BK Mod for the 98/99 exists!! It is called the KL Mod!!

I have been talking to JD and also kept tabs on Clark Mason, who heavily tricked out his 99 WR (and then sold it for an XR650).

I took off the accelerator pump cover, drilled it with a #43 bit, and tapped it with a 4-40 tap. I have installed a 4-40 set screw, locknut and lock washer. This allows me to reduce the stroke of the diaphragm, i.e. the VOLUME of fuel squirted into the throat of the carb. By trimming off the tip of the AP actuating rod, you can alter the timing of the squirt vs. the position of the slide valve. My AP fuel stream DOES NOT hit the slide at all. I need to do the R&D on the set screw to set a reasonable squirt duration. I timed my AP squirt. It was >2 seconds! I am looking at ~ 1/2 second. I ASSUME I'll need to screw w/ the fuel screw/needle to accompany this leaner setting of the AP. JD has recommended fiber washers to seal the set screw against fuel leaks. I will try some teflon tape for sealing the set screw, at least for setting the squirt duration. If this works, I will stick with it for now. I will post everything I encounter. I am working midnight shift (got to LOVE IT!!) and testing maybe slow due to my impaired (more impaired than normal [is that even possible?]) thinking.

Dudes, thesee 99's are on their way to success!! :)

Good show Kevin! Will be doing your mod by the letter over the winter during my yearly tear-down. We will be keeping an eye out as to what you find. :)

[ November 01, 2001: Message edited by: Hugh LePage ]

What about fuel leakage past the screw threads?

Kevin is using teflon tape to seal the threads on the locknut.

I did not use a locknut. Instead, the bolt was cut very short and thin washers were stacked up to set the depth.

This sounds very interesting, I'm looking forward to your results. Do you already have any pictures ?

I have some "personal" pictures of me in my wallet. I ususally send them out to "very close" friends for Christmas!! :)

No, I do not have pictures. I am kind of impressed with the utter simplicity of the mod though.

See my response to Taff's brake question.

I will post everything I find out about this mod!! :D


Can anyone even just post a picture of the part of the carb NH Kevin is talking about? Is it the little diaphram thing on the bottom next to the bowl?

Oh Boy......So kev, has this solved all the issues with your bike? Is that thing finally running 100%?????? Have you been riding??

The "KL" mod is complete and a total success. I have adjusted the "squirt" for ~.75 seconds. The teflon tape on the bolt has provided a completely leak free mod.

I rode the bike for a very short time this afternoon, it is now pitch black outside (5:30pm). Tomorrow I should be able to get more test time.

I would recommend this mod to any 98/99 owners. It is quick, very easy and very cheap. A drill press is recommended for a perfectly perpendicular hole through the AP cover. The 4-40 bolt is a fine pitch, which is THE ONLY pitch I would use. The AP cover is thick enough where this setup, the drill and tap, works perfectly.

Am I correct that the AP is that above that little circular piece next to the bottom of the bowl? If so, you are just drilling and tapping that piece so if you do screw it up you just get to buy that one piece right? Definately let us know how it goes and rides. Also it'll be good to see if it's still leak free after hours of riding and to see if there is any seeping gas around the bolt. Pics would be great, keep the info coming!!!

Sooooo, how does it run? Can you tell a definite difference?


Yes it is the circular cover next to the float bowl. If you haven't taken yours off recently, I'd recommend doing so. The 98/99's are NOTORIOUS for getting crap into the AP diaphragm. Rust and water is VERY common. I do apply a little silicone to the rubber accordian boot that seals up the AP actuating rod, silicone BOTH ends of the boot.


I expected to lose snap off the bottom, why?, but that is not the case at all. I was snapping the throttle VERY rapidly on the road, from full closed to WOT, 1st gear. The front end repeatedly came up. In the past, the bike would wheelie less and less. I would suppose the engine was being overwhelmed with excess fuel...?

I am still working on Taffy's jet recommendations for me. I still have the 220 MAJ installed, Taffster recommends a 180 or <. I am running a 45 Pilot, he wants me to go lower. Our weather yesterday was <45F and dry. The bike was definitely running lean...no doubt. Today it is damp and >50F.

I have to clean the entire house today, ~3000 square feet, excluding the basement, and hope to find time, HOW SAD!!, to test my bike some more.

I will let everyone know. I am on vaca until Monday, 11/19, and SHOULD get all the time in the world to do this. How much time is that, anyways??

Kevin, You da' man!! This is what I have been waiting for. It's a lot to ask but if you could do a step by step with pic's like they did for the BK mod it would be greatly appreciated..


If I had a 98-99 YZ/WR400 I would take some pics for everyone and put them on my site...But I dont I have a 01' 426 (my friend has a 98' YZ400 maybe I can convince him to let me drill a hole in his $800 carb LOL)! If someone can get some pics I will post them on my site (like I did with the BK mod for the 426) and put detailed instructions on there as well!

