fouling plugs

99 WR 400 - Why am I fouling plugs. Plug looks black from to rich of mixture. When I took out spark arrestor and filter cover should I have changed carb settings?

I had a 98 WR - and fouled plugs too until I learned to be very careful with the throttle until the bike was fully warmed up. Quick doses of throttle squirts in lots of fuel - and when the bike is cold the plug can't keep itself clean. Blipping the throttle to warm it up would foul every time - just let it idle and then use slow throttle application for a couple of minutes of riding. I also switched to a "7" heat range plug (hotter) and had no more problems. When cold out I even covered one of the radiators over with a piece of cardboard. I really think the lack of a thermostat overcools those bikes when light loaded in cold weather - to the point that the plug can't self clean and fouls. My KTM has a thermostat - zero fouling problems.


What is your jetting? You're too rich on one or more of your circuits.

Bonzai :)

describe your starting procedure as well. choke? for how long? does it race on choke or just chug? does it 8-stroke and when?


Follow these guidelines, and you shouldn't foul a plug. These have been working for me VERY CONSISTENTLY over the last year.

1.) Know the starting routine inside and out.

2.) use choke only for a 2-5 seconds. If it stalls after you disengage the choke, raise the idle slightly during warmup. If it's real cold out, it could take a little more choke time.

3.) DO NOT touch the throttle during the first minute of run time. After that, roll the gas on very slowly to warm up the engine. WFO in a cold engine is asking for trouble.

4.) For first 2-3 mins of ride time, use only a small amount (1/4) throttle. When the clutch cover becomes warm, you're all set.

5.) Turn gas off 1/8 mile before finishing for the day. Turn the bike off when it starts to backfire and sputter (hopefully you'll already have it loaded. Why? During transport, my bike would bounce around. I think the gas was sloshing into the engine. The next time I tried to start it w/o turning off the gas it would foul.

6.) DO NOT start the bike, let it run for a minute, then turn it off! Sometimes you have to do this for maintenance or testing, but try to avoid starting it up at home to hear it run, then shutting it down for the night. If you start it up after washing, run the bike for a good 8-10 minutes.

Do you need to rejet after those mods? Maybe not, but my guess is "yes" on that one. Also, you didn't remove the spark arrestor, just a sound baffle. The SA is internal to the WR silencer itself.

Tell us what if any rideability problems you have (surging, inconsitent idle, bogging, etc.).

BTW, if you foul a plug, just about the only way to get it going again is a new plug or clean the old one with emory board or oven cleaner.

Good luck!

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