During cruise flight the airplane suffered a loss of engine power. The pilot conducted a forced landing in a field with high vegetation. The airplane nosed over. A subsequent examination of the airplane revealed a leak at the fuel gascolator. The pilot stated that it was his habit to turn off the fuel prior to leaving the cockpit on landing and to turn on the fuel after entering the cockpit in preparation for flight. Leaking fuel at the gascolator is not visible from the cockpit.