On September 5, 1994, about 1226 central daylight time, a homebuilt Varieze airplane, N54WB, sustained substantial damage following a loss of engine power and subsequent forced landing on a muddy field near Millard, Nebraska. The commercial pilot received serious injury, the sole passenger aboard was not injured. The local, 14 CFR Part 91 flight operated in visual meteorological conditions without flight plan. The pilot reported a total loss of engine power while in cruise flight. He was unable to restart the engine and made a forced landing in a muddy field. During a telephone interview on November 16, the pilot said he pulled the carburetor heat to the full "on" position as soon as he heard the engine begin to lose power, but the engine continued to lose power and soon stopped running. The pilot said he believes the engine may have lost power due to vapor lock of the fuel lines. He said the engine was running on autogas, and that the outside air temperature was approximately 80 degrees F. Postaccident inspection of the engine discovered no preimpact mechanical anomalies. The engine was started and accelerated with no observed deficiencies.