Everyone with a 01 fouling plugs please read


I just wanted to voice my personal observation. I have not seen one 01 426 that does not have a plug fouling problem of some kind.

Mine will work fine all day with a new plug, then when it cools off it will not start again.

I am going to talk to my dealer today. I am also going to get in touch with Yamaha USA and voice my concern. I would ask thet everyone do the same. Maybe this way Yamaha USA will do something about it.

Once this becomes widely know it will hurt their sales. The CRF450 may start looking pretty good.

I am starting to think the problem is a weak ignition or mapping problem in the CDI. The carburation seens to be on the money in the 01's. Mine runs great with a new plug, Sea level and 70 degrees.

Also, please leave a reply if you are having a problem. That when we talk to Yamaha we can give them number. Elavation and temepture where you ride would be good.

Rick Evans - Sea level, 70 degrees, Houston, Tx



01 YZ426F #85 Vet C

[This message has been edited by forloop (edited 03-17-2001).]

Hey! I bought one of the plug foulers and after about 12 plugs and a frustrating 45 days I returned to the dealer. He began by swithching the CDI with a new bike on the sales floor. No cure. He then replaced the mapping circuit which turned the electrode and center cone white but still had soot aound the outer dia. of plug. I just got mine back with a new carb off the sales "Parts" bike. I am hoping for a brown plug after I ride it. My dealer has been in contact with Yamaha and has been Great! They are Ramsie Waite Co. in Springfield Or. 541-726-7625. Good Luck!

If you have a plug fouling '01YZ426 follow these jetting steps:

1) Move the needle clip 1 position leaner to EJP#3. The stock clip position is on the edge of too rich at #4.

2) Do not turn the pilot screw out past 2 turns. This is NOT an air screw! Turning it out is richer.

3) Never run the bike with the choke on more than 15 seconds. The idle should be set fast enough to turn it off quickly. Don't twist the throttle more than 1-2 times to prime it while starting.

- A smaller main jet will not stop your plug fouling.

- If you follow these guidelines and your carb is clean and working right you can be confident that it is not jetting causing problems.

James Dean

who said that these bikes MUST have a light brown colour to the plug?


if they're running white in the plug it's alright. if they run black around the outside that's no big deal either.

the YZ's suffer many times worse than the WR's so you may find the problem is in the current that the YZ is able to produce.

if the current is low the plug won't run white.

if you then reverse the argument & try to get a tan plug you won't be jetting properly due to the ignition problem.

don't look for a plug colour. pull the exhaust apart & look inside. far more reliable.


hey Forloop (or anyone living in TX)...if you get your 426 to stop fouling plugs by changing to jetting to the above specs, let me know! I guess I got a "good" 426 because i only fouled 2 plugs (the first 2-3 days, trying to learn how to start it) mine came from NCY (North County Yamaha in Escondido, California) maybe thats why? Hope we get this figured out!



I get my kicks on a 2001' YZ426!

Friendswood, TX

Well I ran it again with stock jetting but paying attention to the choke and warm up rules. It lasted an entire ride and still started the next day. So I think that was progress.

I pulled the plug and there was a little sign of tan on the electrode. Not very much mind you. I talk to my local Yamaha dealer yesterday and they suggested the third clip and leaner on the main. Nothing new there.

I did talk to their head tech. He said he always try's to jet around the accelerator pump first. He also checked to see if Yamaha had any service bullitons on the 01's. There were none.

Well I took the carb off, I moved the clip to the third poisiton and changed the main to a 158. I have the fuel screw at 1.5 turns.

Much better. The best way I can describe it is, it feels smoother though the entire RPM range. I could use just a tad richer on the fuel screw. But, it was not bad.

I just cleaned it, and did the 50 MPH blow dry. I am going to pull the plug and see how it looks.

Thanks for the replies.



01 YZ426F #85 Vet C

[This message has been edited by forloop (edited 03-18-2001).]


I just checked the plug and it looked good. It was tan in color. To be exact. The ceramic around the electrode was tan, the threads were black with carbon. In my book this is fine.

I guess I will just have to see how it goes over the next couple weeks. I am much happier now than I was Friday. The bike seemed to warm up quicker this time.

Thanks for all the info!



01 YZ426F #85 Vet C

I'm either lucky or good, and I know I'm not good. I picked up my machine on 2-20, I've ridden about six 2 hour practices and eight motos of competition. The plug that's in the bike today is the one that came in it. I haven't touched the bike other that the idle (slightly higher). I won't touch it either. The bike came from Nebraska and I live in Texas.

