WR426 Fouling Problem

The Bike:

01 WR426F, PowerNow, P-38 Lightning, Gray Wire Clipped, K/N Air Filter.

The Problem:

Put in a fresh plug and the bike will run fine until I stop for for a long break like lunch. When I restart it will stumble and backfire for a couple miles then run fine until the next long break. Pulling the plug at this point shows light gray. On the second day when first started it will stumble and backfire for the first couple miles and then run great, after a while the plug will foul out and the bike will quit running. Pulling the plug at this point shows black. It will not start again until I replace the plug.

I have jetted, moved the clip position, changed brands of gas and adjusted the pilot screw but have not been able to eliminate his problem. Adding the PowerNow and the P-38 seemed to shorten the overall span of time the plug would last before it fouled fout completly.

Getting pretty frustrated, any suggestions?

Chuck, what jetting are you runing and what's your elevation? Stock airbox and exhaust?

Get a real oiled foam air filter, by the way...


I have the .162 jet and I am at about 7000/ft. I am running the stock airbox with the top cut out. I have the stock exhaust with the restrictor out. Actually i have both types of air filters. I also use a UNI foamf ilter. I have found that the K/N stands up to the dust better when in the desert.

What is so perplexing to me is how well the bike runs initially with the fresh plug and how the problem surfaces after it cools down. Any thoughts?


I have the .162 jet and I am at about 7000/ft.

Hoo boy. You are way too rich, everywhere.

A lot of Denver TTalkers (about 5k ft.) are on a 155 MJ and smaller, 38 Pilot, and have a lean YZ needle like an EKR (Yammy PN: 5JG-14916-ER) on clip 3.

I also heard of a few '01s with mysterious plug fouling problems, but I think yours is jetting related. Simply changing the main jet won't help, you are likely fouling them off the pilot circuit. When the bike is humming it is burning the plug clean, but when you stop and restart the plug is loading up again. That's my theory, anyway.

If you have the stock needle and pilot jet at that altitude you do not know what you are missing.

Hope this helps.

Yeah, Hick is right...

My concern with the K&N filter is that it doesn't stop nearly as much dust and sand particles as your UNI oiled foam filter, which will lead to premature engine wear, especially in the dusty desert.


Thanks for the direction, this has been driving me nuts. My riding buddies are getting tired of the problems as well. I am getting the changes implemented now and will try it out this weekend and get back with the results.

I am getting the changes implemented now and will try it out this weekend and get back with the results.

Thanks for updating your profile and including your location (Taos, NM). That makes it easier to figure out what is happening with your bike: High elevation = unhappy 426 on stock jetting. Take it from someone who has experienced a similar situation, you have no idea what you are missing. Please take the time to get your jetting dialed for that altitude, you could just copy what some of the Denver guys (LarryCO for one) run and your bike will run 100% better, especially at low rpm / low throttle. You may try running a search on this site, WR side for "high altitude jetting" or some variant.

A leaner main and especially pilot should cure the fouling problem, but I would still encourage you to try the needle I suggested (there are probably a few others that TTalk members at or near your altitude would recommend also). If you don't want to go to Albuquerque/Santa Fe for parts North County Yamaha (somewhere near San Diego) has a very good mail-order parts dept. and will most likely have everything you need in stock. 800-225-1629

Hope this helps.


Yeah, I am just up the highway from you a little ways.

I have the 155MJ and the 38 Pilot in but had to order the YZ EKR. This is OK since I have to replace a fork seal as well. Thanks for the tip to LarryCO. I now have his hi altitude setup recommendations. I spent several years in CO before moving here and have ridden Rampart many times. The riding here is much less crowded.


Yeah, I am just up the highway from you a little ways.

I have the 155MJ and the 38 Pilot in but had to order the YZ EKR.

What works at Rampart should work for you in Taos. You may want to try a 40 pilot first, and a 158 main may be better than a 155, depends on how hot it is...

But the EKR on #3 should be a very good starting point, and a big improvement over stock.

I've raced in Española, and ridden in Rampart, so I've got a good idea of what will work...

EKR #3

158 main

38 pilot

That should be a good starting point if you are so inclined. You may also want to do a search on "BK Mod", if you are willing this will also help your throttle response and is a relatively easy modification to perform.

Hope this helps.


I may have seen you at Espanola (Actually, it is Alcalde) since I am there alot. That is primarily where I ride.

I thought the P-38 did the same thing as the BK mod. Is this not the case?

I haven't ridden in Española in a few years, maybe I'll enter the RMEC event there next year, or the NMDRC race...

The BK mod and P-38 accomplish roughly the same thing, but the former is a freebie (almost, you just need to buy a screw and a spring) and infinitely adjustable, whereas the latter is spendy and not adjustable.

I just didn't notice that you already had the P-38 when I recommended the BK...


I believe my "sig" below contains what I found to work best for me when I was living there. If you're not into switching out needles when riding up over 10K ft, I'd just put in the EKQ#2 and call it a done deal (you will start fouling plugs using the EKP over 10K ft for any long duration). Up to you though...as both of these setups worked very well for me (same mods I believe as you have).

Good luck!



Hick pointed me to your profile and I copied the signature. I put the 155MJ & the 38 Pilot in last night but had to order the Needle. I also have the 158MJ as well so will play with it to see what hits. If this works I will be very happy. The interesting thing is I ride with guys who have not made these mods on similar bikes and they do not heve the problems I have. At least not yet.

I am spending most of my riding time between 6 & 8K so I think this setup should work pretty well.

I'm an idiot...sorry, wasted post. :)


Not at all! Better to err on the side of more information. This problem has been driving me nuts. Saturday it died on the trail and I had to hike 4.5 miles over sand, rock and more rattle snakes than I have seen in a long time to get back to the truck. I appreciate all the information I can get at this point. If you have ANY other recommendations, be sure to pass them on.


Here' my problem. Bike ran great first 6-8 months. Then started having sputtering and backfires at midrange that came and went. Would get a bit lean when the weather got colder and do some popping on decel. Otherwise ran great. Usually, when I'd stop and restart, or start for the first time, It'd do it for a short time and clear up. Never really lasted, so just kept rockin.

YZ timed a few weeks ago. Initially seemed a little flat in the mid, but better low end for sure. Then started sputtering mid to full throttle and became mostly unrideable. Usually worse after stopping the bike for a short time.

I cleaned the carb completly and changed plug and ran great for a while. Then Wham! same old s h i t again.

I'm poking around with the ignition currently and trying to dial in the jetting. At least I haven't had the pleasure of 4 miles of pushing/towing. Wonder if it has something to do with the Taos humm that has recently plagued a guy in the foothills near me. LOL

When I fix it I'll let you know. :D:)


Hmmm, sounds familiar. There was not way I was going to push the bike so I parked it under a tree where it was nice and comfortable while I made the trek.

I got the bike in March and had my first problem surface in October so it sounds like we are on the same time line. The problem happens more frequently with each modification that I have made. I am down to getting about three hours of riding before it fouls out completely. I have discussed the possibility of there being an ignition problem but the CDI and Voltage Regulator pass the tests listed in the manual.

I am hoping that Hick is right and this is a jetting prob. I will post the results of this weekends ride.

If its the Taos Hum then I am screwed!

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