400 ex carbon fouled plug

[KDracing]

2003 400ex engine all stock, engine was running rough so changed the plug, now enigne runs great. when i pulled the old plug it was full of carbon, and fit the definition of carbon fouled according the the shop manual, now the shop manual says it could be one of a few things.

1) running rich mixture
2) prolonged idleing
3) faulty ignition control module
4) spark plug heat range too cold
5) clogged air filter
6) poor compression

i am pretty sure it is not #6, engine has plenty of power; #5, almost brand new UNI foam filter; #2, only really idles when warming up, and a minute or two after hard ride

so my question is how can i figure out which one of the remaining options is what caused my fouled plug and how to fix it????

# 1,3,4???

[storms400ex]

they all go bad after awhile i wouldnt worry about it just ride and have fun

[yfzfelix]

if its all stock I doubt its #1, could also be old gas or maybe oil fouled ? does it use any oil, might want to do a compression test and see if its within spec if it keeps fouling the plug, I would buy the correct plug for it, the one Honda recomends and make sure its gapped rite, run fresh fuel and a good grade like super unleaded, not the cheapest gas you can find, like the other guy said, plugs don't last forever, buy a few spares and check your plug before each ride, good luck with it, Steve

[bwamos]

Are the intake and exhaust also stock?
Has the caurberator been rejetted?

My first though would be that you are probably running rich.

Originally posted by storms400ex
they all go bad after awhile i wouldnt worry about it just ride and have fun

yeah after 25 years or so.

[KDracing]

everything engine related is 100% stock (UNI foam filter in stock intake box but i don't really think that counts at an aftermarket intake, just a new filter) carb is stock, never touched the srews. stock exhaust. i run Exxon mid grade, it doesn't burn any oil