What happened
While operating in cruise flight, the engine experienced a loss of power. In response to the power loss, the pilot attempted to apply carburetor heat and switched to a different fuel tank, but these actions had no effect on engine operation. The pilot subsequently performed a forced landing into a hayfield. During the landing rollout, the Cessna (type not specified in source) encountered a soft area of the field, causing the aircraft to nose over.
The investigation
An engine test run was conducted following the accident. During this test, the engine failed to operate on the left magneto. Upon disassembling the left magneto, investigators found the timing gear keeper lying loose inside the case. Further examination revealed that the keeper, the timing gear keyway slot, and the magneto drive shaft were all worn. After replacing the timing gear and keeper with serviceable components and reinstalling the magneto, the engine operated correctly on both magnetos with acceptable RPM drops.