The pilot stated that he was in cruise flight when he heard a loud noise and the helicopter yawed to the left. He initiated an autorotational landing into a residential area where the helicopter impacted trees and the terrain. An examination of the engine found two pieces of plastic in the induction tubes. The pieces were found to have come from the induction housing, but the reason for the fracture was not determined. The two pieces fitted together and could be positioned to block the fuel injection control.