"David" wrote
I'm not sure, but i think he said it was carbon.

You likely have a cylinder running too rich. First off, are you
leaning for taxi? You should be - and I mean leaning to the point
where you can't advance the throttle to runup power without killing
the engine.

Second - do you have a carbureted engine or an injected one? If
carbureted, not much you can do besides leaning, but if it's injected,
the injectors come in several sizes and you can go to a narrower bore
injector.

Of course it could be lots of other reasons as well. Bad plug, bad
contact in the mag, bad wire - but all those things cost money to fix.
Leaning costs nothing, so you should try that first.

Michael