Jetting conclusions with the BK mod

Well I got around to doing the BK mod on a 01 WR 426 That is YZ timed, open box, baffle out, cliped wire,of course the throttle stop cut aswell!! I spent about 3 hrs last night in a field trying a number of different combinations and this is what I ended up with:

50 FT

78 degrees


162 Main

EKP needle at #4

Tryed position # 2,3,4,5,6 and ended up at #4. Clip #5 felt like more power at 3/4 trottle but made the bike burble at lower throttle positions.

MAJ #200

PAJ #100


2 3/4- 2 7/8 on the fuel screw

tryed a 45P and liked the results. felt like it had more grunt off the bottom but the fuel screw wanted to be at 1 turn out. So if I wanted to ride at up to 3000Ft altitude at this setting I would not be able toajust my fuel screw any leaner. hopefully when the weather cools a bit I can run the 45P with the screw turned out a touch more.

The acc pump was timed to .3 seconds like suggested. I used a 6/32 screw with a spring from a pen. Once timed to .3 sec I put a drop of white out on the top of the screw head so I would be able to come back to the same spot every time.

The pump timing was way off. The spray would hit the slide every time. I adjusted that so it would miss the slide. I had to go back and adjust it even further after a test ride. I checked it and it was still hitting the slide. This made a big difference so don't miss this step. I had to retime the squirt after I adjusted the pump timing. It was way off again.

I adjusted the jetting with the pump off (thanks Hick) and then adjusted the pump spray set screw untill I had the best throttle response which was 1/8 turn out from the .3 seconds

(CAN YOU SAY HUGE F'EN DIFFERENCE) oh sorry got a little carried away!!

Still a little hesitation(I wouldn't call it a bog) if you really whack the crap out of the throttle. To be expected......

The 100PAJ made a big difference compared to the 75PAJ. Thinken I will get the PAJ screw from sudco and open it alittle more.

I think I would like to experiment with the EKN needle. What about the DVP?

Any other needles I should try?

I love to tinker and this is a good challenge. I would like to learn more about the tapers of the needles and how it relates.

This mod is worth the time just to be able to turn off the pump for testing.

Sorry this is so long but I thought I would share it with you guys. My friends think I'm a complete fruit loop (wasteing my time) They want to know what I am running but I am a little reluctant to tell them because of the time I have invested and there complete refusal to do any testing on there own.

Thanks to JD and Hick and everyone else who has shared there info on past posts. It has gotten me where I am at. I can now remove the carb out the side and change needles in under 5 mins.



[This message has been edited by Jason Williams (edited August 29, 2001).]

i hear ya, ive been tinkering with my carb for the past couple of days trying to get the perfect settings all the while having fun and learning much. the bk mod was a huge improvement, but i msy go back and retime the squirt, i never did after i adjusted the timing of the squirt. hey do you know if there is a huge difference between .3 and .8 secs. on the duration of the squirt? i have my needle 1 clip lean from center and a42 stock pilot at 1.5 turns and a 152 main. im not sure of going to a bigger pilot, i live at 8000 ft. what do you think? -blaze

Holy toledo batman, that front wheel never sees the ground at that alt.............

jason if you go to they have a decoder that tell you what each letter means in what posision it is in. i printed one and it has been a big help finding the right stuff.



not yz timmed yet

some mods

Blaze, I didn't go back and time the squirt after I turned my Bk stop screw out that 1/8 of a turn but I know that very small moves on the screw can be noticed pretty easy. If I retime it at 1/8 out I'll let you know what the squirt lenght was.

I would think that you would be running a smaller Pilot at that altitude especially in the summer when the temps are up. Have you tried a 40P? I would think I would foul a plug at that alt. at the summer temps. (have come close a couple af times) started to cut out and had to stop doggin it!! I was running a 75PAJ with the same setup as above and had to have a 42P with 1 1/4 turns out to keep the crispness. but when I climbed up in alt and the filter started to get dirty I would be running way rich on the pilot and would be at 3/4 of a turn or so. (not good) What needle are you running?

John H, This is very true!!!!! 13/50 and great jetting makes this thing point at the sky!! :)

Slashman, I will sheck that out. I can't seem to get enough of this!!!???? I need more I need more I need more!!! oops lost track of reality!!

