So basically, when your carb is mixing the fuel and air, you have too much fuel in the mix. The engine can't burn it all cleanly, so you get the black deposits on your spark plug. This can be fixed by either increasing air flow or reducing the amount of fuel that gets mixed in.
If your bike is stock, start by checking the air filter and making sure it isn't dirty/blocked. If your bike is stock and you think a previous owner may have tried to make it go faster, they may have drilled out the jet, which would cause you to get too much fuel. You wouldn't usually be able to tell if the jet had been drilled just by looking at it, so if you suspect that someone may have messed with it, you might just want to get a new one. And while you're looking at the jet, report back with the number (size) stamped on the side.
You could also get a high-flow air filter, which would increase airflow and probably solve the problem. Just be sure to check that it isn't running too lean after you do that.