Jetting with the BK mod...

12 replies to this topic
  • motoman393

Posted August 07, 2001 - 05:37 PM


Hey guys,

Well after putting up pics and info on my site about this "BK mod"...I finally got around to actually doing the mod! I took a quick 2 min ride down the street after I completed it (the ride was quick because I was stopped by a cop 10 houses down the luck right LOL) man this mod really does work I now have instant response like a 250 2 smoke! I only got to about 1/2 throttle and it seemed to carburate clean. But what I was wondering was, what have you guys changed your jetting too after doing the BK mod? Or did you even need to? I live in Houston, TX (sealevel) right now the jetting is stock (except 1 3/4 turns out on fuel screw) and it worked/ran great before I did the mod.

I have researched this topic and saw where James Dean and BK said "if you dont lower the needle clip by 1 (richen it up 1) position you are only doing half the steps"! I have also printed out old posts back from 2-1-01 through 2-19-01 (which was when the BK craze started) LOL! I would really appreciate any feedback you guys have! And if you have any useful pics/tips I would be happy to put them on my site to help others! I will also post my results when I get through "dialing it in"


Garrett Berg

I get my kicks on a 426!
Motoman393's MX Site

[This message has been edited by motoman393 (edited 08-07-2001).]

  • Hick

Posted August 07, 2001 - 07:56 PM


I remember hearing that about the clip position. It is hard to say if that would apply to everybody, it did not work for me, but I may have been a half clip too rich already (I had rejetted before I did the mod).

My advice to you would be to try it a clip richer. Then try two. That is a big handful of difference, and as soon as you feel what that is like you should be able to determine what is best. In my experience the winter/summer weather change is worth a clip position on the needle (but again this may not apply to the rest of the state :) ).

If you still have the stock pilot in there you may also want to try a 45.

Not likely, but I hope this helps.

  • Tom_Higdon

Posted August 08, 2001 - 07:11 AM


Is this mod for the 01', 00' or both?

  • motoman393

Posted August 08, 2001 - 07:43 AM


Thanks Hick, I will try that (I already have a 48 pilot...but it didnt work to well)! Just out of curiousity what are you jetting specs? I know you live in El Paso but still I am curious to know! So you only changed the needle one clip to compensate for summer and winter? Thanks

My site tells what bike/years this mod works on

Any other jetting comments (James Dean, Mikeolichney, or other cyber jetting gods LOL)? Thanks,


I get my kicks on a 426!
Motoman393's MX Site

[This message has been edited by motoman393 (edited 08-08-2001).]

  • mikeolichney

Posted August 08, 2001 - 09:10 PM


Sorry but my spreadsheet doesn't help with mods. It only predicts how to change your brass when Temps and Alts change. You will need to jet it the old fashioned way. Then I think the spreadsheet will work well once you establish your new STP jetting.

I go first for the pilot circuit. I use a tach, set the idle to 1700 to 1900 RPM, and then find the maximum RPM position of the fuel screw. If it is more than three turns, it needs the next size pilot up. If it is less than 3/4 turn, it needs the next size pilot down. In between, leave it at the max setting. It taks some fiddling but I have gotten good at it.

I next do the main. I wouldn't think the BK mod would affect the main jet, since the throttle is wide open pulling alot of fuel and the fuel squirt of the acc pump is probably insignificant. James Dean may have alot more insight into this, especially if he has done the mod himself.

I would just change the needle by seat of the pants. Move in one position either way, and see what runs best in the 1/4 to 3/4 throttle position. I havent done the BK mod (but you guys have me thinking pretty seriously about it, even though my bike has good response).

If you make big changes in the main or needle, go back and set the pilot again. Hope this helps some.

  • jaybird67k

Posted August 08, 2001 - 01:04 PM


I had my clip set at 1 position lean when I first did the Mod, and moved it 2 clip positions rich today(1 position richer than stock)and this made a noticable difference in the bog area.
The only problem is that now when I am running on the street at a constant RPM it seems to burble and surge a bit, so I may have went to far on the clip and may need to move it back to the stock position.
Before I do I am going to play with the acc. pump delay screw to see if this helps.
Right now I am running 160 main,1 position rich on the clip,45 pilot,and 1&3/4 turns on the fuel screw.
I will keep you informed over the next couple of days of my proggress.

# 67

Motoman 393......Its looking like Highlands Saturday if your interested.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • YZThump

Posted August 13, 2001 - 02:09 PM


Hey Mike,I tried to download your spreadsheet with no luck,any chance you could e-mail it to me,thanks in advance!

