'01 426 plug fouling issue: more info

Some will remember that I had problems with plug fouling and odd running behavior with my '01 426. A CDI swap (free from Yamaha) seems to have mostly cured the problem. During the 'discovery' period Yamaha had me try different pilot jets and also a different accelerator pump diaphram. Initially I didn't notice much change from the diaphram but as the weather warmed up the bike developed a very rough off-idle condition. I changed the pilot jet from a 45 back to the stock 42 but no amount of tinkering with the pilot screw could get rid of the problem. In fact, if I gave the throttle just a small blip it was possible to stall the motor. However, when I replaced the 'trial' diaphram with the original the problem mostly went away. The motor still feels a bit rich - 'burbley'- off idle but it still runs pretty well.

Now just yesterday I was talking with one of the folks at the local Yamaha dealership and he mentioned that another new '01 426 had developed a bad plug fouling problem. Yamaha is going to swap the carb because the bike idles even when the pilot screw is turned all the way in. Ah ha! So naturally I went home and tried the same trick on mine and it too idles with zero turns on the pilot screw. This indicates an internal fuel leak in the carb. The leak in my carb is probably smaller as the bike works best when the pilot is 1 1/2 turns out but the problem will probably get worse as the weather warms up.

Has anyone else noticed this on their bikes? If you get a chance try turning the pilot all the way in and see if your bike still idles. I'd like to correlate the condition with those who can't seem to get a tan colored plug (always a little sooty).

Good to hear from a fellow plug fouler. Mine has run great since the new CDI. Plugs are dark brown now, sooty on the threads. I do remember being told that Yamaha had problems with some carb body seals leaking-sounds like possibly a source of fuel to bypass the screw. I imagine that the guys here who have had the carb apart could confirm if this is a possibility. I am on a two week trip but when I get back I will see if mine idles with the screw all the way in. I know my wr250F won't because I was setting up the screw on it the other day and it dies at about 1/2 turn out.

It appears I also have a plug fouling 2001 426. :) The last three times I have been out riding, I have had to change the plug to get it to start. The last time out, it also fouled the plug twice while I was riding, and again after taking a break. The bike was running like crap all day, missing and popping. Plugs are black and some times sooty. This is very frustrating! I had cleaned the air filter and let it sit over night. I never blip the throttle to start it. The bike really likes the hot start. I have been reading all the old posts on this problem. Jetting is stock. My plan is to (change the plug AGAIN) move the needle clip to position #3, and go one smaller on the main. Should I change the pilot jet? one bigger? I haven't moved the pilot screw yet. How do I set the pilot screw to help this problem? No more then 2 turns out, right? I guess I should also completely go through and clean the carb.

I am going to try jetting then look into a new cdi. Did you guys get a new 2001 CDI or a 2000? What if I just get another bad 01 cdi? or should a guy just order a 2000 cdi?

I have talked to the dealer and he was talking about adjusting the accelerator pump. Would this entail changing the diaphragm? He doesn't think it would be the cdi....Hmmm??

I am going to see if the bike idles with the pilot screw turned all the way in. What else should I try?


Racing this weekend. I think I will have to keep a couple plugs ready to go on the line.

I have owned the 98 yzf , and a 00 and have never had this problem. So I am banking on the faulty cdi. I want my 2000 back!! Bad clutch and all.


Try turning the pilot screw in to lean the jetting and check the response. Second choice would be 1 clip position leaner on the needle (#3). This will be close to a '00YZ426 carb needle setting. Also, did you check the TPS resistance against the spec?


I'm not sure if this is the answer, but perhaps it will help.

I have found the start and idle circuits will leak between the two upper halves of the carb.

This is basically the result of the lower section bowing from the outer force of the screws. In other words, not enough center pressure.

The use of too much sealant or uneven application of the sealant can also cause or contribute to this.

The fix seems to be, and perhaps this is how the good carbs are manufactured, is to apply a thin and even film of sealant on the gaskets (there's 7 of them), then tightly clamp the two halves together where all the force is applied at the center of the carb. You'll need to make a fitting that goes up into what is the top of the float bowl usd together with a large clamp - like a wood clamp, (ViceGrips makes some cool ones).

This seems to flatten out and seal the middle section before the four screws are put into place, thereby maintaining a more even center pressure.

If you simply place the two halves together and tighten the screws, the middle portion of the lower section has a tendency to bow away from the upper section. This results in not enough center pressure, which causes the start and idle circuits to leak.

