Prevent spark plug from fouling?

I have a year old Troy-Built 2600 max psi pressure washer with Honda GVC160 engine. I have replaced spark plug, air filter, oil and fuel. Unfortunately, after about 20-30 mins of run time it quits and will not restart. The spark plug has already accumulated a significant amount of black carbon (or whatever the stuff is) in this short time. It restarts, runs with clean plug but only 20-30 mins.

Could it be the gas?

Dave
September 2013