BK mod - this time in metric

Ron, This is very cool to have your thoughts confirmed. This definetly helps to have a second opinion.

As for possibly being able to take a larger main. This is probably been the only change that I am making that is outside of James Dean's recommendations. James has been scarily accurate. When I have tried james's recommended settings for my bike they were perfect first time out. As for our DSP exhausts needing larger exhausts, who knows? It sure is one large, meaty pipe running through the frame rails. And maybe the tapered header helps exhaust the gases as well?

The other possibility is that I have drilled out my MAJ and MAJ passageway, so maybe I can take a larger MJ.

But here is what I am thinking. The bike ran great at 8,000 ft when on the 172 main. I could feel the bike load up on the needle EKN#5 = EMN#6 and then when it came on the 172 main, nirvana, so how can a 175 not be better at sea level?

This weekend I am going to race the SRA GP at Carlsbad. 30+ beginner class. All right I am not the fastest guy. So for this weekend I will atleast keep the EKN#5=EMN#6 , 175MJ settings.

But if when I go up to silverwood/ arrowhead 4,000-5,000 the EKN#5=EMN#6 setting is still too rich, then I am going to go to an EKN#4.5 and increase the accelerator pump.

After the BK mod (w/.3 sec squirt)this is what I noticed about the needle settings.

EKN#4=EMN#5 - Bike flat just dies off of idle.

EKN#4.5=EMN#5.5 - It is pretty hard to stall it gasing it right off of idle, but still possible. (maybe curable with more squirt).

EKN#5 = EMN#6 - Throttle snap problem of idle solved. But maybe too rich at 8,000ft.

Being lazy by nature. I want to get a setting that is perfect 0-,6000 ft, so I don't have to do any jetting changes.

Thanks for you input as well.


2000WR - EKN#5, 172MJ, 226MAJ, MAJ Passage drilled to 278, 48PJ, 100PAJ, YZ timing, BK mod, 3/4 turn pilot screw, DSP tapered header and pipe, IMS tank & YZ seat, Scotts dampener & triple clamp & Renthal Fatbar 971, White Bros suspension, Dunlop 756's.

John and Ron,

A few ideas for you to think about from testing done many months ago searching for good throttle snap without bogging.

At Sea level, You can run EKN#6 and completely turn the pump off. The throttle can be snapped open and feels like a husaberg 501. Problem is it runs borderline rich, especially on a long downhill at 1/4-3/8 throttle, a risk of fouling plugs.(I know) rolleyes.gif

The EKN#5 will work with a small amount of accel pump flow and give good quick response. Maybe .3-.5 seconds of pump (~.7mm stroke). Higher elevations are risky again on long downhills or with a lot of throttle turning at low speeds, tight woods for example. I would not take a long adventure away from the truck with EKN#5, or at least carry a spare plug(s).

The EKN#4 will run cleaner over a more broad range of altitudes and temperatures but needs a little more fuel from the pump to get a quick response without bogging. Try backing out the BK screw 1/2-3/4 turn with clip #4 to see if the response comes back. This gets to close to the range I recommend for stock accel pump bikes (EKN#3) but with some adjustment both lower and higher altitudes can run great with the reduced pump flow.

Note that the '01YZ426 needle option EJN will split the clip positions in half. This is 1/2 clip position richer than EKN. (EJN#4=EKN#4.5)

I also suggest the pump delay (top) screw be 3/4-1 1/4 turns to make sure the pump refills adequately. If you're a gear high lugging, the throttle may not get closed. When coming out of a corner the pump can be devoid of fuel with this condition. Always reset the BK screw after adjusting the delay screw, it affects the total stroke.

It may over dramatic to say fine tuning will make the jetting work perfect everywhere, but you have all the adjustments to make the most of it.


James thanks for your comments, they kind of confirm that an EKN#4.5 might be the optimal setting for 0- 5000ft in altitude. I am going to run a race this weekend at sea level with the EKN#5 / 175MJ, and then maybe try to dial in the EKN#4.5 by increasing the squirt until snap comes back.

It is surprising that just a .5 clip adjustment makes a significant difference with regards to throttle response.


