On October 2, 1999, at 1040 eastern standard time, an Enstrom EN-28, N602H, sustained substantial damage during an autorotational landing in a construction area, near the Downtown Indianapolis Heliport, in Indianapolis Indiana, following a partial loss of engine power. The pilot and one passenger reported minor injuries. The personal 14 CFR Part 91 flight was operating in visual meteorological conditions. No flight plan was on file. The flight was originating at the time of the accident. The pilot said that prior to initiating the flight he conducted a "run-up." He said, during the magneto check the engine ran rough and exceeded the normal magneto drop. He said he continued to run the engine at high RPM and leaned the fuel mixture for a minute. He said he then conducted another "run-up" and the magneto drop was within specified limits.. He stated that during the initial climb the engine began to "pop" and the rotor RPM was dropping. He said he elected to attempt a turn to the heliport; however, due to the continued loss of power he elected to conduct an autorotational landing in a gravel construction area. During the autorotational landing the helicopter landed hard and received substantial damage. Subsequent to the accident an examination of the helicopter and engine was conducted. The flight and engine controls were found to work correctly and have continuity. Both magnetos and harnesses were examined and no discrepancies were found. All the spark plugs were removed and examined. The bottom spark plug from the #2 cylinder was oil fouled and did not test fire. The top spark plug from the same cylinder (#2) fired with a weak and intermittent spark. All the other spark plugs test fired. No additional anomalies were found in the helicopter or engine.