[This message has been edited by YZThump (edited 08-13-2001).]

  • MGR8

Posted August 22, 2001 - 05:07 AM



How are you liking the BK mod? Have you gotting your jetting straightened out? If so, what did you end up with? I am about to have it done to my 01 426....I am also going to jet it to run VP C12. Are you running race fuel? Let us know. Thanks

  • jamracing

Posted August 22, 2001 - 06:42 AM


"when I am running on the street at a constant RPM it seems to burble and surge a bit"
Surging (at a constant throttle opening) is a telltale sign of running lean, yet a burble is a sign of running rich. That's pretty cool to have both happening at the same time. I would try going one size up on the main, or shim the needle half a clip richer.
Right now my WR426 is running a 45 pilot, 2 1/4 fuel screw, stock needle (and position), and a 175 main. It's also YZ timed. Pro Circuit header and moto-x can installed.
Hope it helps.
Oh yeh, I'm in Florida, which has a total of about 20 feet of elevation changes...

01' WR426
99' YZ400
87' FZR1000
Uh, like twist the throttle...

  • forloop

Posted August 22, 2001 - 09:34 PM


I have not done th mod yet. But, I have been thinking about it. This is the way I am going to approuch it when I get around to doing it. You guys can tell me if I am off base.

In theory the acc pump should not affect the jetting. It is only used to compensate for quick throttle openings. Once you are at the throttle opening it does not come into play.

What I am plaining to do is leave my jetting like it is. It works great at all throttle openings. I am only planning to adjust the accelrator pump to get the best roll on power.

I guess the best way to test would be to have the bike going at low speeds then roll that bike on at a given point and time the bike to a second point. Of course a chasis dyno would be good too.

01 YZ426F #85 Vet C

  • motoman393

Posted August 22, 2001 - 11:51 AM



You are not going to believe how much of a difference this mod makes! I have done the BK mod on my bike and 2 others and they all were WAY out of spec stock (my delay was at 4.2 friends bikes were both around 2 secs, the recommended is .3 secs)

My jetting is still stock (except 1 3/4 turns out on fuel screw) it runs really strong everywhere (the other bikes I did, did not need rejetting either) I used a 6-32 drill and tap and a 1" matching screw (and a spring from a writing pen). One little hint: put a hacksaw blade inbetween the black plastic acc. pump half moon thing and the exit hole of the drill bit (I scuffed mine up a tad when the bit went through)

Let me know if you need any help with the mod! You will understand how the pump works alot better when you take the carb off and time the duration of the spray (at least I did)! The timing of when the acc. pump sprays was also WAY off on my bike (it was spraying gas when the slide was half way up the throttle bore.) It should just barely clear the slide (which can be adjusted by the screw above the one you drill and tap)

Oh yeah my bike also starts 1st kick now after a stall or fall. It used to be a pain in the butt after I fell to get it restarted, now it is a breeze. Hope this helps,


I run 93 octane from the pump. I cant afford race gas since im only 16! I will take donations though LOL :)


I get my kicks on a 426!
Motoman393's MX Site

[This message has been edited by motoman393 (edited 08-22-2001).]

[This message has been edited by motoman393 (edited 08-22-2001).]

  • forloop

Posted August 23, 2001 - 11:11 AM


Hi Garrett,

Cool, I was thinking I would not have to mess with the jetting. I may give it a shot before this weekend. I am riding RIO in the early am on Saturdays. Much cooler that way and its a good track to get into shape.

Just to clarify,

You set the delay so that the acc pump stream just misses the slide.

You set the duration of thee stream to .3 seconds.

01 YZ426F #85 Vet C

[This message has been edited by forloop (edited 08-23-2001).]

  • motoman393

Posted August 23, 2001 - 01:05 PM



Yeah I set the acc. pump spray duration to .3 secs and so it barely hit the slide (wipe off the slide before you test makes it easy to see if it hits it or not.)!

Instead of leaving the carb on the bike and squirting tons of gas into the head (with the subframe off) just take the carb out of the left side of the bike (with the tank and seat off) and run a 2-3 ft hose from the tank to the carb and hold the carb in your hand and adjust the screws accordingly. This method IMO is the easiest! Let me know if you have any more ?'s.

I have to work Saturday or else we would head out to Rio. We are riding Sunday though. Later,


I get my kicks on a 426!
Motoman393's MX Site

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