On March 20, 2008, about 1430 Pacific daylight time, an Ercoupe 415-D, N3942H, made an off airport forced landing following takeoff from Flabob Airport, Riverside, California. The pilot was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The certificated airline transport pilot was not injured. The airplane sustained substantial damage to the firewall. The personal cross-country flight was en route to Santa Monica, California. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that he fueled the airplane with 12 gallons of gas, sumped the wing tanks, and departed from runway 24. The engine lost power at the end of the runway on climb out. He made a forced landing on the grass infield of the airport. The nose wheel went into a gopher hole, and the airplane nosed forward. Post crash examination established that the firewall sustained damage. The pilot reported that he also found water in the gascolator; he said that it had recently rained. He opined that a more thorough preflight could have prevented the accident.