What happened
The aircraft lost engine power shortly after initiating takeoff. The instructor pilot assumed control of the aircraft and, recognizing the insufficient altitude for a return to the airport, elected to land straight ahead on the grass adjacent to the runway.
The investigation
A teardown of the engine revealed that a valve was stuck in place. Examination showed the valve had been struck by a piston, causing it to break into several pieces. These fragments entered and obstructed the exhaust system.
Findings
The mechanical failure was caused by a valve that had broken due to impact from a piston. The resulting debris lodged in the exhaust system, leading to the loss of power.