02 426 YZF pipe, 168mj 45 pj Runs like a banshee just above 1/4 throttle. Pulls like a mule from the bottom. Hits that 1/4 throttle and gets that lean backpop/misfire. Did plug reading its very lean right there at that throttle position but outside diamater of plug was black. went to 42pj.That was worse. back to 45=better

4th pos on needle=Big time fouled plug

3rd better but still fouled. 2nd was probably best but still lean @ 1/4 throttle. 1st notch=lean back popping at 1/4 throttle. Air screw never seemed to change anything. Needle I'm useing is a OBEKR (?) part # 5JG-14916-ER-00 providing I read the needle correctly. So I now lay at the feet of the jetting God's on this site hopeing they have mercy on me and solve my jetting problems.


Originally posted by SoCalWR426:

Air screw never seemed to change anything.

I would guess you mean fuel screw??? Check the duration and timing of the accel pump to make sure it is correct. If that checks ok, I would try the following. I might try the needle in the 2nd from the top position and try a 48 pilot. If that makes it feel better try the needle in the 3rd from top position.

If I am correct, the 02's do not need the BK mod??


i would try 48 pilot.i live in calif. right now im running 185mj,50pilotjet,dvp needle 4th clip,pilot airscrew 1 1/2out.

[ January 01, 2002: Message edited by: freestyle111 ]

didn'tn read your post, but, wonder why jetting is your nemesis? doesn't take a rocket scientist to screw a jet in and out!

Have to clarify that one. Took the damned carb off for some reason or another, and realized how EZ it is.

[ January 02, 2002: Message edited by: John H ]

I had the same problem dude. Changed to the EKN needle Pos#3 and everything else cleaned itself up.

Bonzai :)

Thanks Guy's,

I Will try both Ernie's and Yamakazi's sugestions. If anybody else has idea's I would like to hear them.

Ernie..Mr BK said 02 does not need BK mod...Heard both good reports and no change reports. If this won't work that will be next. I can alway's go back to stock.


Definitely check the APJ TIMING!!!!

On this forum(?), I read one guy's 02 bike had a APJ duration of 4.5 seconds!!! :)

You'd think after a few blips, HE WOULD RUN OUT OF GAS!!


This is exactly why I wrote the tuning guide. There is nothing more frustrating than wasting time changing the needle all over the place and not being able to solve the problem.

Try needle EKN or EMN or ELN or EJN... they are all the same except for clip position. EMN is from Sudco, the others are from Yamaha.

The last letter is needle straight diameter and mostly affects jetting near 1/8-1/4 throttle. EKR has a pretty lean straight diameter (2.755mm). The CRF450 uses EKR#4, the '00YZ426 used EKR#4. You should have been able to run the EKR#4 and adjusted the pilot screw to compensate to avoid any plug fouling. A slight 1/4 throttle lean condition will still be present though.


James Dean,

Once again Thanks for your help.

I went to both Yamaha Dealers in my area and none of them can get me any of those needles without Yamaha part#!! @*&&%# I couldn't believe it!


EKN is 5JG-14916-EN

ELN is 5NL-14916-EN

EJN is 5JG-14916-JN

EMN is from Sudco (1-800-998-3529) and is made from brass, unlike the others.

These part numbers are listed in the tuning guide & spreadsheet too. :) Tried to make sure it was complete.

Here is my current setup:

* EMN Needle @ #4

* 175 Main Jet

* 48 Pilot Jet

* Pilot Screw 1.75 Turns Out

* 100 Pilot Air Jet

* Accelerator Pump Timing Screw 1.9 Turns Out

* Accelerator Pump Squirt 0.7 seconds (+-)

Hope this helps.


Thanks JD,

Obviously your spread sheet is worth the mere sum of $19.95. It sure would save alot of hair pulling. P.M me your address and I will get $ in the mail...Only sickler though is I have web-tv and no printer. It's a good place for my pocket fisherman to sit when not in use :)

See the web page: www.JDJetting.com

[ August 29, 2002, 09:36 PM: Message edited by: James Dean ]

After reading this article and several others on jetting problems, including my own, I decided to give James' Jetting Guide a try. WOW! James has done an excellent job putting together a valuable resource for anyone who wants to tweak their jetting. In just one morning, I have learned more about my carb then I would have ever imagined!

The Jetting Guide is an excellent resource! Thanks James for your great work.


Thanks Blackie,

It took many hours with late nights and the help of an Excel spreadsheet expert to write it. Both of us are engineers.

You made my day!


Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now