plug fouling

I have a 2001 YZ 426, its 4 weeks old now, i am having problems with plugs fouling out, they wont foul when riding, only when trying to crank it after a few days of sitting, the plug looks black and sooty when i pull it out, anyone have any suggestions???????

Do a search on foul plugs/foul mouth and it should bring up more info than you will want. I had one of these bikes and yamaha knows about the problem,and get to your dealer where you bought it. At four weeks your warr. is about to expire!!

Good Luck

Two things come to mind...

One is the obvious...follow the drill when starting...touching the throttle is BAD.

Two is...when you're done riding...turn off the gas and let it die on its own. This is good for your carb and should ensure that you're not dumping gas on the plug at the last minute.

Otherwise...make that dealer pay...And I agree that you'd better get on it since the extensive 30 day warranty (bogus imho) is almost up.

good luck




Rip the knobbies clean off :)

The above advice is great.

Mine had the same problem. I fixed it by moving the clip to the third position and running 158 main jet. I'm in Houston Texas.

I also run a 50/50 race fuel mix. This helps keep everything clean. BK suggested this. I have not fouled a plug in a long time.

Good Luck,



01 YZ426F #85 Vet C

if the plug is black and sooty, then the carb is too rich, and this will cause the fouled plug. So id advise leaning the carb out some


I have been thinking the exact same thing on a fouled plug, black equals rich, and white equals lean. Especially since I came from riding a Kawasaki 2-smoke. I am also in the midst of a sever plug fouling epidemic, may be due to something besides jetting. But,CO Powersports here in Boulder are adiment that this is a lean mixture, denoting a miss-firing situation. Does anyone out there know for sure which it is.


If it is only fouling on start up, then pull the header and look for a carbon build up in your ehaust port and in the header it self.

I had the same problem with a new 01 426 and jetting did not help, but after explaining my problem to BK(Tim Ferries engine mech)he said this is a classic example of carbon build up which holds moisture in the cylinder causing the plug to short out.

He suggested running race fuel, and since I did I have been on the same plug every since.


# 67

Ok, guys i have called my dealership, and they couldnt help, but they did tell me to go back to the manual and check to make sure that everything is in order when starting, i did this and this may be a problem that i am causing, now someone let me know on this, when i crank it i run the throttle up and down......(old 2 stroke habit) anyways the manual says not to race the engine because of the accerator pump, is this plug fouling my fault because i am racing the engine up and down on start ups?????? thanks all :)

I am not positive what you are doing. But, the 01 426 does not needs any throttle to start it. It may need a little choke but that is it.

I always let my bike warm up a bit before putting the spurs to it. I just let it sit there and idle for about 30 seconds.

If you wick the throttle open when it is cold it can foul the plug.

Do a search on starting and see what you come up with.

Good Luck,



01 YZ426F #85 Vet C

Here's a little history and what I've learned WRT jetting

on the '01 (and 01 ONLY) jetting:

I got mine in December. It ran great on a fresh plug. Would

foul a plug the very next time I tried to start it. I went through

8 plugs in 8 cold weather rides. As the weather warmed

up, the problem went away. This would *seem* to imply that

I had a jetting problem but read on...

About 3 weeks ago I was in Apex Sports in Colorado Springs.

I spoke to Paul Zinke, on of my most trusted mechanic friends.

He stated that the only way they've been able to jet the '01

426's is to put each and every one on an EGA. He said

every one he's seen is different.

That's the sum of what I've experienced and what I've learned.

YMMV. I'm waiting for the cold weather to return to see if

my plug fouling returns.


turn your idle up!

To follow up/confirm what merfman said, I was having the same problems with plug fouling. It took about a month to get in, but Paul at Apex did a fantastic job on my '01, and like Merman said, he used a EGA, he said it took about 2 hours to do. I have had 5 good hard rides on mine since he worked on it and have not experienced any of my past plug fouling problems.

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