On June 16, 1997, at 1228 hours Pacific daylight time, a Bell 47G-3B-1 helicopter, N1335X, lost engine power while returning to a refueling site and was substantially damaged during the ensuing forced landing at Fallon, Nevada. The commercial pilot and two passengers were not injured. Visual meteorological conditions prevailed. The aircraft was operated by High Desert Helicopters as a public-use aircraft under contract to the U.S. Department of the Interior and was engaged in a census of wild horses. An investigator from the Department of the Interior, Office of Aircraft Services (OAS), examined the wreckage and drained 1.9 gallons of fuel from the entire fuel system. The aircraft has a 2.5 gallon unusable fuel quantity. The OAS examination of the aircraft found no evidence of mechanical malfunction or failure. The pilot's load calculation form shows that the aircraft departed with 60 gallons of fuel, however, the aircraft flight manual states that the maximum fuel load is 57 usable gallons and that when the fuel gage indicates EMPTY the fuel remaining is not usable in flight. According to the OAS investigator, the engine hour meter recorded that the engine lost power after 3.0 hours of flight. The owner/pilot acknowledged exhausting the aircraft's fuel supply to the OAS investigator, but felt that the aircraft should have had more endurance. He reported that the engine should have consumed 18 gallons per hour and the endurance should have been 3.5 hours. He felt that the higher fuel consumption was due to a faulty carburetor. He did not report any error or abnormality in the fuel quantity gage. The engine carburetor was sent to the manufacturer by the OAS for a flow test examination on July 26, 1997. It flow tested within the manufacturer's specifications. The carburetor was not disassembled for examination by the manufacturer. In October, 1997, the operator returned the carburetor for examination by the company which last overhauled it. A representative of that company identified an air leak past the inner throttle shaft seal below the butterfly. The inspector advanced a theory that leakage past the seal could have resulted in a false reference pressure signal to the automatic mixture control which might have caused the engine fuel mixture to become abnormally rich. He reported that the damage to the seal might have resulted from a blow on the end of the throttle shaft sometime after the carburetor was overhauled. The inspector also advanced a theory why the enriched mixture would only occur in a supercharged pressure environment and would not have been detected in the flow bench test at atmospheric pressure. Their report is attached. The density altitude was approximately 6,500 feet. A representative of the aircraft manufacturer told the Safety Board that an autorotative landing at that density altitude would likely result in a "hard landing," and that there is no information in the aircraft flight manual to tell the pilot at what density altitude an autorotative landing without damage becomes unlikely.