On June 4, 1998, at 1500 hours Pacific daylight time, a Cessna 150M, N704ZA, experienced a loss of engine power and made an emergency landing in an open field 3 miles west of the Livermore, California, airport. The aircraft, operated by Flying Vikings under 14 CFR Part 91, sustained substantial damage. The commercial rated flight instructor pilot and student pilot, the sole occupants, received minor injuries. Visual meteorological conditions existed for the instructional flight and no flight plan was filed. In an interview with a Federal Aviation Administration (FAA) inspector from the Oakland, California, Flight Standards District Office, the pilot reported that they were in level flight at 2,500 feet above ground level (agl) when the engine rpm dropped. They attempted to land at the Livermore airport, but had to make an emergency landing in an open field. The aircraft came to rest inverted. In the instructor pilot's written statement, he reported that after performing air work they were returning to base when they experienced a loss of rpm's from 2,300 to 1,100. He stated that he took over the controls and noted that the "power level [did] not respond to any of my applications." The instructor stated that the power setting remained constant at 1,100 rpm's. He completed the emergency checklist and informed the tower of their situation and landed in the open field. The instructor reported that on the landing rollout the nose wheel hit a hole in the ground and the aircraft flipped over. The instructor reported that due to high vegetation he was unable to see the rough terrain before landing. The aircraft was examined by the FAA on-scene. The FAA inspector noted that the bolt was in place, but that the nut and cotter pin were missing from the carburetor throttle attachment assembly. According to the FAA, the aircraft had flown approximately 10-15 hours since the last annual inspection and maintenance had been performed. The Army National Guard was conducting maneuvers at the time of the accident and rescued the flight instructor and student pilot. They also extinguished a postimpact engine fire.