What was found
FLYING, NUMBER ONE ENGINE CARBURETOR STARTED TO ICE UP. APPLIED CARBURETOR HEAT BUT IT HAD NO EFFECT ON CONDITION. ON APPROACH, PILOT HAD TO SHUT DOWN ENGINE. POST FLIGHT INSPECTION FOUND THE CARBURETOR AIR BOX DOOR WAS WORN (COULD BE PUSHED UP ABOUT .5000 INCH). REPLACED CARBURETOR AIR BOX WITH SERVICEABLE ONE.