the amount of gas getting into your engine is regulated by the fuel injectors or the carburetor.
the amount of gas in the tank is irrelevant(unless your on emty) because the amount of gas getting to the engine is controlled and consistent
so the answer is no
NO, overfilling will not foul the spark plug.
I would not think that overfilling the gas tank would cause a fouled plug. The carb controls how much gas gets in. Check the plug to make sure it is indeed fouled. Also make sure you don't have the choke on.
dd
Put a new spark plug in drive to gas station and overfilled my tank then wouldn't start. Tried starting it with the cap off. Nothing.
