The airplane collided with rugged terrain after a loss of engine power. It struck the north face of 20 degree slope of an east-west ridge, just below the crest. The airplane then slid about 20 ft down the slope where a post-impact fire destroyed the plane. An exam of the engine spark plugs revealed the #1 & #3 cylinder spark plug gaps were bridged with cinder-like deposits with metallic globules starting to fill the firing end cavity & worn center electrodes. According to the spark plug manufacturer, severely fouled plugs will operate colder causing misfires, and will also misfire at higher power because of the conductive nature of the deposits at elevated temps. Bridged gaps will short out the spark plug. Such a malfunctioned plug will misfire at all power settings in a manner similar to a oil fouled plug. Bridged gaps bridged by beaded lead globule will not clear up by engine operation & can be corrected only by replacing the malfunctioning spark plug with a serviceable unit. The acdnt time was estd be at 1530 PST.