On May 30, 2011, about 0800 central daylight time, an experimental amateur-built Jones Cozy Mark IV, N204TJ, experienced a partial loss of engine power while maneuvering. The pilot subsequently diverted to Sioux Gateway Airport/Col. Bud Day Field (SUX), Sioux City, Iowa; however, he performed a precautionary engine shutdown due to a fuel smell and landed in a field near SUX. The certificated private pilot sustained minor injuries. The airplane sustained substantial damage to the wing. The airplane was registered to and operated by the pilot as a personal flight under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan had been filed. The local flight originated from SUX about 0730. The pilot stated that the engine began to sputter while the airplane was maneuvering at 7,500 feet mean sea level. The pilot then diverted to SUX and about 10 miles from the SUX, he turned the fuel selector off because he smelled fuel and became concerned about a possible engine fire. He stated that a strong headwind prevented a glide to the airport and when he realized that the airplane was unable to reach runway 13 at SUX, he performed a forced landing to a field near the approach end of the runway. The pilot stated that the gascolator gasket dislodged during flight and air entered the fuel system resulting in the loss of engine power. The pilot performed the last inspection of the airplane, which was a condition inspection on May 7, 2010. The pilot stated that he did not properly install the gascolator gasket during the inspection.