SHORTLY AFTER TAKEOFF AT AN ALTITUDE OF APPROXIMATELY 50 FT AGL, A LOSS OF ENGINE POWER WAS EXPERIENCE. A FORCED LANDING WAS MADE IN A FIELD DURING WHICH THE LANDING GEAR COLLAPSED. POST ACCIDENT INSPECTION FAILED TO REVEAL ANY MECHANICAL FAILURE. IT WAS NOTED THAT THE SPARK PLUGS WERE HEAVILY SOOTED. METEOROLOGICAL CONDITIONS WERE CONDUCIVE TO CARBURETOR ICING. AUTOMOTIVE GAS WAS BEING USED IN THE AIRPLANE AT THE TIME OF THE ACCIDENT.