On September 8, 2012, about 1330 Pacific daylight time, a Great Lakes 2T-1A-2, N3702F biplane, experienced a loss of engine power while maneuvering, and the pilot made a forced landing in an open field near Monroe, Washington. The pilot/owner and certified flight instructor (CFI) operated the airplane under the provisions of 14 Code of Federal Regulations Part 91 as a local area aerobatics instructional flight. The pilot/owner and CFI were not injured. The airplane was substantially damaged. Visual meteorological conditions prevailed for the flight, and no flight plan had been filed. The flight departed Harvey Field Airport (S43), Snohomish, Washington, about 1245.According to the pilot/owner, the purpose of the flight was to improve his aerobatic skills. He spent a couple of hours on the ground with the CFI going over the maneuvers and flight, and then they went out to the airplane. The CFI was seated in the front, and the pilot was seated in the rear. Start up, taxi, and run up were all normal, and they departed the airport to the north. They flew to a valley that was situated between Monroe and Duvall and commenced with a series of aerobatic maneuvers that included advanced stalls, slow flight, aileron rolls, and inverted emergency roll recovery procedures. Once the aerobatic maneuvers were completed, they headed back toward S43. The pilot reported that the airplane did not feel right to him, and shortly thereafter, the engine began to lose power. He adjusted the mixture and throttle; however, this did not have any effect on the engine. The engine quit, and he asked the CFI to fly the airplane while he unsuccessfully attempted to restart the engine. The pilot stated that the CFI made a successful 3-point landing, but during the rollout on soft dirt, the airplane landing gear sunk in and the airplane went over onto its back, which resulted in structural damage to the rudder and vertical stabilizer. According to the CFI's written statement, after about 40 minutes of flying they began the flight back toward S43. The CFI stated that in level cruise flight, the engine started to show signs of a loss of power. The pilots decided to continue toward S43 and remain over open fields. After the pilot was not able to return full power to the engine, he asked the CFI to fly the airplane. The CFI set the airplane up for best glide and identified a plowed field to make the forced landing. The CFI reported that they were at 3,000 feet when the engine quit. Following a successful 3-point landing and roll out, as the airplane slowed, the main landing gear sunk into the freshly plowed field and nosed over onto its back. A Federal Aviation Administration (FAA) inspector examined the airplane engine, along with maintenance personnel. The report is attached to the public docket for this accident. A visual examination of the engine revealed no obvious damage and the engine controls were manipulated to their respective minimum and maximum stops with no binding noted. A visual inspection of the exhaust and induction systems revealed no blockages. Fuel from the quick drains at the header tank was taken with no contaminants noted; it smelled and appeared to be 100 low-lead aviation fuel. An engine run was performed, and the engine appeared to run normally with no mechanical anomalies.