The accident occurred during an aerial application flight. The pilot reported that during a course reversal the engine "felt" as if it had no power. He referenced his engine instruments and noted no anomalies. He assumed that the airplane had encountered an area of disturbed air, and elected to continue onto the next spray run. During the next course reversal, the engine started "popping" like an engine misfire. The engine instruments were referenced again and no anomalies were noted. The pilot switched magnetos, verified that the mixture was full rich, and selected a full low pitch propeller setting. Despite the pilot's corrective actions, there was no change in engine performance. The pilot performed a three-point landing in a nearby grass field, and the airplane nosed-over during the landing roll when it encountered tall grass. The pilot had flown over 10 hours on the day of the accident without any engine discrepancies. The airplane had departed approximately 40 minutes before the accident with 35 gallons of fuel on board. A post accident engine examination failed to identify the cause of the engine power loss. There was compression on all cylinders as the propeller was rotated. Both magnetos provided spark on all leads when rotated by hand. There were no obstructions to the air induction system.