2000WR - EKN#5, 172MJ, 226MAJ, MAJ Passage drilled to 278, 48PJ, 100PAJ, YZ timing, BK mod, 3/4 turn pilot screw, DSP tapered header and pipe, IMS tank & YZ seat, Scotts dampener & triple clamp & Renthal Fatbar 971, White Bros suspension, Dunlop 756's.

James and John:

I have been following these posts and was going to do the BK mod on Saturday, but now you've got me rethinking if I want to. I don't want to start fouling plugs. My current setup is good, but it does bog some off of the initial snap and a bigger hit would certianly be welcome.

My current setup is: Needle EKN # 4; Main Jet 168; Pilot 48; Air Jet 100; Fuel Screw .75 turns out; YZ Timed; Air Box Removed; FMF Powerbomb header; Stroker Exhaust; Stock Air Filter; Octopus removed.

I'm in So Cal. It sounds like John and I ride at similar places and altitudes and do some trails and mx.

It sounds like if I do the BK mod, I should leave the current jetting as is but adjust the BK screw? Is this the screw that I will be tapping in. How would I know if I was 1/2 to 3/4 turn out since its a spring. I guess the other option is to try and buy the EJN needle and install. Advise please and Remember- I'm mechanically challeged. Thanks guys. I must say that the original change to the DVP was great, then to the EKN was much better and more fun. So thanks for making my bike run great.


why is it caled the BK MOD. also will this work on a 1998. thanks


If life is not a blur. You aren't going fast enough

Nope, it won't work on the '98. That's not the right carb. It's called BK because that was the username of the guy that told everyone about it. Supposedly he was Tim Ferry's factory yamaha mech. You could probably do a search for his user name.



"What's that?"

"It's a Yellow 99' YZ400!!"

so there is nothing that i can do with the carb for my bike right

No cool secret things like that so far. Just jetting it and that's about it. I'm in the same situation. It seems like the new bikes came with all sorts of hidden power increases for free!!!



"What's that?"

"It's a Yellow 99' YZ400!!"

I've been fiddling around with the pump timing & bk screws and it has helped quite a bit. Also trying mj's & clip positions. At least I'm having fun in the garage - it keeps me out of trouble and I'm not out spending money on stupid junk I don't need. Best of all, I'm not working on someone else's computer systems!

I think I've got it close for low elevations. Will find out for sure tomorrow. After I get it right I will let you know where I ended up.

James & John, you both have been a tremendous help. Thanks again guys for the input.


N 33'41.156" W117'59.749"

HelmetCam Videos

Ron - Thanks for the thought. When you talk about timing you are talking about the screw that is on the other side of the new BK screw. Correct? If so I haven't messed around with this because it seemed like the squirt was exactly where it should be, immediately, but not into the throttle slide.

Harold - when you do the mod, go one clip richer on the needle at the same time.

As for the benefit. I feel like you do. Why fix what isn't broken. I felt like my bike worked so well before the mod that it was an unnecessary improvement. But "In Thumpertalk I trust" So I thought I would give it a try. And there are benefits, so I will spell them out.

The bottom line with the accelerator pump squirt is that it last about 3 whole seconds. This is long time. This creates two problems.

1) With the squirt at 3 seconds this requires a leaner needle setting, so that after the squirt is gone the needle feels a little leans. In other words the squirt masks the correct setting. I only really noticed this on my bike in a long uphill climb at Glen Helen during the SRA GP when the track gets routed up a longer hill than normal. The bike had power starting at the bottom of the hill.. Then 1... 2... 3... , "hey why is the power dropping off". (no more squirt!). The bike now feels meatier in the mid. With the EKN#5.

2) The second area of improvement is the uncontrollability of having gas dumped into the carb. Previously the bike seemed kind of twichy to throttle changes at slow speeds. Either in tight trail riding or when I would get tired towards the end of a moto and hit braking bumps, the bike would feel a little like a bucking bronco. Nothing teribble, and yes it is mostly my fault anyway for being a spaz. BUT THIS IS THE MOST IMPORTANT REASON TO DO THE MOD, in my opinion. More controllable throttle response off idle. Now the bike feels softer or more manageable at small thottle changes during low speed operation, WITHOUT a sacrafice in POWER or SNAP. When I want power it is still there, but now I only get it when I want it, not when I unintentionally twitch the throttle.

Hope that helps.


