The pilot reported that the aircraft was flying at 50 knots at about 500 feet above the ground when the engine stopped. He pulled the engine start cord and the engine restarted. The aircraft was then climbed to 2000 feet as the pilot was concerned about operating over rough terrain. A short time later, the engine stopped again and rather than attempt another restart, the pilot concentrated on landing the aircraft. Substantial damage resulted from the landing on unsuitable terrain. Examination of the engine revealed that the carburettor was misaligned and that the piston at the magneto end of the engine had failed. The piston exhibited signs of detonation which was probably the result of an overly lean fuel/air mixture. Recent advice from the engine manufacturer included the recommendation that super grade petrol be used instead of unleaded petrol with ehtyl alcohol added. Further, new engines are fitted with a baffle or air guide under the cylinder cowl and have a new carburettor setting. This accident was not the subject of an on-site investigation.