It’s obvious that Yamaha does not want these two halves serviced based on the manual and the filling of the screw heads. This perhaps indicates that the manufacturing process is a bit more complicated than a simple join and bolt.



I had the same problem with mine. I'm sure you read my posts. So just a update.

My bike continues to run great after the jetting changes. stock pilot, 42 1.6 turns. 3'rd clip and 158 main. I am at sea level 80 degrees. My bike does not ping on pump gas.

I also use the choke very sparingly. If I have not started my bike in a fews days and it will not start with the choke off. I will pull choke, then as soon as it fires I turn the choke off. A high idle helps things. If I start the bike once a day it does not need the choke at all.

Good Luck,



01 YZ426F #85 Vet C

[This message has been edited by forloop (edited 04-18-2001).]

Oh no....What have I done???

Last night I toke the carb apart and moved the needle to clip #3, checked the pilot screw it was set at 1 1/2 turns out. Put it back together and now I can't get it to start. Tried a new plug, no go. It would fire 2 or 3 seconds after kicking then die. I was able to get it to start, by using the hot start and the choke. The second I closed either one (choke or hot) of them it died instantly. The plug looked good not wet and tan in color. This morning I tried to start again, but no thing. What did I do? I am going to take the carb apart again and check the float height, and the TPS. Needle has nothing to do with starting, so what is going on? To me if it need the hot start to start then it is too rich. Will turning in the pilot screw help starting? I know how to start these things, some thing is wrong.

What did I do? Thanks for the advice

Did you turn the air screw in and forget to turn it back out?

Another fun fun night of tinkering. I wish I forgot to screw the pilot screw back out.

I have been able to get the bike to run with the hot start and the choke. I have to give it allot of gas to be able to close the hot button and choke, then it runs like crap. Once you let go of the gas it dies instandly.

What does it mean when the choke and hot start have to be out? I checked the float level and it is bang on 8mm. I went through the whole carb, all the jets are clean. I have put the jetting back to stock. Same thing, I don't know what has changed. Any more ideas? I think it is time to bring the bike back to my dealer. I don't know what else to try. Looks like no racing for me. :)

Are you sure you put the slide back together correctly? It's pretty easy(if you aren't paying attention) to put the slide plate upside down. It will still fit together and you can reassemble the slide assembly back into the carb. Make sure the cutaway is at the bottom and not the top. I discovered this when I was cleaning my carb and was waiting for some parts to soak and was playing around with the slide plate. Just for giggles, I checked to see if the plate could accidently be flipped uspide down and still fit. It did. Someone who is not familiar with the intricasies of this carb could put it back together with the cutaway on top. Where the arc of the cutaway should be, you would end up with a straight edge and a small gap for the air to pass. It occured to me that if some did this, they would try to start an engine that would run very oddly...if it ran at all.

Boit, you know that makes alot of sense. When I was moving the clip the plate fell on the bench. When I put it back together, I just put it back not thinking. Now that I think about it, I believe the cutout IS at the top. That has to be it, cause the bike ran before I toke the carb apart. Now I want to go home right NOW and fix it. Only eight more hours of work to go, Woohoo I am pumped now. Thanks alot I will let you know.

This may be stupid, but did you reset your idle after taking the carb apart?

Thanks Boit, I did have the plate upsize down. Bike runs good now. :) Amazing how that little cut away makes such a difference. I moved the clip back to position #3. Bike ran all last night with the same plug. Stock pilot screw is at 1 1/2, should I move it to 1 1/4?

Thanks again.

Great call Boit!

Who knows how long this puzzle would have taken without your observations. Nice post. :):D:D


Thanks for the kudos. Glad I could help... :)

Bike ran great all weekend. I don't use the choke anymore.

Since that slide plate isn't held on by anthing other than the carb bore, I think I might apply a little bit of Yamabond the next time I have the carb apart for cleaning. Keihin could have machined the plate such that it would have an oval shaped ridge instead of a perfectly round one. That way, it would be impossible to reinstall it upside down. Just like you, when I first disassembled the carb, the plate fell out in my hand and caught me by surprise. Luckily, I had some time to examine it and see exactly how it fits. I hope others read this thread and remember this little bit of information. When I first suggested this to you, I felt like I was going on on a limb. Like James dean commented, it could have been a very frustrating puzzle to solve.

[This message has been edited by Boit (edited 04-25-2001).]

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