I have never fouled plugs

I just found this site yesterday and am impressed by the by all of the kind people who have replied to questions.

I have a 98 YZ400 and am going to pick up my 01 YZ426 this morning. I have never fouled a plug on my 400. I am a bit concerned after I read all of the posts regarding fouled plugs on the 01 426.

1. What causes the fouling?

2. What is he solution?

I do know that the air filter (when freshly oiled) can cause some difficult starting. I

cannot believe that the 426 would be that much different than my 400.


Let me start by saying I have owen and wrenched on 6 yz400/426's and untill the latest '01 426 I/we had never fouled a plug.

My son picked this last one up in Colorado

and it has fouled plugs and didn't run right

at all untill the needle was changed. It still has fouled plugs but runs ok now.

My borther has an 01 also and has not had

any problems at all.

What I'm trying to say is it's the luck of

of the draw. And hopefully yours will be a

good one. There is no dout there are

some bad ones there, most in Colorado from

what I'v read. Good luck,



Did you have to go to a bigger or smaller needle?


Originally posted by yzernie:


Did you have to go to a bigger or smaller needle?



The needle was leaner but I don't know the

exact size. The shop in Boulder put it in

and they had been working with yamaha. So

with any luck your bike should be fine.


I never fouled a plug in 2 years of riding my 99. Changed the plug once for the fun of it. My 01 would foul a plug every 10 minutes or so, no matter what I did to jet. Since my new CDI, the bike has run perfectly. Starts first kick and never a fouled plug. Yes I am in CO, but I bought the bike mail order from the Great Outdoor store in TN.

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