BK Mod - Net Jetting Impact - James Dean?

18 replies to this topic
  • LarryCO

Posted October 31, 2001 - 05:08 AM


I finally have time to complete the BK mod and start monkeying around with my jetting this weekend. :) After reading pretty much all of the posts about the mod, it's never been clear to me as to the net impact on one's jetting in terms of pilot/jet needle sizes.

Being here in CO (riding 5K - 10K mostly...40F - 60F now), I was going to start with 45PJ/85PAS,EJQ#3,160/200MAJ...moving from my current 42PJ/75PAJ,DRR#3,155/200MAJ setup...based on recommendations from the crew...and go from there.

As you can see, I'm going one size richer on the jet needle diameter (xxR to xxQ), staying about the same on the jet needle taper start (DRx to EJx), and going up a bit on the main (155 to 160) to try to compensate for the BK mod, but didnt know if that was generally enough.

I'm sure it depends on the where each bike's currently at in regards to squirt duration before/after the mod, but assuming average numbers (say 3 secs to 0.3 sec), can one equate the mod to changes required in jetting?

Please advise oh wise ones...

[ October 31, 2001: Message edited by: LarryCO ]

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  • James_Dean

Posted October 31, 2001 - 12:25 PM



It looks like you have the goods and settings covered. A single step richer clip position is normally a good idea when reducing the pump stroke. However, you are adding some fuel with the needle straight and pilot, so jump in and give it a try as you've described. Work with the pilot screw to get the response and idle working best. Start at about 1 1/2 turns out.


  • LarryCO

Posted October 31, 2001 - 12:51 PM


Would you start at the low end and work your way up...meaning get the idle running well (pilot jet/screw/PAS), then good response off idle (pilot jet/screw/PAS/jet needle straight), then strong running thru midrange (jet needle straight & taper/main), then strong high end (main)?

It seems to me like this is the best way to proceed, but I didnt know if you had better luck dialing everything in by focusing on things in a different order...

Also, I have an EKR and a DRQ needle to play with as well...do you think either of them is a better option to start with instead of the EJQ?

Looking forward to this weekend...I'll of course post results.

Thanks in advance!

  • James_Dean

Posted October 31, 2001 - 09:51 PM


Just use the EJQ needle. Work the pilot screw first, then move the needle clip if necessary. Back to the pilot screw until satisfied. Main jet is far enough from the bottom that it can be checked and adjusted at any point, depends how much WOT riding you do.

  • LarryCO

Posted November 01, 2001 - 04:45 AM


Thanks JD...I'll let you know how things go this weekend...

  • XR4hun

Posted November 01, 2001 - 11:19 AM


Rode Rampart last Sunday after my new jetting. Currently at EJP#3,152main,42pilot,75paj,200maj,throttle stop shaved to 18mm, no BK mod, no Greywire Snip, Stock pipe and Can(uncorked), Airbox Lid gone. Didn't run that great from the bottom parking lot up the powerline BUT as I gained elevation up and around trail 681 or is it 688? (you know....down "ball riser" slick rock) the bike ran more pure through the revs. The decel backfires became non-existent. That's a good sign. Idles fine. Sounds to me like it's a little lean overall right now due to running better at slightly higher elevation. I need to go out to Watkins or something to really see where I'm at. I was just soooooo happy to be out riding again so fair performance was OK for me that day. My bike was running soooooo bad before with a DRR#3 160main,142pilot. But.......a month earlier it was running fine with that jetting at Berthoud, Thundervalley and Watkins. Well....that was at DRR#4. I moved the clip to see if that would fix things but didn't.

Anyway. Sounds like you mostly ride at Rampart mostly. I'm sad that winter is almost here. I'll be looking forward to hearing what your setup is. I was going to take a couple of hours off work and go up to Thundervalley just to run the bike and make some notes but I guess there winter hours are weekends only now. Damn. Can't wait for the weekend.


