On July 10, 1993, at 1830 Pacific daylight time, a homebuilt experimental Stephens VP2, N31349, sustained substantial damage during a forced landing in a corn field near Lodi, California. The forced landing was initiated following a loss of power in the takeoff initial climb from the Kingdon airport, Lodi, California. Visual meteorological conditions prevailed at the time and no flight plan was filed for the operation. The aircraft incurred substantial damage. The certificated airline transport pilot, the sole occupant, was not injured. The flight was originating at the time of the mishap as a local area personal flight. The pilot stated that during the takeoff initial climb the engine RPM's dropped from 3,150 to about 2,600. The power level produced by the engine was insufficient to maintain altitude and the pilot landed in a corn field about one-half mile from the airport. The aircraft sustained damage during the on ground collision with corn plants and soft, plowed dirt. The Volkswagen 1800cc engine and related subsystems were taken to an FAA certified repair station for a detailed examination. During disassembly of the Posa carburetor, the main metering valve was found rotated 20 to 25 degrees from it's normal position. A technical representative from Posa stated that while the valve could have been a factor, the factory was aware of cases of much greater valve rotation where the engine continued to produce full power. The repair station noted that the engine had been modified with the installation of a single Slick model 4216 magneto for the ignition system. The magneto was removed from the engine and disassembled. A crack was observed in the coil at a location close to the magneto case. Black ash deposits were noted on both the coil and the magneto housing.