The purpose of the flight was to test the aerial spray equipment. After starting and warming the engine, the pilot took off, flew about 1/4 mile south, then flew north. A motorist stated that he saw black smoke coming from the helicopter. Two teenagers heard popping sounds, and reportedly, the engine was backfiring, stopping and starting. About 1 mile north of the departure point, the helicopter impacted and traveled 82 feet before stopping. An investigation revealed the engine had operated about 17 hours since a major overhaul on 5/20/78. The last reported inspection was on 3/24/82. An engine teardown revealed heavy rust and corrosion in the carburetor, an extremely dirty air cleaner, old and severely worn spark plugs, moderate rust and corrosion in the cylinder barrels, heavy deposits on the piston heads, and moderate scoring of the piston skirts. According to the pilot's parents, the engine would "pop" when it was first started and the pilot had the idle set faster than normal to keep it from running rough.