Maybe check to see if it is fuel or oil fouled. If you rub an oil fouled plug on the palm of your hand it will be like black paint and very hard to rub off. Fuel fouled is much easier to get it to lighten in color when you rub it off your hand. Most likely fuel though. It can be caused by so many things including weak spark or even a bad plug. I would try flip flopping the plug. If the problem stays at home then go to ignition troubleshooting then carbs. I am thinking carbs though and the black cylinder carb in particular. You didn't say how it ran. Did it seem like it missed on one cylinder? It can be kind of hard to even detect if you don't have a good feel for what is normal for these bikes. They can sound fairly smooth with one dead cylinder.