Follow Pablo's lead. The oil ratio you are running is fine if that it what your particular oil of choice recommends. The jetting of the carburetor has a profound effect on proper combustion and spark plug fouling. The heat range of the spark plug is important too.
If it is too cold it cannot keep itself clean, if it is too hot it can do damage.
I too ask, can you share a photo of your spark plug, the sparking end of it?
It may be too cold or your carburetor is tuned too rich. The color of the combustion residue left on the spark plug will help determine the next course of action.