No I don't think that the EKN#5 is too rich, in fact it is probably just about perfect for sea level. However, I can say is that it is too rich at 8,000ft. I have yet to see if it is too rich at 5,000ft.

As for one-half clip adjustments. Do the following, get a very small washer (I just picked up a #3 washers at Lowe's) and put it under the c-clip to raise the needle 1/2 a notch.

The washer should be the thickness of the groove (in the needle) or the thickness of the distance between grooves, I think both are about equal. This will give your fine tuning ability.

On the #3 washer I used I did have to open up the hole a little with a dremel tool as the needle was a little too big to fit through.


2000WR - EKN#5, 172MJ, 226MAJ, MAJ Passage drilled to 278, 48PJ, 100PAJ, YZ timing, BK mod, 3/4 turn pilot screw, DSP tapered header and pipe, IMS tank & YZ seat, Scotts dampener & triple clamp & Renthal Fatbar 971, White Bros suspension, Dunlop 756's.

Ditto what John has said, similar experiences on long uphills.

Harold, I am a little more conservative on richening the jetting, about 1/2 clip leaner on the needle and a little out on the pilot screw. It may be the difference in exhausts or simply production tolerances because the bikes are the same otherwise.

[This message has been edited by James Dean (edited October 19, 2001).]

Thanks guys. Will probably work on this weekend.


John- From reading your post and James' post, it looks like the EKN #4 is too lean and the #5 is too rich. Did you make any other adjustments- I see that 4.5 but not sure what it means. Have you though about trying that EJN #4?


[This message has been edited by Harold in So Cal (edited October 19, 2001).]

Well, did the BK mod today. What was wierd was that before the mod, my spray time was only .5 to .75 seconds- no where near the 3.5 to 5 seconds that everyone else is talking about. I did the mod anyway and set it at around .5 seconds. Hard to measure it happens so fast. Also droped the EKN from 4 to 5. It ran great around the block. Will update when I hit the dirt and a little more altitude.


Ron, I just read your post in "Jetting Party" results. You really got a EMN#4 (EKN#3) to work? This does surprise me, as we have nearly identical set ups.

I was screwing around yesterday to try to get a EKN#4.5 (EMN#5.5) to work. I just wasn't happy with the throttle response, it still had a slight tendency to die when given quick throotle openings right off idle.

I even tried backing the Squirt out to 2 seconds, and backed the screw out to two full turns, and still not happy with the results.

As for the 175, I like this better. I raced Carlsbad today, and there is that big straight away. So I had the chance to really test it. No drop off when it comes on the main like with the 172.

As for the EKN#5 (EMN#6). I was digging this needle setting. There is a large hill that has quite a few bumps going up. I had a hard time keeping the front end down, the bike has tons of torque on the needle. Wheelies are extreeeemely easy now.

I wonder why you haven't got similar results?


2000WR - EKN#5, 172MJ, 226MAJ, MAJ Passage drilled to 278, 48PJ, 100PAJ, YZ timing, BK mod, 3/4 turn pilot screw, DSP tapered header and pipe, IMS tank & YZ seat, Scotts dampener & triple clamp & Renthal Fatbar 971, White Bros suspension, Dunlop 756's.

I was too fat at #5 with the 175 main. #4 is spot on at sea level. I had to really tune in the ap timing screw AND the pilot screw to get the response to not bog. Now the bog is completely gone. I found the timing screw to be real finicky. I had to go 1/8 turn at a time and reset the pilot screw until I found it. I tried to separate the needle/main jet from the slow circuit, one thing at a time, and found the clip position that worked with the 175 main and then set out to fix the bog. It is amazing how much snap it has now.

How was the GP?


N 33'41.156" W117'59.749"

HelmetCam Videos

The GP was great, cool and overcast most the day. I felt like I finally am getting faster. Standing up through a lot more stuff and rarely sitting down. Next time you go riding let me know. We can swap bikes. It would be fun and interesting.


2000WR - EKN#5, 175MJ, 226MAJ, MAJ Passage drilled to 278, 48PJ, 100PAJ, YZ timing, BK mod, 3/4 turn pilot screw, DSP tapered header and pipe, IMS tank & YZ seat, Scotts dampener & triple clamp & Renthal Fatbar 971, White Bros suspension, Dunlop 756's.

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