BK Q&A (I'm back)

Hi guys! Sorry I havn't posted in a while but I havn't had a computer hooked up for a long time. Anyways I bought a new one and hooked up to DSL so we'll see how it goes. I also want to apologise to everyone who emailed me in the past, I was only able to answer a few before my system took a crap. That said, I would be happy to help whenever I can.

P.S. I don't claim to know everything but I'll do my best to research your questions.

BK (Brian Kinney)

Factory 426 mechanic

(I must ad that I do not represent Yamaha Motor Corp. USA when I reply to any questions posted on this web site.)

Hi Brian and welcome back!!

Many of us here have done the accel pump mod you suggested. I for one am a VERY happy racer now. I cannot believe the difference the mod has made on my bike. Keep up the great work and we wish you and Timmy the best for the coming season.


Welcome back! I just entered the wonderful world of strokers @8 weeks ago, and love my 426....

The only thing I have not done is the "BK" mod :)

It is next though, I was concentrating on my suspension mods first....

Cya around, and welcome back again!

BK - I also did the mod and I'll never look back bigok.gif This mod is going to severely impact my tyre life laughing.gif

Thanx a bunch


I welcome you to join us on the WR400/426 websight!!

You can bail us out of carb problems on our de-octopussed, YZ timed, airbox de-lidded, throttle stop removed WR's.

Or, just say "Hey". Just because our bikes have headlights, doesn't mean we don't ride hard. Many of us are ex-mxer's as well!

Kevin Lysdahl

Hey Brian,

I just finished replying to your email. You wouldnt believe how many people your mod has helped! Just do a search ok "BK carb" and you will get seriously hundreds of threads! Thanks a bunch,


Hey Brian,

Just out of curiosity. There has been a bunch of hype about the new CR450 stroker being a bunch lighter than the 426. What does Tim's bike weigh with all the trick mods you've set him up with and how much do you think that the 16 or so pounds the Honda is claiming makes for a real advantage?

Thanks and keep Timmy up front!!

Bleed Blue.


Hi Brian!

I was wondering if you've worked with the 250F any? I've heard some talk of using .6 second squirt times for the 250F instead of the .3 used on the 426. Have you any info on this?


Brian,I did the Mod and like it alot!!but,I still have the bog when I wick it on hard . I"ve tried fuel screw adjustments,clip positions so far the best jetting I've found is 165 mj,42 pilot,clip in stock position with pump gas and at Sea Level.Any suggestions? I'm a Senior B rider so I'm not exactly lighting the world on fire!!!

Thanks for The great Outdoor MX this year!!!!

Brian, that mod is unbelievable!!! By far the biggest bang for the buck. I've never done a mod that minor that made such a dramatic differance. For that matter, ANY mod that made that kind of differance. I don't know about everyone else but I did your mod on mine and three others which turned the '01 426 from an already bad to the bone motocrosser to a down right violent (in a good way) ride. Wow, everytime I ride with the other guys I did the mod for, they always get off their bikes hootin'. These are all A-B riders too. I told them I would pass it on the next time you posted.

If you have any other mods like that stuffed up your sleeve let us know. Anything to really blow by those other colors. As if thats hard to do on the 426. By the way, with your mods that CR450 is nothing!!

Anyway, keep Timmy roostin!! We're rootin for you both.


Welcome back and congrats on a great season. Can't wait to see you and Tim whip up on RC and his Honda!

Thanks for the great tips and tricks.

What else ya got up your sleeve? :)

Hi Brian,

I have read articles about wet sumping the 426. Do you know what mods are being made to the oil pump to do this. I can't beleive it's too difficult. Any ideas?


Don Lackey

[ December 04, 2001: Message edited by: Lackey-92 ]

Wow lotsa questions to answer it looks like. I'll make mine quick. I'm riding a 98' YZ400 and I'm thinking about doing the P-38 thing, since I can't do your mod. Some say it works great and others say not at all. My question is, in your experience does the P-38 work for us or not?



Hi BK,

Congrats on a great season. Thanks as well for the helpful tips. I have let folks ride my 426 and they are quite impressed with the difference.

I saw Tim's bike at the Moto show here in Seattle and had a few questions.

1. What is the device attached to the header that leads back to the cases? Is this an exhaust sucker for pressure equalization?

