spark plug problem

2001 426 took my plug out and it was black and full of carbon.I have the BK mod .also if i crack the throttle it sputters or stalls.I replaced the plug but no change.any ideas

Did this problem start out of the blue, or did it start immediately after some chage that you made or some event?

just got the bike is alot of carbon on a plug too rich or too lean

how do I correct eiher

A poor running 4-stroke can have a black plug regardless of being lean or rich. Typically extra carbon means rich but if something else is causing it to run bad then that can also make the plug black, even if it's too lean.

Have you ever seen the bike running properly? If so, is there any way to find out what happened to it between then and now?

If you really don't know anything about the bikes past then probably the best place that I can think to start would be change the gas to be sure it's good, clean out the carb and check the valve clearances and adjust if necessary. If that doesn't help then let us know.

the bike runs great except when you crack the gas it either sputters for a second or stalls.

If it runs great otherwise then try cleaning the carb out first. If that doesn't work then post back here and I'm sure some of the more jetting knowledgeable people can help you play with the slow speed jetting / fuel screw to try to clean it up. If the BK mod was done to it then it may also be worth inspecting the accelerator pump to be sure it's working properly. If that doesn't work then hopefully someone more knowledgeable about these things than me can help.

It sounds like the air screw is off. Off idle, it should be nice and crisp, however, thumpers will cough for a second because of the stroke you catch it on when you hit the throttle.

Some of the 2001 models had faulty CDI units that caused constant plug fouling. The only way I know of to test the unit is to install one from a bike of the same year that doesn't foul plugs.

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