You wouldnt believe how many emails I get from people asking, "can I do the BK mod for my YZ/WR400" and I have to say I dont know because I havent looked at the setup of the 400's FCR slant carb! This mod Im sure would benefit all the 400 riders, just like it did the 426 guys! So if anyone can get some pics please email me them @ motoman393@hotmail.com and I will post them ASAP! Thanks,


Kevin, how did you come up with this mod? Anyways, I read your instructions and it sounds pretty easy, just like the BK mod. Like all requests for pictures would be greatly appreciated, just to see how the process goes. Hope all the testing goes well and that we 400 owners can reap the results from your Mod.


No picutes, yet, but this is how I did it:

1 - First the AP timing must be set. Ensure you do not have the fuel stream pointed to your face. You want to observe this looking into the airbox side of the carb. The AP nozzle is the small brass nub in the throat of the carb, next to the slide valve. With the carb off the bike, and float bowl filled with fuel, wick the throttle WFO, HOLDING the throttle wide open. You will be observing the fuel stream as it sprays out the AP, and possibly onto the slide valve. It is also a good time to rough guess the length of time the fuel sprays out, just for reference sake.

If your fuel stream DOES hit the slide, you will have to trim the tip of the AP actuation rod. This is the rod that attaches to the carb linkage and goes to the AP via the rubber accordian boot. To trim, I used a felt tip marker to mark a one milimeter mark on the STRAIGHT end of the AP rod, the part that enters the AP itself. I think I trimmed mine a total of 3 mm's. PLEASE, PLEASE, PLEASE don't go and just shave off 3 mm's. It is easier to take off metal than add it. Trim off milimeter by milimeter, ALWAYS testing the timing after each cut to ensure you trim off the right amount.

2 - Prior to removing the AP cover, please note you WILL find some small O-rings when the cover is removed. If you lose either of these O-rings you will have to go in the house, grab 3 Sam Adam's beers and commence to drink them all, IMMEDIATELY!! :)

Remove the AP cover. On the inside, you will see in the center of the cover it is slightly raised. You want to drill your hole in the very center of this. Use a #43 drill bit, and a drill press to ensure a perpendicular hole. Tap it using a 4-40 tap. I used a 1/2" X 4-40 bolt with a lockwasher, washer and locknut. When you are installing the bolt, first screw on the nut, then drop on the lock washer, followed by the washer. You will have to wrap the bolt with a couple wraps of teflon tape. Looking straight down at the bolt, threads pointing up, wrap the teflon tape in a clockwise pattern. When you screw the bolt into the cover, the teflon tape won't be pushed off.

Before you slap this thing back together, be sure you have NO teflon tape hanging loose inside of your AP cover. Murphy's law will have the teflon tape tear off while you are shooting for the 200' QUINTUPLE JUMP. Of course your endo will start 6' after the launch ramp. Maybe if you are lucky, you will correct it, after completing your forward flip, and someone catches it on film!

I started off my testing with approx. 2 mm's of bolt sticking through the cover. Fill the carb with fuel, and do the throttle blipping thing again, timing the squirt. Where to set it? I set mine about 3/4 seconds. Remember the Taffster has removed his AP and is happy. I think my 3/4 second is the max I'd go.

I don't see a reason you can't use a larger diameter bolt, but IT MUST BE a fine pitch. If you go with a course pitch, you may not have enough thread to grip into. My 4-40 bolt works grand, and I had zero problems.

I can answer anything specific at lysdahlbunch@netzero.net or feel free to call, 603-942-7105.

Or, feel free to just post it. Everyone can benefit this way!!


[ November 20, 2001: Message edited by: NH Kevin ]

Credit is earned where credit is due. THe "KL" mod is a selfish, self absorbing thang.

James Dean recommended I try this, so I now declare this the "JK" mod!! :)

Here is another way to do it. I made a small bracket in an U shape out of aluminum. Drilled two holes in the bottom to bolt to the AC pump screws. Another hole in the top for another screw that is used as a stopper for the AC pump arm.

Here are a couple pics. They are pretty bad but you should get the idea. If you want more let me know and Ill get some with the carb off.

<a href=http://www.geocities.com/npuza/BK1.html>BKMods</a>


[ November 13, 2001: Message edited by: npuzajr ]

[ November 13, 2001: Message edited by: npuzajr ]

[ November 13, 2001: Message edited by: npuzajr ]

"KL Mod" is a good name Kevin, you can keep it. :)

Garrett and others,

The pump cover is all that you will be drilling here. If you mess it up or want to have a spare they are only $17.50 from Sudco (P/N 021-057). From Yamaha they are about $30 (P/N 3TJ-14293-40).

I made this modification just before the BK Mod was posted last year on a '00WR400. Then I did the BK Mod also, which is only for the '00 and newer models. They both have the same end results.


(Sudco's number is 1-800-998-3529)

[ November 13, 2001: Message edited by: James Dean ]

When my 01 YZ426 had a bad plug fouling problem, the yamaha dealer did this exact mod to my bike (I guess he didn't know about the BK mod then). If you look at posts about a year old on the YZ side, I know at least one other guy had this done. It did not solve the problem. and a new cover was placed on my bike. The problem was a bad CDI.

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