On May 22, 2013, about 1050 mountain daylight time, an Allen, Jan A - Challenger II, amateur built, single-engine airplane, N269MM, was substantially damaged during an off-airport forced landing during climb to cruise near Greeley-Weld County (GXY), Greeley, Colorado. The pilot and a pilot-rated passenger were not injured. The airplane was registered to and operated by a private individual. Visual meteorological conditions prevailed at the time of the accident and a flight plan had not been filed for the 14 Code of Federal Regulations Part 91 personal flight. The flight departed from GXY about 1048 and was destined for Erie Municipal Airport (EIK), Erie, Colorado.After engine start, the airplane had been taxied to the runway at idle power. Shortly after takeoff, after climbing to about 1,200 feet above ground level, the pilot reported a complete loss of engine power. He was twice able to get the engine briefly restarted, but each time it ran for only two or three seconds. The pilot performed a downwind off-airport landing to a flat grassy farm field. During the landing roll the airplane impacted a 12 inch berm hidden in the grass which resulted in a collapse of the nose gear, a complete separation of the left main gear, and substantial damage to the lower fuselage. The airplane came to rest upright and the two occupants exited the cockpit unassisted. There was adequate alcohol-free gasoline on-board, and there was adequate oil in the engine oil injection reservoir. The electric fuel boost pump, engine driven fuel pump, fuel lines, fuel filter, carburetors, spark plugs, and other components were examined with no defects noted. The airplane was not equipped with provisions for carburetor heat, nor was it required to be. A postaccident examination showed that the two-cylinder, two-stroke carbureted engine was seized and could not rotate because of a mechanical restriction. There was evidence of vertical scoring on the piston and cylinder wall which appeared to originate near the intake port on cylinder number one. A tear-down examination of the engine confirmed further evidence of vertical scoring on the pistons, damage to the piston rings, and evidence of vertical scoring on the cylinder walls. The closest official weather reporting station was at GXY, about 1 mile north from the accident location. At 1055 the Automated Surface Observation System at GXY reported wind from 110 degrees at 5 knots, visibility 10 miles, sky clear of clouds, temperature 18 degrees Celsius (C), dew point 4 degrees C, and an altimeter setting of 29.96 inches of Mercury. The carburetor icing probability chart from Federal Aviation Administration (FAA) Special Airworthiness Information Bulletin (SAIB): CE-09-35 Carburetor Icing Prevention, June 30, 2009, shows a probability of icing at cruise and glide power and a probability of serious icing at glide power at the temperature and dew point reported at the time of the accident.