An old trick we used to do to clear fouled plugs was to put the plug wire on them loosely so the spark had to jump a little to the plug before lighting the plug off. Theory was that it made the plug run hotter and cleaned it up, don't know, but had success with the method a couple of times way back when I was a young man, if the plug wasn't too fouled.

Doesn't cost anything to try it and only takes a two or three minutes to see wether it is going to work or not. Course back then, we used to lightly sprinkle borax down the carb to re-seat the rings on a worn motor too. I'm not recommending that though. LOL!