BK mod - What am I missing?

I just did the BK mod.

Here are my bike's specs

2000WR, YZ timing, no air box lid, EKN #4 needle, 172 main, 200 MAJ drilled to 226, MAJ passage drilled to 278, 48 pilot, 100 PAJ, 3/4 turns out, DSP tapered pipe and header.

So I made the changes, but when I crack the throttle off idle, it just wants to cough and die. So knowing that things were leaner I opened up the pilot screw to 1.5 turns. Still the bike just dies when I crack the throttle.

So I set things back to original, because my bike worked great before the mod.

So maybe I just don't get it. Do I need to tweek the jetting in some other area to accomodate less gas from the accelerator pump? What is this mod supposed to solve or enhance? Where is the rich condition that less pump stroke helps? And finally what does BK stand for anyway?

Thanks John


"BK" comes from the inventor's initials, Brian Kinney (Tim Ferry's factory mechanic). Do a search of "BK pictures for a link to motoman's website which explains and shows good pics of the mod. As for the problems you are having, I have no clue as I have not run my bike since completing the mod. Hopefully someone here will have some info for you.

Did you try different settings? I’m amazed that anybody would actually prefer stock over a shorter accelerator pump stroke. It wouldn’t surprise me to see bikes or riders who wanted a bit more than most, but am surprised you went ALL the way back to stock.

That is what confuses me about your post, are you saying you tried it once and then took the screw all the way out? Why didn’t you try different settings?

This mod is supposed to improve throttle response, and just about everybody has enjoyed some success with it. The best thing about doing this is the limitless adjustability, a side benefit is you can easily turn the pump all the way off, jet your bike, then turn it back on in small increments until you like the response you are getting.

Jetting the bike with the pump all the way off made selecting the right needle and clip position very obvious in my case.

A lot of guys here ended up a clip position richer after limiting the pump stroke.

Hope this helps.


Yes thanks that does help. I guess I just don't understand what needs to be richened when the accelerator pump is reduced.

Yes I did try about a .3 second squirt, and then backed it out checking it at different intervals, but the bike simply worked best with my previous settings.

If I reduce the duration of the accelerator pump back to the 3. second mark, what do I need to richen up to keep the thing from dying when I crack the throttle open just off idle.

The first item to probably change would be going from EKN #4 to EKN #5, Yes? Any other changes? Then what proceedure do you use to dial it in.

Thanks again,


EKN clip #5 and retry it the same way as before. This was part of BK's original tip that is consistently neglected.

I had the same thing happen and it ended up being the timing was off and it was just hitting the slide. I reajusted and then had to retime the spuirt. worked great after that.

Jason LOL

Thanks everyone,

I made the change to EKN #5 from #4 and reduced the pump to a .3 second squirt. Now it works like a charm.

First I have to say that the previous jetting, provided by all of you on the forum worked great. So I was a little reluctant to fix something that worked great, but you guys have never let me down.

The bike now seems a little stronger in the midrange and transitions to the main better. With the previous jetting I noticed a problem on a long uphill at Glen Helen Motocross track. The bike seemed way down on power in the middle rev ranges, but only after the squirt ran out. The bike would start out stong and then halfway up the hill it was clear that I needed a lower gear. I don't think that I really needed a lower gear, I just think the squirt ran out and the hill kept me from getting to the main jet quickly enough. I think the BK mod will fix this.

The other area where there may be improvement is in the throttle control area. I have noticed that when I get tired and a little sloppy with the throttle that the huge squirt might make the bike a little harder to control when I am in tight stuff or hitting braking bumps and the thottle is just off idle. With a quick drive around the neighborhood I think the control just off idle seemed a little better but no less sharp should I want to get on it.

It could just be that I am a crappy rider and incapable of modulating the throttle just off idle, but it would be a nice to have the bike come off idle a little smoother to accomodate my tired lack of control. I think BK mod will make this happen.

Any thoughts from the rest of you? Thanks again for the help.



Interesting thoughts. I have zero throttle control, I always have a handful of either throttle or brakes. I think this is because of where I ride (sand, dez), and say that knowing I didn’t realize the difference until an extensive ride in the Pine Tree lined trails of Rampart, SW of Denver.

I’ve had the shortened AP (Accelerator Pump) on two different bikes long enough to forget what doing w/out it is like. My friends ’01 YZ wasn’t jetted quite as well as mine, but riding his bike when we were up in CO at Rampart it was obvious that the extra AP delivery was the main factor in the hard-to-control ride. When I grabbed a handful or two things got extra snotty, and this ruined the feedback I was getting to my right wrist. I ended up using a lot more clutch and riding it with the revs up to “ride around” the uncertain feeling at lower throttle openings.

Yeah, a lot of that was the slightly richer jetting but it was obvious that, without a small section where you could spin the motor briefly, it would tend to load up progressively until you did encounter a section where you could bring the revs up a bit (failing clutch use). Once I hopped back on my ’01 I realized how good I had it, the response was much more.....

....crisp, consistent, manageable, perceptible. Or something along those lines, especially noticeable during this “watch out for the tree” type riding. For me, anyway.

