The pilots operation of the airplane with known deficiencies in the equipment based on witness statements describing an excessive rpm drop and rough running engine after starting, during the engine run-up, and after becoming airborne, his failure to abort the takeoff after an excessive takeoff roll, and his failure to maintain airspeed resulting in an inadvertent stall, uncontrolled descent, and in-flight collision with terrain. A factor in the accident was the loss of engine power due to undetermined reasons.