your bike is running too rich, try cleaning your air filter, and adjusting the mixture screw (if your bikes carb has one, most moped carbs dont, some do though) if it dosent have one and the air filter is clean, try buying a smaller main jet, also clean out your airbox while your at it, it may need cleaning as well. anyway, its somthing keeping good air flow from reaching your carburator, causing the mixture to be too rich on fuel, not enough air, making the mixture very oiliy, fouling up your plug quickly with a nasty oiliy slick. when its running on the proper mix, your spark plug tip will be dark brown/tan like the color of coffee.
-Garret