What was found
(CAN)DURING TAKEOFF ROLL A LEFT YAW WAS NOTICED AND ONCE AIRBORNE THE FUEL FLOW AND MANIFOLD PRESSURE ON THE LEFT ENGINE WAS NOTED TO BE LOW. THE ALTERNATE AIR CONTROL WAS NOTED IN THE HEAT POSITION. WHEN THE ALTERNATE AIR WAS RETURNED TO THE FILTER POSITION FULL POWER RETURNED. INSPECTION FOUND THE AIR BOX WAS ICED UP IN THE CLOSED POSITIION. THE VANE WAS THAWED AND THE AIRCRAFT RETURNED TO SERVICE.