I think it's funny that all jetting gods on this site figured out all the good set ups and then Yamaha changed to within half a clip of them. after all the testing I did I still ended up really close to the stock settings.

Just ordered a EKN and a DVP needle. I'm going to try these as suggested here at this site and see what kind of power delivery I get. I'll let you know after I test.

Thanks for the ears :D

This is a great mod that you can chalk up and add to the great TT free ( or almost) upgrades.



Try a search using FHN FHP both or individually. (FHN#4-5 = FKN#5-6) Compare FKN#5-6 to EKN#4 and DVP#5.

OBFKN is P/N 5JG-14916-FN

The Golden Egg :)

[This message has been edited by James Dean (edited August 30, 2001).]

Has anyone read my recent post's, both here and on the YZ side? I know nothing compared to James Dean, but I spent many,many,many hours on this. (With J.D.'s help)

posted August 27, 2001 06:15 PM by John in Granite Bay WR Side; "Monty and Friends"



It was great meeting you. Thanks for the offer to ride your 426. It was the first time I've ridden one. Within 20 feet it felt sooo comfy as the front end eased up ever so gently. Sorry about the 4 gear wheely! Your jetting is spot on. Look forward to riding with you again. Maybe up in Tahoe sometime???"

I live at 6300ft,at Lake Tahoe. If I ride downhill, I drown. (Average depth of Tahoe is 1645 FEET!) My 3-4 time weekly ride is from 6300-9000ft, and this is what I jetted my bike for. However, John's post refers to a ride from 2500-4000ft.


Rick Fuller

'01 yz426

'00 yz426 destroyed by fire in desert race 4/22/01

'81 490 Maico

[This message has been edited by ricky1 (edited August 30, 2001).]

Man, you guys have me wanting to tear into my carb. It runs pretty well really but now I'm curious to see what throttle response I can get. Not that I need much more. The 450 is plenty strong as it is.


MX Tuner


If you search ALL the posts refering to the mod I know I have said more than once the BEST BENEFIT of the BK MOD is the infinite adjustability. You can turn the accelerator pump off or you can cause hydraulic lock. I think :)


Rick Fuller

'01 yz426

'00 yz426 destroyed by fire in desert race 4/22/01

'81 490 Maico

[This message has been edited by ricky1 (edited August 30, 2001).]

James, I will order a FKN and see what I think. Thanks for all the help!!! Glad to see your still checking up on us.

Ricky1, I have read your posts. Just thought I'd share my results of what I found with what was suggested here at this site.


i would like to know how all of those other needleds are working out i really want to try them out. i think i will order the #EKP needle and see how it works in my bike. i do not have the pump mod in my bike. how much difference is there really? My front wheel is barely on the ground now. do not know if i really want it or need it.



not yz timmed yet

some mods

Is there an archieved post on what the BK mod is, and does? I've been away for a while. The last mod I did was the gray wire disconnect, and love it.

All I know is that it seems to only work on a 426. If I'm wrong let me know.


2000 WR400F - Thumper Racing Exhaust Insert, Throttle stop shortened, Grey Wire Cut, Filter Cover Removed...

It seems like there are alot of new guys here on thumpertalk, which is great! If you want to find info on the BK mod do a search for "BK", "BK mod" or will find 100+ topics on it! For you guys who are is the link to my website that has the BK mod pics and info! CLICK HERE for the BK mod pics and info! Hope this helps,


This Modification is only for the 00'-02' YZ426, 01'-02' YZ/WR250F and 01'-02' WR426...this modification WILL NOT work on 98-99 YZ400's and 99-00 WR400


I get my kicks on a 426!

Motoman393's MX Site

[This message has been edited by motoman393 (edited September 02, 2001).]

motoman, reminds me of pics of an old Holley 4bbl.

Weather was a little cooler this weekend. around 60-65 degrees ran a 45P and it wanted to be at 1 3/4 turn out ran sweet. any cooler and I might go one clip richer on the needle. This mod is great. seems to get slightly better mileage???? another couple of rides should tell. Just another added benefit!!! Still waiting on the above needles to come in.


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