I'm In Jetting Hell

I have a 2002 wr 426, I have the grey wire removed from the plug, throttle stop removed and an uncorked exhaust. The bike has a dead spot for a split second when you twist it up quick, and when you slowly roll it on it starts to break up from about 1/2 to 3/4 throttle on up. I currently hav a 165 mj, 50 pj, stock obdrr needle in the 4th clip, air box lid on. Nothing I do seems to have much of an effect on the problem, I have installed Main Jets from stock to 175, now I'm back to stock, I've raised the needle, now I'm back to the stock clip position, and I,ve repeated all of these with the airbox lid on and off. The reason I have the 50 pj in now is because I could start the motor and rev it a couple of times and the header was glowing red right at the head, the larger pilot jet stopped this problem. Help Please!!!

Bigfoot, first off... you need the right main jet. To check it, get the bike nice and warmed up. Once it is, run it wide open in first gear. After you reach the rev limiter then pull in the clutch, kill it, and coast to a stop. Pull the spark plug. White = too lean (need bigger main jet) Black = too rich (need smaller main jet). If you get the main jet problem fixed then you should have good power from 3/4 to full throttle. If your plug is a good color and you still have problems, then it is not your main jet causing the problem.

As far as the glowing headpipe goes, that is completely normal. It is made out of thin metal and heats up very quickly if you stay in one spot. I would take the pilot jet back to stock, and start fidgeting with your fuel screw. Try 1/4 turn at a time either way and see if it helps your low-mid power.

Also, here is an excellent site regarding jetting. It is mainly for 3-strokes, but also applies to thumpers...


Whats a 3 Stroke?

Yes - do NOT jet to get rid of the glowing header. The 50 pilot will be pretty rich down low - even in Arkansas! Do as acarter920 says and go back to stock pilot. THEN work on your main and needle clip position. It sounds like it is rich if it is sputtering...

Thanks for the info guys, I'm gonna give it another try today. I'll let you know what happens.

I reinstalled the 42pj and ran the bike as you described, the plug was kinda furry right at the base, but the tip and the electrode were shiny and clean. Maybe it's fat on the way up and then leans out on the top end? Also maybe I should further explain the circumstances of the problem. When on the bike riding it it has tons of power down low and will easily wheelie in 1st - 3rd, I haven't ridden it flat out yet. The problems I described are when slowly rolling on the throttle while the bike is in neutral.

Did you try the DRR needle in clip #3??


NEVER make any jetting assumptions from the bike being stood still on the stand.

you'll be damned to jetting hell forever. the choke jet is the only thing you should set on the axle stand and that comes last.


As JD said, Lower the needle (raise clip to #3 from the top)!! If it's sputtering at constant throttle, it's more than likely too rich on the needle. One clip position on this carb can be the difference between "what the hell is wrong with this POS?" to "man, this bike is awesome!!" literally.

And as Taffy said...ride it to tune it!! Remove the shroud bolts at the radiator, use just one tank bolt, one seat bolt...this makes it much fast to get to everything.

[ March 18, 2002: Message edited by: Scott in KC ]

Sounds more like jetting club-med. You need to add 3 cylinders, about 50 extra hoses, and vacuum operated throttle valves (packed tigher than a Honda Civic) to be in jetting hell.

Buy a bag of jets and experiment until it works. We're lucky to have the info available on this site. It's pretty rare to have any help at all without paying by the hour.

Good luck

AK, you're right about that! I'm helping a buddy jet a slant FCR on his modified 400ex quad motor. If it wasn't for the info I've learned on this site HE would be lost. Half of the quad sites I searched, looking for baseline jetting info, refer to the pilot fuel screw as "the AIR screw on the bottom of the carb". Two local guys here had NO idea that they could adjust the fuel screw (on guy continued to argue that OUT was leaner) or the actual pilot air screw, let alone change needles to increase performance even more. Talk about getting lucky out of the box!! Seriously, how the hell these guys can get the things to start, let alone run good (not great) is beyond me. The FCR is great because it's so tunable...it can also be a royal pain in the a$$ because it's so tunable. :) I will say there is nothing more rewarding than getting through a jetting problem and coming out with great performance!

I agree AK this site is worth a mint, I don't think I've ever seen so much model specific Info in one place, All of those trial and error tests by taffy and JD (at the expense of their own time, patience and money) should save the rest of us a lot of grief. I called Sudco today and ordered the PAS Kit, several MAJ's, PJ's, and MJ's.

I'll keep you up to date on the progress.

As for doing anything in neutral, HOLD YOUR HORSES!!

You have a wire from your tranny that feeds your ignition. This wire will lower your rev limiter when in neutral.

You can (I did) disconnect this wire.

I'm so close I can smell it. I got my parts today and changed to a 160MAJ, installed the pilot air screw and adjusted to 1 turn, installed a 45 PJ, and Raised the OBDRR needle to the 3rd clip. After testing I had to drop to a 160MJ from a 165.This is not even the same bike that I bought, and is now way faster than I am. Thanks for the input guys. I'll keep you up to date on the fine tuning. :)

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