THE PILOT WAS EXECUTING A DOWNWIND TAKEOFF, THE WINDS REPORTEDLY GUSTING TO 25 KNOTS, LOADED WITH TWO PASSENGERS, WHEN JUST AFTER TAKEOFF THE PILOT STATED THE HELICOPTER EXPERIENCED A LOSS OF POWER. THE PILOT THEN ATTEMPTED TO EXECUTE AN AUTOROTATION, BUT DURING THE EMERGENCY DESCENT THE PILOT WAS UNABLE TO MAINTAIN DIRECTIONAL CONTROL. AS A RESULT THE HELICOPTER STRUCK A MOVING VEHICLE WITH ITS TAIL ROTOR AND SUBSEQUENTLY COLLIDED WITH THE TERRAIN. THE HELICOPTER WAS APPROXIMATELY 130 LBS UNDER GROSS WT AT THE TIME OF TAKEOFF. WHEN EXAMINED AND TESTED THE ENGINE OPERATED NORMALLY AND NO MECHANICAL MALFUNCTIONS WERE FOUND. EXAMINATION OF THE ENGINE DID REVEAL THAT A FUEL NOZZLE HAD BEEN INSTALLED IN THE AIRCRAFT'S FUEL SYSTEM WITH A RESTRICTOR MISSING BUT TO COMPENSATE FOR THE MISSING RESTRICTOR THE FUEL SERVO IDLE MIXTURE WAS SET AT A VERY LEAN SETTING. THIS WAS DETERMINED NOT TO BE A FACTOR IN THE ACCIDENT.