Also, I don't blip the throttle at all for starts of any kind. Most of the time my bike starts on the first kick, but I've never kicked in more than three times to start it. Most of the three kickers have been when I was tired.

I have owned several jet skis. My skis are all stock with the exception of a ride plate and impellar. All the guys with modified skis at the lake always have the engine covers off having to monkey with the darn things. My point is if you can leave the machines alone you should. Change oil, clean filters etc. but the machine is set the way it is at the factory for a reason.

There are always exceptions to the rule, but my 2 cents are leave the carbs alone and follow the start drill exactly but don't blip the throttle.

Good luck and I wish everyone the good luck that I've had.

A bunch of us have been through this and if you're fouling plugs it is possible that a jetting change will cure it. That's the normal diagnosis. I tried a wide range of jetting changes on my '01 426 and had it so lean it ran/idled poorly but it had no effect. Also, in my case the engine performance was not consistent during a ride but exhibited a weird 'twilight zone' after a few minutes of running in which the bike would start popping/backfiring and the idle would drop so low that it would quit going into a corner. After just a few minutes the problem go away but would reappear after stopping for a break. No jetting change affected this. A different accel pump diaphram had no effect either.

The cdi from my '00 426 cured the problem as did the replacement cdi that yamaha eventually provided. I've now had the same plug for 3 months and no problems.

I must also be one of the lucky ones. I've had my '00 426 for almost a year, and I've only been through one plug. And that was due to an overall tune-up, not because it fouled. I check it all the time and it's always perfect. My friend however was fouling plugs left and right. It turned out that we was starting his bike with a freshly oiled filter. The bike would suck the oil right into the carb and foul the plug. once he stoped that he hasn't fouled a plug since.

blbainb and I had the same deal. My 99 had a yearly plug change, whether it needed it or not. My 01 would foul a plug in 5-20 minutes. It always started fine, just loaded up and fouled while you were riding. Jetting did not help, I know how to jet. The problem caused me to DNF and ruined this years HS series for me. After a new CDI, there are no problems. All that blipping throttle stuff is not an issue. With the new CDI, the engine is stone cold reliable.

If you get one of the plug foulers, jetting WILL NOT CURE THE PROBLEM!

I was under the impression that all of the people having fouling prob's were not following the procedure.........until now!

I got my 01 426 in November and it always started on the first kick, unless the ambient temp was below 60 deg or so, then I would blip it depending on the temp and after 2 to 3 kicks no problem.

For the first 3 and a half months the thing ran great and I thought all of you were crazy, now it sucks to start.

My wet fouling problem started out of the blue and I have tried everything I know to figure it out.

I am at sea level in Texas so it was jetted correctly to start with, as I said it ran great for 3 months.

I tried to do adjustments in steps to cure the problem....and here they are.


I took the carb off and cleaned and inspected the the entire carb, using 30psi compressed air to clear all my passages.


I tried going richer on the pilot,needle, and main, all separatly....it ran very sluggish.


I tested my CDI unit per the manual and everything was within the parameters.


My stock setting on the fuel screw was 1&1/4

I have tried from 1 to 2&1/2 no good.

After this problem started I noticed while the bike was hot it would hang on high idle going in to turns and I talked to Russel at Thumper Racing and he suggested that since I was doing alot of jumping I should pinch down my float level a bit because the tang was probably sprung.....I did and it seems to have cured that.

Keep in mind I have never fouled a plug while riding all of mine were when the bike was cold on the first kick, instantly you could smell gas and I would pull the plug and put in a new one and restart.

This made me think that the accelerator pump was set to close and it was getting a little gas from the bike shaking on start so I backed it off a half turn and then a full turn....no dice.

It is 65-75 deg down here just like it was when I bought it the only difference is it takes 8-10 kicks to get it to light.

I have checked for air leaks and have found none.

When this started I noticed a distinct dead spot when whacking the throttle, I know they have a little dead spot off idle but it is much worse now. When I'm rolling and hit it it bogs and pops back through the air box.

There has to be a reason for this, when I'm racing its not that big a deal, I would still like to solve this.

Like I said this started overnight

Should I go leaner than stock ?

Is this a valve clearance prob?

My dealer is doing what he can, I hope we can fix this.

I should of asked Tim Ferries mechanic at the races last Saturday. LOL




hey jason...you may want to try what forloop did, by changing the main to 158, screw at 1.5 turns out and needle at 3rd clip! Since we all live in the houston area are jetting should pretty much the same!



