The pilot took off at 1805 with 150 lbs of fuel remaining, intending to fly to Beckley, WV to refuel. He stated that after he was in level flight at about 3000 ft MSL for 3 to 4 minutes, he noted an unusual jerk or vibration in the rear. Suspecting a possible tail rotor problem, he lowered the collective and turned toward an open area. He then noted that the engine had lost power. During an autorotative landing, the helicopter touched down hard without enough RPM to cushion the landing. An exam of the fuel system revealed there was about 20 gallons of fuel remaining. The engine and fuel pumps operated normally when tested. However, further checks revealed that when fuel in the aft tank was below the stand pipe, fuel would leak to the front tank; also, there was virtually no induced flow in the left transfer system. Disassembly revealed the standpipe in the aft tank was not properly aligned, thus the O-ring was not properly seated, which allowed a leak. A piece of masking tape was found in the inlet to the left transfer pump. As the front tank approached empty (during operation), air was entrained in the fuel.