fouled plug

Does anyone else have a yz426 that fouls plugs constantly???

Do a search on fouled plugs, there were many posts about this problem, luckily I have never had a problem!

Another one!!

What is going on here? I’ve probably heard of at least five or six bikes ‘01s with this problem here and at DRN and two more in the world. The two I know about were not operator error, and older bikes commonly run forever on the same plug. Something is amiss.

I believe they all have defective CDIs, but I don’t yet know what is wrong with the two bikes I know about personally. I’ll post what I know when I find out.

The point is, out of curiosity, I, and others I’m sure, would like to know whatever the problem turns out to be.

Finally, do you ever check the plugs for spark, or are you assuming they are fouled just because the bike runs good when you put a fresh one in there?

Here is an interesting thread on the topic, more or less, from a few months back. I believe in a few cases the fouling went away with a new CDI.

When my bike doesnt start,i do check the plug for spark. Put in a new one and it starts the first kick.My dealerhas mine now and he seems to think it is in the carb(float) but the service manager says the float is not adjustable.

Originally posted by yz426mx:

...the service manager says the float is not adjustable.


You say you check the plug for spark. How do you do it? Just curious...

when i bought my 99 400 used it would to foul plugs everyride, i took it too a dealer and the carb was way out of wack, said it was too rich everywhere and the float was stickin. He cleaned and re jetted it on a dyno for 60 dollars and havent had a problem since.

I had this same problem with mine. The cure was changing the jetting. I went third clip on the needle, 158 main and 1.5 on the fuel screw. Sea level 85 degrees.

BK from this forum emailed me about what was happening. Carbon builds up on the exhaust valves and causes them not to close correctly. This reduces compression and makes it hard to start. Once you get the bike started it will run fine, but carbon builds on the plug and exhaust valves. A new plug always got mine going.

I have not had a problem since jetting my bike. I also have switch to a 50/50 race fuel mix. This did not help HP but, thing are a lot cleaner now.

Good Luck,



01 YZ426F #85 Vet C

[This message has been edited by forloop (edited 06-05-2001).]

ricky1 , one of the local dealers in las vegas had there own, he went ahead and put mine on to see if that was the problem with the foulin

Mat, thanks


Rick Fuller

'01 YZ426

'00 YZ426 destroyed by fire in desert race 4/22/01

'81 490 Maico


Where did you have it dyno tuned?


Rick Fuller

'01 YZ426

'00 YZ426 destroyed by fire in desert race 4/22/01

'81 490 Maico

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