I get my kicks on a 2001' YZ426!

Friendswood, TX

Just fouled my first plug after the first day of riding my 01 426. They had some weird brand plug in the bike, I believe it was Demol or Desmol something like that but I never heard of it before. I through an NGK in after cussing at the amount of work it took to replace a plug in the damn thing. Does anyone have any tips on this, I pulled the tank and I can't get an 3 inch extension and the plug socket to come out at the same time, so then I'm stuck with a socket that still on a plug and trying to somehow pull that out of the black hole! So I practice the starting routine following it exactly how everyone says and then I smell the gas and no I'm through. I live in N.E Pennsylvania temp was approx. 45 degrees, anyone have any input that has had this problem in my area. I normally ride a 01 CR 250 and if this is what I'm going to have to go through to change a plug. I couldn't imagine being in a hare scramble and needing to change a plug in the middle of the woods. Come on Yamaha, help us out here!!

Good forum though, I look forward to reading through all the tips and stuff. Keeps me busy while at work!!


I would suggest you try the carb settings I did. They worked pretty well. I no longer use the choke or blip the throttle to start mine. With the CR8E plug the bike can still foul when cold.

I am going to switch to a CR7E plug as soon as I can find one. I have tried a couple places ans they did not have them. I think once I get the CR7E plug in the bike will be in good shape.

My fouling problem was similiar to yours. My bike ran perfect for a month, then it started eating plugs when it was cold.

The problems is the plugs are not doing a good job of self cleanning. Even when you run the bike hard it does not burn enough of the carbon off.

I think a better ignition would do the trick also. I have a suspicion that the 01's have marginal ignitions. I think the CR7E plug will be enough though. Mine is close now.

Good Luck,



01 YZ426F #85 Vet C


Go to Sears and get a 5/8 plug wrench witht he built in swivel. This gives you enough angle to put any extension on it that you wish. Works great.

Wish I had some sage advice for your fouling problem though. I ride a 400 and change the plug once a year whether it needs it or not. With all this fouling with 426's there has to be a hidden issue other than jetting. I actually test rode a 426 last year thinking that I might want to step up from the 400. It was pretty much brand new but ran like crap (acted like it was real rich). The guy who owned it only had taken it out a few times. I decided to keep my 400 after that. Was not impressed.



99'YZ-400 (Mine)

00' TT-R 125 (Hers)

91' KX-125 (Son #1)

93' CR-80 (Son #2)

99' PW-80 (Son #3)


When you back off the accelerator pump screw you are increasing it's flow and pumping more fuel. Turning the screw in is decreasing the flow and delaying the pump. Be sure you go the right way. About 1/2 - 1 turn delay - turning in from squirting at idle.

I think the forloop jetting of clip #3 and pilot screw 1 1/2 - 2 turns is a good idea.

The richer pilot screw setting helps starting to a point. Too rich of a mixture can foul but is less likely to bog. If the pilot is too lean it seems to get fuel from the pump instead as all or none and can also foul part time. The hot start is a good "bailout" if it doesn't cold start in 3-4 kicks.

Your bogging may be a result of the changed pump setting and lean pilot screw mixture. Moving 1 clip leaner (EJP#3) will run cleaner while you ride and make the plug last much longer.

I still think Yamaha went 1 clip too rich on the '01YZ426 (EJP#4) and this summer there will be a constant stream of complaints.


I was afraid that ya'll would think that when I said that I backed off on the acc pump, I meant that I backed off the stroke of the pump by screwing it in.

I screwed it in just to let you know James Dean.

I should have been more clear.

I ordered 2 leaner pilots and 2 leaner mains today........hope it helps.

Got a new D.I.D X-Ring today, any suggestions on a durable steel rear sprocket ?




I have an '01 426 and the only time that I had a problem fouling plugs was after I cleaned my air filter. I think two things caused it:

1) Did not let the filter air dry.

2) Put too much oil on the filter.

I now avoid doing those things above and I have not fouled a plug since. However, I must have fouled three cosecutive plugs before I cleared all the oil out of the head. I hope this helps.

Hey Rick,

How is your new jetting working for you? Does your bike run/start better? I am seriously thinking of rejetting mine (since it is startin' to get HOT again...arrghh) since it runs richer in the summer! I dont think the White Bros R4 pipe I have would really affect jetting (since it runs fine with the stock jetting)

Let me know what ya think! Thanks,



I get my kicks on a 2001' YZ426!

Friendswood, TX

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