Watkins finish line jump

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  • LarryCO

Posted November 02, 2001 - 05:52 AM


Yes, I too am a bit sad about the whole pending winter deal...but ya know, Fruita is only a 4 hr drive... Not to mention, snowmobiling up at Rampart would be pretty cool...

I will let you know about my results...I'll be doing the mods at my home in Genesee (about 7200 ft.) so I'm guessing that's pretty comparable to Rampart Range Rd./Powerline trail.

I agree with your assessment on the "overall lean" comment. A couple of points...

1. With a 152 main in there...but using an EJP needle (P being a fairly rich straight diameter), I wonder if the 152 main is restricting fuel flow enough to not really make a difference what straight diameter is used. Perhaps going up on the main would help. I'm going to try a 160 and go from there...

2. On your (and mine currently) pilot system (42 pilot/75 PAJ), are you able to turn the PScrew out to lower your idle using 3 turns or less? I found when using the same setup that I'm unable to pump enough gas through there to lower the idle. According to the "Kouba tool" instructions, you should be able to turn the Pscrew out enough to lower your idle...then turn it in until idle lowers...and the approx. correct setting is 1/2 way between the two. Thus, I've made the decision to go with a larger pilot jet (45PJ/85PAJ). Just curious...


  • Taffy

Posted November 02, 2001 - 10:15 PM



i urge you to try the 160MAJ. have a look at jetting Q's for my latest results.


  • LarryCO

Posted November 02, 2001 - 11:19 AM


Have you done any tests comparing the use of say a 160main/200MAJ vs. 152main/160MAJ? Overall, the effect of more juice/more air vs. less juice/less air?

Other than you Taffy, I havent noticed anyone talking much about using a different MAJ other than the stock 200MAJ...

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  • LarryCO

Posted November 02, 2001 - 11:41 AM


Just read your post in "I'm getting weary" about MAJ usage.

I guess what you're saying is that by keeping the SAME fuel/air mixture at max torque by using a smaller main/MAJ (say 152main/160MAJ vs. 160main/200MAJ), you'll do better on the last few revs after max torque due to the use of the smaller main jet (i.e. smaller hole to suck gas through for those last few revs). I guess conceptually that makes sense to me...just need some time to think on it more.

By chance, do you have a link to that study you're referring to on the Keihn carbs by Patrick Burns? I'd be curious to read up on it...


  • Taffy

Posted November 02, 2001 - 12:29 PM


the patrick burns write up is good. one or two things don't apply to us, such as the MAJ's due IMHO to the fact that patrick dyno's roadracers and they probably run 'F' and 'G' needles.

<A HREF="http://www.lifenet.com/brm/carbkei.htm" TARGET=_blank>http://www.lifenet.com/brm/carbkei.htm[/url]</A>

i'm absolutely useless with these contraptions so if it works it's a bloody miracle. if it doesn't-key it in.

you're right larry but let me have another crack at it.

max torque is 8-9,000revs
max revs is 11,000revs.

let's say that your bike needs a 162 MJ at max torque. but it won't rev out. you go to 172 MJ and it revs out. brill!

two things here. if you reduce the air at max revs it makes the bike rich, rich let's say by 10.

so now we're rich by 10 at max torque AND at max revs, so it's easy, jet down by 10. simple-i hope.

don't forget the MJ controls the needle pretty tightly so you may need to go lift the needle 1/2 a clip.


  • LarryCO

Posted November 05, 2001 - 10:20 PM


Thanks Taffy. I'll go take a look at it...

Quick question for ya though...somewhat unrelated though...

Regarding the pilot circuit, I'm trying a 45PJ/75PAJ combo...and I've found that I cant cold start the bike using the choke...only without the choke or even with the hotstart. Does this mean that my pilot circuit is way too rich? Should I either jet back down (42PJ) or upgrade the PAJ (85 or 100) to be in the correct range for use of the choke?