2. Why do you run the axle so far back in the adjustment range (lengthening wheelbase)? Or was this just a result of the gearing/chain setup. Does a longer swingarm improve the suspension action?

I was surprised by how stock the bike actually looked compared to some of the other 2-stroke factory racebikes.

I will understand if you cannot answer in a public forum.



Outta pure curiosity (and I'm not looking for any secrets here) could you tell us about how far ahead of the public you get your hands on the next model year bikes? If not, no biggie, like I said, I was just curious.

Hi Brian, just wondering how well the BK Mod works on a 2002 426. I have not seen any posts on the new model.

Hey BK. Looks like from all the questions, you'll need a holiday real soon. bet you wish you received a royalty check from each BK user!

Thanks to all of you for your support and praise.

Let my try to answer some of your questions.


Timmy's bike weighs around 224-226 lb. We could stand to loose a few more pounds but almost everything is ti already. I'll just have to put Timmy on a diet. Maybe I can drill holes in his boots. That should be worth a 1/4 lb. Haha. The weight advantage of a lighter bike depends on the location of the savings. When the bike "feels" light, it usually has a lower center of gravity. Thats where a ti pipe and carbon fiber airbox can really help to make your 426 feel lighter.


I have tried the accel pump mod on a few 250Fs and I haven't had a lot of luck. I usually end up backing the screw all the way out and just adjusting the timing of the pump. Also the fuel screw is pretty critical on the 250F. If it isn't set right you'll never get it dialed. The 250F likes to be a little on the lean side when idling otherwise it is a pain to start. The Doug Dubach remote hot start works killer to get those bikes lit when hot.

YZ Thump:

If your bike still doesn't respond well enough for you then I'd try lessening the timing and adding a little mor spray. Maybe 1/4 turn in on the timing screw and 1/8 turn out on the pump screw. Give that a try, and make sure your fuel screw isn't too rich. Good luck!


Wet sumping the 426 takes quite a few modifications to the cases and oil pump. I really can't go into detail about how we do it. But I'm sure you could fing someone out there that knows how. Sorry.


I'm not sure wat you are refering to with P-38. You can limit the spray with the old style carbs by adding a little stopper to the lower side of the diaphragm. I used to use a small hollow piece of aluminum tubing that would press on to the little nub of the diaphragm and then shorten it to length on a sander or with a file. It takes a lot of trial and error to get it right but its worth a try. I think adding 2mm might be close. Give it a try and let me know how it works for you.


That device you saw on timmy's bike is just a crankcase vent. The reason for the wheel being so far back in the swingarm is for outdoors the longer chassis has a little more stability. It also lets the bike get a little more wheelspin off the gate and helps keep the front end from coming up on starts. The oposite is true for SX. When you move the wheel all the way forward you shorten the wheelbase and make the bike turn quicker. It also adds traction a little bit for loose starts.


I really can't say, but I do see them sooner than most. Again sorry I can't elaborate but I have gotten a little heat for being a little loose lipped on this site so I have to play my cards right!!

tony t:

I have adjusted 02's that can benefit from the adjuster and some that don't. Not sure why, but I suspect the fuel. Put it this way, if you do the mod and it doesn't help after some experamentation then simply remove the screw! Back to stock in 3 seconds.


You are absolutly right if a had a dollar for everyone that has don this modification I'd have fifty bucks in my pocket!! HaHa. Seriously, I just like to help out when I can.

Thanks again for the words of encouragement and I hope to see y'all at the races. If you get a chance, pop in the pits and say hi!

Take it easy,


[ December 11, 2001: Message edited by: BK ]

Hey Brian,

Thanks for all your wisdom/tips. Hopefully, the powers that be will understand that your presence here can do only good for Yamaha. You're a terrific ambassador for their program and their products. Thanks for giving us a peek into the coveted world of a factory rider. I am very greatful for your tuning tips so I'm gonna pass along a tip for you and timmy, try the Thunder Alley pipe (hell, I'll loan you mine) I'm sure Tim will be pleasantly surprised! :)

Good luck to you and Timmy, you guys will have quite a cheering section as evidenced by all the folks posting here and the sea of blue out at every track in So Cal.

[ December 11, 2001: Message edited by: thumpy ]

What a great website! Where else would we be able to access such awesome info!

BK, I'll be at the first Anaheim race. I'll look to thank you there for all your help and insider info!


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