On September 28, 2002, at 1100 central daylight time, a Jonas Nieuport, N678DJ, was substantially damaged when it contacted soft terrain during a forced landing in Creve Coeur, Missouri, following a partial loss of engine power. The airline transport pilot was not injured. The 14 CFR Part 91 personal flight was operating in visual meteorological conditions without a flight plan. The flight originated from the Creve Coeur Airport, shortly before the accident. The pilot reported that during the flight, the engine cylinder head temperature began to rise and the engine began to lose power. He reported that he did not have enough altitude to return to the airport so he elected to make a precautionary landing in a dirt field. The left main gear dug into the soft dirt during the landing and it collapsed. The airplane came to rest on its left wing and nose. The pilot, who was also the airplane builder, reported the airplane has a total time of 17.6 hours. The engine that was installed in the airplane at the time of the accident was the second engine that had been in the airplane. The pilot stated the first engine experienced high oil temperatures and low oil pressure, so it was replaced. He stated the first engine did not have a cylinder head temperature system installed. The pilot reported that after the accident, he determined that the carburetor jets were not the proper size for the engine. He reported that this resulted in the mixture being too lean and the engine running hot.