Still Fouling Plugs???

I have a 98 Wr400 and it all of a sudden started fouling plugs. At least 1 each time I ride. I have performed all the OHM tests outlined in the service manual to no avail. Everything checks good. Once the bike is ran it seems to have a miss while at a constant RPM. Once under acceleration it goes like it should until it fouls the plug. I have cleaned and checked the carb. I am thinking one of the charge coils in the stator is going bad but I hate to take the $180 chance. Any ideas, any known problems with this kind of thing. I am currently using lots of NGK plugs.

Mine was eating plugs until I disconnected the blue wire. Have not fouled a plug since.


A friend of mine had the same problem with 2002 wr426f and he switched to CR9E plugs and he was fine after that.

Air filter? Did you change anything?

check carb vent lines too.

Make sure no one is turning the throttle while it's sitting. Pull the carb and check the float height, clean the carb, accelerator pump and vent hoses. Drain the fuel bowl prior to transporting the bike. Never clean and oil the filter "the night before".

Bill :)

What is the blue wire and what does it do?

The blue wire affects the voltage out of the CDI while the bike is in neutral.

motoman393 has good web page on this under "how to make your YZF start better"

I have not fouled a plug since I unhooked the wire.

If you push the wire out of the connector instead of cutting it you can always hook it back up.

Hope it helps.


If your bike was fine until recently, then something must have changed. Have you tried cleaning out and overhauling the carb? Sometimes the oil from the airfilter builds up in the jet opennings and clogs the works. Might be worth a try to just clean out the carb or have it done by a shop mechanic. It's much cheaper than buying a bunch of plugs or playing "guess the problem" on your bike. I had that problem... changed jets, bought zillions of plugs and scratched the hair off my head, then cleaned the carb. Good luck.

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