel was aimed at the 2-strokers, it is also an excellent choice for the 426 due to its light-ends. MR2 allows this engine to rev to its redline cleanly like pump gas can't. I had been using C-12 with good results but Rich suggested that MR2 would even step it up further. He was right. It takes some jetting changes to get the most of this fuel but it certainly has been worth it. Not only does this fuel provide very crisp throttle response but it also has allowed me to learn to jet accurately.

What I'm getting to is that to make the most of performance from this machine, consider using the correct fuel. While MR2 is not the definitive fuel for the 426, any good quality race fuel will allow this machine to operate at its maximum design. Along with the correct fuel, jetting crisply is essential. Consider that along with the BK mod, the correct fuel and spot-on jetting, you end up with an engine that responds exactly as you wish with no dead spots or other quirks. You know exactly what will result if you whack the throttle open at ANY given RPM range. Aren't we searching for an answer to make our engine respond to accurately to our right hand?

[ November 15, 2001: Message edited by: Boit ]

Hey I have been reading about the Bk Mod for a month or so. I have a 2000 426 stock. I took off the subframe just to get the original settings. BTW my bike runs absolutely great no bog or other symptoms. Anyway I look down the throat and gas it, nothing happens no shot , no spray , nothing. I checked the gas, etc. Everything was fine just no spray. I called the original owner and he told me he had only changed the pilot and the main and nothing else! I have not checked into the diaghram and I am not sure I want to because it does run so well! I would like to play with the jetting a bit. I was so shocked not getting the spray or squirt I stopped working on it for the moment. I will find out tomorrow what jets are in it and the needle setting. Has anyone heard of this? This bike runs a hell of a lot better than I do. But if it is free I was going to take advantage of it. Thanks All

Went to the track tonight (first time since the mod). Before I went I set the pilot screw two turns out and installed a 168 main jet (from a 162). I didn't change the pilot jet, and I'm not sure what stock is.

Anyhow, something wasn't right. It wasn't running clean and a few times it hesitated when I opened the throttle. I didn't feel like messing with it at the track (can't start your bike in the pits at El Cajon) so I put everything back to stock. Most likely I didn't have the jetting right.... but Kirtwell's idea of leaving a strong bike the hell alone is becoming more appealing.

I'll probably work with it more over the weekend.

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