On April 5, 2011, about 1700 Pacific daylight time, an amateur built experimental Zenith 701, N128CF, sustained substantial damage during a forced landing following a partial loss of engine power during takeoff initial climb from a private airstrip near El Centro, California. The private pilot was not injured, and his passenger sustained minor injuries. The airplane was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight. The local flight was originating at the time of the accident. The pilot reported to the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) that during takeoff initial climb to the south, he noticed a partial loss of engine power. The pilot initiated a forced landing to a wet, muddy, wheat field adjacent to his position. During the landing roll, the nose wheel of the airplane submerged within the soft terrain, and the airplane nosed over, which resulted in substantial damage to the vertical stabilizer and left wing root. Examination of the recovered engine by the pilot and a Federal Aviation Administration (FAA) inspector on June 17, 2011, revealed that when the exhaust stack for the number two cylinder was removed, visible damage to the number two cylinder wall and piston was observed. The engine was subsequently removed by the pilot and shipped to Recreational Power Engineering, Tiffin, Ohio, for further inspection. Examination of the three-cylinder Hirth 3701 engine, serial number 898687, by a representative of Recreational Power Engineering under the supervision of a Federal Aviation Administration (FAA) inspector was conducted on October 24, 2011. The inspection revealed that the number two piston exhibited signatures consistent with overheating. The number two piston pin and underside of the piston head were blue in color. The number two cylinder piston rings were seized within their respective ring lands and were worn smooth with the side of the piston, consistent with oil starvation to that cylinder. During disassembly of the engine oil pump system, the number one shaft/bearing assembly for the oil pump system was intact and operational. The number two drive coupling was found damaged, out of alignment, and not functioning. The representative from Recreational Power Engineering stated that the number two oil pump drive coupling would result in the oil supply system partially functioning and that the oil pump system would only supply oil at a sporadic rate below minimal volume and pressure, which would result in oil starvation primarily to the number two cylinder assembly. There was no evidence of a failure of the number two oil supply tube from the pump to the cylinder. The reason for the failure of the number two oil pump drive coupling was not determined. No aircraft maintenance records were located during the investigation. The pilot reported that at the time of the accident, the airplane had 44 hours total time on the engine and airframe.