The aircraft was being used for mustering when the engine began to run roughly and lose power. The pilot turned towards the nearest cleared area but he was unable to maintain rotor rpm with the power that was available. The pilot initiated an autorotative descent, from approximately 40 feet above ground level, and the aircraft touched down in the cleared area on the heels of its skids. The aircraft bounced forward, onto the toes of the skids, and rolled to the left. The pilot lifted the aircraft clear of the ground, in an attempt to regain control, however it turned through 360 degrees several times before touching down again and coming to a stop. The tail boom and tail rotor were damaged during the landing. The precise cause of the loss of power could not be determined however the movement of the exhaust valve in No. 3 cylinder was found to be restricted by a build up of gum on the valve stem and one spark plug was found to be inoperative. Most mustering operations take place at low level and at a slow speed, as was the case in this accident, and this lack of performance potential makes a successful, damage free, autorotational landing unlikely.