One more BK mod question!

12 replies to this topic
  • omnivortex

Posted February 06, 2002 - 06:10 PM


After the BK mod, what are the typical jetting changes that need to be done? right now my bike is running good aside from the quick throttle bog...

  • yzernie

Posted February 08, 2002 - 06:41 AM


The most important change to eliminate the bog is to the pilot jet. Since you have decreased the squirt duration, you'll need to richen the pilot one or two sizes. Fine tune with the fuel screw.

I went one leaner on my needle and one richer on the main. The bike rips!!!


  • Taffy

Posted February 08, 2002 - 09:08 AM


sorry YZ, i can't agree with that!

so if they have a 45PJ they should possibly go to a 50! no way

just do the BK mod, turn the pilot screw all the way in and all the way out trying the bike every 1/2 turn until you feel which is the best position. then report here.


  • andy_b

Posted February 08, 2002 - 12:34 PM


yeh have to agree with taffy on this one did the mod 0.5 secs duration then went up one on the needle clip position and just messedwith the pj screw till it felt right .hope this helps let us know how u got on mate.!

  • yzernie

Posted February 08, 2002 - 08:25 PM


Originally posted by Taffy:
sorry YZ, i can't agree with that!

That's fine, you do what works for you and I'll do what works for me.


  • Greg_in_Atlanta

Posted February 09, 2002 - 06:41 AM


I ended up going LEANER on my pilot jet after the BK Mod..........but, I had previously changed to an EKN needle and my bike was running WAY rich at partial throttle.
I think I have it straightened out now.
Just my 2 cents worth.

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  • SealClubber

Posted February 09, 2002 - 03:02 PM


Just got my bike back together after doing the BK mod and cutting the grey wire.

My original specs were a 42 PJ, 155 MJ, 75 PAJ, 200 MAJ, DVR#4, Screw 2 1/8 turns, the squirt was about 4 - 5 seconds.

I ordered a EKP needle but it hasn't come in yet.

So I moved my needle to #5. Set the squirt for .5 seconds and messed with the screw a bit.

Lots of back firing and stuttering around mid throttle. Also, If I twist the throttle too fast from idle the motor stalls. If I roll the throttle on a bit and then floor it, I can hardly keep the front down. The bike has some hesitation on acceleration and backfires when I let off the throttle. I was testing in my neighborhood so I couldn't go wide open.

I put in a 45 PJ to see if that would help. The bike still stalls when I twist the throttle too fast from idle.

What is causing the bike to stall, too rich or too lean? It didn't do it before the mod. I am going riding tommorrow so I couldn't wait for the new needle to come in.


  • Taffy

Posted February 09, 2002 - 04:08 PM


you've been told so many times it hurts!

lean it off. your bike is to rich,



  • omnivortex

Posted February 10, 2002 - 09:15 AM



when adjusting the pilot screw..the Kouba page says to turn in until idle slows, then turn out until idle slows, counting the number of turns..then turn back 1/2 the number of counted turns and that should be where the screw should be. I did it (havent done the BK mod yet) and seemed to help the bog off the bottom a little bit. What do you think?

  • h_bomb

Posted February 10, 2002 - 01:49 PM


On my 2002 yzf426 I did the bk mod at .4 stk was 3 secs my bike bogged off idle at the track with the bk and I tried a lot of different jetting so I put it back to stock and went with a 45p and a 168m stk was 42p and162m The bike has no bog anymore it rips,I also have the thunder alley pipe.There right for 2002 not all bikes benifit from this mod.

  • SoCalWR426

Posted February 10, 2002 - 03:53 PM


Originally posted by h bomb:
On my 2002 yz There right for 2002 not all bikes benifit from this mod.

SoCal :)

  • Taffy

Posted February 10, 2002 - 11:32 PM



we've all done different mods, we're from different continents etc. you sound to have done the right thing.

if you're happy then leave it! if you aren't you must be brave enough to try other settings. as can be seen by the above posts it's a major headache for some and rather like swimming; you're best heading back for the side of the pool if you're drowning!

all of these bikes would benifit from a lower PJ AND PAJ. when i did my bike it always felt sharper when i went down one PJ size (adjusting the PS out at the same time and then working it back in again). going down in PAJ size made barely any noticeable difference. i know that most riders wouldn't feel the slight vibration at about 2-3,0000 revs.

but going down on PAJ meant that i could yet again go down on the PJ size and it just kept getting better.

i've recently reflected on why most of you haven't been after this improvement, i realised that unless you are in a bog or in tight woods you just don't need to be riding softly at 5mph anywhere.


  • omnivortex

Posted February 14, 2002 - 08:33 PM


thanks for the advice..I will give it a try.


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