Small Engines (Lawn Mowers, etc.): fouls out sharp plugs, lawn mover, spark plug

Question
I have a craftsmen riding lawn mover 42" cutting deck w/B&S intex plus 18.5 ohv engine m#31P7770348E1 i bought it 7/05. I have to clean or change the spark plug after each mowing because it fouls out. It also back fires a lot. Help!

Answer
Backfiring typically indicates a lean fuel/air mix but fouling plugs generally indicated a rich mix...this is contradictory.

What color are the spark plug tips when you clean them?  Is the air filter clean?  What brand and spark plug number are you using?  Has the carburetor ever been cleaned or serviced?
Let me know.
Eric