Keep in mind this was at high alt., making any rich condition even worse, but to summarize I would agree with what you said about off-idle throttle control. :)


i would have thought that your pilot jet was a larger than required now?

possibly time to try the 45 again or even the 45 with a 75PAJ that would make even more sense.

this is one for both you and hick (or anyone else of course); ride along on a closed throttle in first gear.

1)snap the throttle and what happens?

2) open the throttle slowly and when does the bike jolt forward?

on my bike with EKP clip 4 it is 3mm of travel at the grip (that's the same distance at the slide as well)before i get this jolt.

if you have a chance lift the needle 2 or 3 clips and make no attempt to do anything other than just play at snapping the throttle from that roll-a-long test? within 3 yards it will be too rich and misfire. but it's those first 2 yards?

the front wheel will be like one of those wild mustangs!

one for you hick. i ran my EKP on clip 3 at the weekend and an EMM has been ordered and should arrive today. the EM_ #4 is equal to the EK_ #3 so it's that thinner 'straight' that i hope will set the juices flowing.

let you know. also hick run by me again how come your running such a low PJ PAJ? what is your overall jetting?


[This message has been edited by Taffy (edited September 13, 2001).]

Taffy & Hick,

Thanks for the feedback, it is helpful to have your thoughts confirmed. As for being a little rich down low with a 48/100, I will keep an eye on this. It felt pretty good around the block, but that was only about 3 minutes of riding and only after a real ride will I have a better idea.


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


What are your jetting specs?

There is only one questionable area to my current jetting. It is this. In second gear cruise steady at about 2,000 rpm for a while, roll on the power and about 1 second into the acceleration I get a mild hick up (misfire), for a split second.

With a 48PJ/100PAJ and 3/4 turn, the bike warmed up very quickly, so I thought maybe too rich? Per Taffy's suggestion I tried the 45/75 combo. It ran cleanly but lost a lot of the grunty power that the 48/100 had.

I will probably be able to tell if it gets worse or better as I go up in altitude whether this is s lean condition or a rich condition.

Any thoughts.



[This message has been edited by John in Long Beach (edited September 13, 2001).]

[This message has been edited by John in Long Beach (edited September 13, 2001).]


I had the exact same impression on the 48/100 versus the 45/75 combination. The 48/100 was clearly what I wanted for that extra grunt off the bottom although the 45/75 seemed clean and sort of "street" worthy. Past experience told me the 45/75 would have met with lean stalling in technical low speed stuff.

Not sure about the roll-on hick up. When using a short stroke pump the START of the pump needs adequate delay to ensure a refilling at low throttle. Otherwise the pump has nothing to give (BK mod condition only). I think it was about 1 turn (in to delay) on the stock screw.

Originally posted by crs:


"BK" comes from the inventor's initials, Brian Kinney (Tim Ferry's factory mechanic).

HAHA And this whole time I thought it stood for Burger King.



Darin in Missouri - 1999 WR400F

Enduro Heaven - Ozark Mountain TrailRiders

John & Taffy,

Here is my jetting:

1.5 out on idle mix

40 pilot

Approx. 80 PAJ

162 main

EJP # 3


Modified Accelerator pump, approx. 1 mm of pump stroke at the rod.

Bike is otherwise stock save for one of those new FMF “Q” silencers, this may serve to richen things up slightly, I would think, as it is more restrictive than stock. I replaced the stock silencer and rejetted the bike at the same time.

I don’t think this will help you any John because of where I live/ride.


I live and ride at 4k ft. plus, and current temps are always 90 + F (97 + last month), don’t know what that is in Celsius, but it is hot. Believe it or not but I’ve only ridden a total of three days in my entire life below this elevation (Imperial Sand Dunes, Glamis), and that was over a year ago, so I don’t have any jetting experience at lower altitudes for a basis of comparison, but obviously not too many on this board run this lean. But I have ridden in nearby mountains (Sacramentos) and Rampart way up in Colorado (both areas 8k ft. +) and found the above too rich so I went even leaner with good results.

I also have several friends with blue thumpers at this elevation and higher who have also reported good results with my jetting “tips” (the kind where I go over to their garage and jet their bikes).

I do know that a 40 pilot is better than a 42 in the summer, while the 42 is slightly better in the winter here. I am very confident at this point in my jetting because I went through a lot of trial and error, with invaluable help from this board and the likes of you and JD, on my “old” 2000 model 426. I rode my new ’01 a total of one day before I changed things and was amazed all over again at the improvement over stock, although I’ll say that a stock ’01 is worlds better IMO than the stock ’00.

As for the PAJ, I guess I don’t understand fully the dynamics it operates under. I did read somewhere, probably here, likely from you or JD, that a smaller pilot likes a smaller PAJ. After trying several settings with the adjustable PAJ I bought (again based I believe on a tip from one of you guys) I believe it. I haven’t run this in the winter yet, but with the 40 pilot the bike DOES like a smaller PAJ. It is not a huge difference, but it isn’t all that obscure either. My bike does not pop at all on low throttle openings and high revs, or surge at low openings low revs, like many others do (and like it did stock).

On the other hand I’d rather ride it than jet it so perhaps my findings should be taken with a grain of salt, I’m not the most meticulous guy around (that would be JD).

BTW I also ended up with increased pump delay over stock with the BK mod, but I believe only ¼ turn (definitely not more than ½). I did observe the timing with the carb off the bike initially but then reset it based on riding impression.

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