By the way, one of the four different changes I made over the weekend managed to make my bike absolutely rip...like scary fast! I felt just like the day I first sat on a YZ at the dealer...shortly before putting my new WR on order...tears in the eyes and all! :) Probably mostly due to the BK mod...or even the EJQ needle (from a DRR). I'll post jetting results when I have a couple questions answered (like the one above) and a few more data points.


[ November 05, 2001: Message edited by: LarryCO ]

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  • Taffy

Posted November 05, 2001 - 11:38 AM



it was me that came up with the idea that the MAJ may have to be bigger, it was tested by three others and they all concurred that bigger was the way (230MAJ). i followed because i had the gearbox frag on me. i don't usually do something without back-to-back testing but i did with this. if you read my october 7th test on jetting Q's you're left with no doubt in MY mind.

http://www.thumperta...ic&f=3&t=003434 is the one thread it was all done on.

we're talking about what? $5 for a jet?

now, i don't think you're really reading the posts! because i've said twice in jetting Q's what the PA#/PJ ratio should be.

a 75PAJ is begging for a 40 PJ and PS to 1.5 turns. but we've still got this problem of the needle? i'm telling you that at see level there is no way you woud need bigger than a xxM or xxN needle.

would you describe to me your bikes antics at choke please?


[ November 05, 2001: Message edited by: Taffy ]

  • James_Dean

Posted November 05, 2001 - 11:50 AM



The EJQ needle is what has transformed the power delivery. If you put the DRR back in it will change the whole character of the bike back. I would turn the pilot screw in and test ride, at 3/4 turn or below go back down on the pilot jet.


  • yamahalic

Posted November 06, 2001 - 03:41 AM


James Dean,Why don't you give me a call and lets go riding sometime.I live in Port Orchard. My phone is 253-857-7714

  • Taffy

Posted November 06, 2001 - 04:26 AM


JD's right, the E bit is the bit that made it go!

with a xxQ you are lean on the straight so i would still like your could start symptoms Larry.

don't waste your time with that 45PJ, stick the 42PJ back in. then buy a 40 ready to get it even better because if you're out on a limb on 45 then a 42 will probably not be quite enough and it'll be a 40PJ you need.


  • Stefe9999

Posted November 06, 2001 - 10:03 PM


Interesting, you use a 160MJ and EJQ3 and run US uncorked, and seem to like it. That is almost identical to what I *was* running (EKQ3). It ran ok, but sputtered a lot. Does yours sputter at constant RPM, you sound happy with it?

I am now at 160MJ and at EKQ-3, but now using my Euro exhaust. It has a 1" outlet vs your 1 3/4"! The Euro pipe however, has a larger can on it, so the flow isn't exactly proportional to just the difference in outlet diameter because exhaust gasses now have a larger chamber to fill before they exit.

I did a big ride at Rampart this weekend and just loved it. Only thing I thought is that throttle response off idle was a little mushy, and a just a little less in the low-mid area. Given we are at about the same on the main, needle and MAJ, seems like one of us still has something to gain because you are flowing more air than I am.

If you'd like to ride sometime, let me know.

  • katoom300

Posted November 06, 2001 - 01:33 PM


something you Yamaha guys may be interested in:

I have an 01 KTM 400/EXC, our bikes share the same carb with a few differences, the accelerator pump diaphram on my bike came with a "stop" on the gas side to shorten the stroke, or pump duration, they both share the same part # NOK 055, and are dimentionaly the same with the exception of the stop. this might be a quick fix to shorten the stroke on the Yamahas.Posted Image

[ November 06, 2001: Message edited by: katoom300 ]

  • LarryCO

Posted November 07, 2001 - 09:49 AM


Probably me...regarding the one who could achieve a performance gain. I just put in the EJQ needle and only ran around the neighborhood a little to try it out (enough to piss off my neighbor though!). Before last weekend, I was running my crappy DRR needle...

On that other thread (Jetting ?'s), let me know about any trials with using a 40PJ/75PAJ with that EKQ needle...

Supposedly heading out to Rampart Sunday morning (630AM?). Let me know if you're interested in hooking up out there...



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