On May 24, 1998, at 1007 mountain daylight time, a Mader Wag-Aero 2+2 homebuilt airplane, N6272A, was substantially damaged during a forced landing near Hurricane, Utah. The private pilot and his passenger were not injured. The airplane was being operated by the owner under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the local flight which originated from Hurricane Airport approximately 7 minutes before the accident. No flight plan was filed. According to the pilot, he was practicing touch-and-go landings on runway 18 (3,410 feet x 40 feet, asphalt). While on downwind, the engine began to run rough, and smoke entered the cabin from underneath the instrument panel. He turned off the electrical system, then executed a forced landing in an unplowed wheat field. Prior to touchdown, he turned the fuel selector off, and the engine quit. Upon touchdown, the landing gear became entangled with wheat, causing the tail section of the aircraft to be lifted, and the aircraft nosed over. The propeller and the vertical stabilizer were damaged. Postaccident examination of the aircraft's engine revealed that the exhaust stack had fractured, allowing exhaust gases to envelope a fuel line.