BK mod for Accel. pump? What is it? Thanks

10 replies to this topic
  • MotoHero

Posted November 17, 2001 - 01:04 PM


I see you all referencing the mod but I am not familiar with it. My '00 426 runs like mad with a 2-step enrich of the pilot and the main with WB taper headpipe and 12-disk e-series. Does the mod help the initial roll-on?


  • Boit

Posted November 17, 2001 - 01:30 PM


In short, the BK mod is a little adjustment screw that you can install that you adjust to give an accelerator pump fuel squirt duration of ~ .3 seconds. To get the benefit of this modification, you also need to adjust the separate timing screw so that the squirt begins when it JUST clears the throttle valve(slide). Not all 426's need this mod. It's really for those machines that have a bog or hesitation problem. I was able to achieve perfect response by installing a Factory R&D P-38, using VP MR2, and crisp jetting.

  • thumpy

Posted November 17, 2001 - 03:45 PM


Yup, that's what it is. I just did mine today (BK mod). It took about an hour (mostly to figure out how to get the carb out of the right side of the motor, removal of the coil and exhaust coupler fitting were necessary). My "01" 426 didn't have the bogging problem but I tried it anyway (It bogged once in 20 hours of riding). My initial squirt duration was about 2-3 seconds and now it's .3 and she launches! I didn't have to change jetting at all. I'm heading to the track tomorrow for a thorough test session and we'll see how the plug looks as well. It cost me about $10.00 to buy the tap, the tap wrench, the screw, washer, and spring. All were purchased at Ace hardware. It's a very easy mod and the payoff is huge. This little mod will also save my clutch as I was having to feather it to get the rpm's up for a good drive out of the corners. All I can say is this little mod is exactly what everyone said it was, very impressive!

  • mikeolichney

Posted November 18, 2001 - 07:33 AM


If you think its good now wait until you change the main. Brian Kinney recommends a 168, I presume for standard conditions. Unless you were too rich before, you are now too lean. Do yourself a favor, try the next two larger main jet sizes and see if you like it better. You dont need to remove the carb, just the drain bolt on the bottom. There even is a allen head screw for draining the gas from the bowl.

  • thumpy

Posted November 18, 2001 - 02:54 PM


Holy smokes!!!

What a difference in my day. I was at Elsinore today and knocked the hell out of it. That mod made a world of difference in how that bike pulls out of corners. I also noticed starting was vastly improved to virtually problem free. Try this mod fellas IT WORKS!!!!

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  • motoman393

Posted November 18, 2001 - 04:02 PM


I agree w/mikeolichney...mine ran ALOT better with no change in jetting after the BK mod. But when I put a 168 main jet in, it was even faster!

  • thumpy

Posted November 18, 2001 - 07:09 PM


I'll have to give the 168 a try. I didn't change my jetting after the mod, it's at 165 main, 42 pilot, needle in fouth position. The dealer I went to didn't have a 168 in stock. I'll check around tomorrow and see if I can locate one. I can't imagine it working better than it already does but hey, I'll give it a shot....

  • speedharris

Posted November 19, 2001 - 08:27 AM


I'm new to this site and have already fallin in love with it. My friend and I have recently purchased '98 YZF 400 and I'm wondering if this mod will work on the older 400's and where I can get the specifics on doing the mod. Thanks Speedharris

  • Boit

Posted November 19, 2001 - 03:36 PM


Motoman393 has a link to a detailed decription with photos of this mod. Check it out. It's at the bottom of his post about 3 posts above mine.

  • motoman393

Posted November 19, 2001 - 05:43 PM



yes you can do it on the older 400's here is the link to a thread about how it is done: http://www.thumperta...ic&f=3&t=005746

Here is the link to my site which has the pics and procedure http://mx393.tripod.com/carb.html

  • john.hilton@cape.k12.de.u

Posted November 20, 2001 - 02:45 AM


I'm really getting more and more curious about this BK mod.

Thumpy, I also have an 01' YZ 426 that seems to run PERFECT with stock jetting. I don't ever notice it bog. My friend has an 00' that I do notice bog every so often. You're saying that your 01' didn't bog, but since you did the mod it is running stronger off the bottom?

Up till now, I wasn't sure whether or not it would be worth it to do the mod to an 01'. But from what you guys are saying, it seems like it would.


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