On September 9, 2006, about 1730 Pacific daylight time, a Schweizer 269C-1, N323CP, made a hard landing following a loss of engine power at Van Nuys, California. Group 3 Aviation was operating the helicopter under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The certificated flight instructor (CFI) and the private pilot undergoing instruction (PUI) were not injured; the helicopter sustained substantial damage to the tail boom. The local area instructional flight departed Van Nuys about 1615. Visual meteorological conditions prevailed, and no flight plan had been filed. The operator reported that the CFI and PUI had gone to Burbank, California, on an instrument training flight. On the return to Van Nuys, the tower controller cleared the CFI to land on taxiway Alpha abeam the Group 3 facility. The CFI and PUI agreed to perform a practice autorotation onto the taxiway. They turned to final over the taxiway at 500 feet above ground level (agl). The PUI was in control, and preparing to initiate the maneuver with the CFI following on the dual controls. The airspeed was 65 knots indicated airspeed (KIAS), and the manifold pressure was 21-23 inches. Shortly after initiating the maneuver, the low voltage warning light illuminated. They scanned the operating gauges and tachometer, which indicated a loss of engine power. The CFI took control, and told the tower that a malfunction had occurred. The helicopter landed hard, and the tail boom separated. Maintenance personnel examined the engine under the supervision of a Federal Aviation Administration (FAA) Aviation Safety Inspector. A review of the airframe and engine maintenance logbooks revealed that the last 100-hour inspection occurred at a hobbs time of 1,699.8 hours; the hobbs read 1,769.1 at the examination. An entry noted that maintenance technicians adjusted the idle setting at a hobbs time of 1,737.9 hours. They replaced all eight spark plugs at 1,707.7 hours, and installed and adjusted new points in the left magneto at 1,709.3 hours. The inspector examined the fuel injection lines, fuel shutoff valve, fuel bowl, fuel boost pump, fuel injector servo screen, and took a fuel sample. All items checked satisfactory. The mixture control traveled freely from idle cut off to full rich. The throttle traveled freely from idle to full open. Maintenance personnel removed the lower spark plugs. They were all clean with no mechanical deformation. All cylinders were satisfactory on a compression check. The magnetos were timed to 25 degrees before top dead center on the compression stroke. The oil was free of debris. They installed the spark plugs, and prepared the engine for a ground run. On the first run, the engine started easily, but ran rough and very rich. The operator warmed the engine up, and performed a magneto check. Both magnetos had a 250-rpm (revolutions per minute) drop, and the engine tried to die at idle. On the second run, it started easily, and the operator leaned the mixture with the mixture control until the engine ran smoothly. He ran the engine to 2,000 rpm. He performed another magneto check and observed a 200-rpm drop. Fuel pressure, oil pressure, oil temperature, and cylinder head temperatures were satisfactory. Maintenance personnel removed the fuel injector servo for further examination. Precision Airmotive personnel examined the fuel servo under the supervision of a Safety Board investigator. They noted that the idle mixture spring was not on the servo. The purpose of the idle mixture spring is to control the movement of the mixture adjustment. If this spring is not installed, the mixture setting can vibrate to higher or lower settings. As received, the mixture link length was 2.685 inches. Specified flowmeter limits at idle were 11.3 to 12.3 pounds per hour (lb/hr); the test result was 25.8 lb/hr. Limits at maximum throttle were 98.0 to 102.5 lb/hr; the test results were 98.8 lb/hr. Technicians adjusted the unit to the proper specifications on the flow bench so that the idle link measured 2.660 inches. At idle, the resultant flow was 12.2 lb/hr; at maximum throttle the results were 98.5 lb/hr.