What was found
PARTIAL LOSS OF ENGINE POWER OCCURRED JUST AFTER TAKEOFF. THE MANIFOLD PRESSURE DROPPED TO 15 INCHES HG, RPM DROPPED TO 1900. PRE TAKEOFF ENGINE RUN UP, ALTERNATE AIR CHECK WAS REPORTED AS NORMAL. A POST FLIGHT INSPECTION REVEALED THAT ALTERNATE AIR (HEAT BOX) DOOR HAD SEPARATED FROM ALTERNATE AIR BOX ASSY, WAS INGESTED INTO FUEL SERVO INTAKE, PARTIALLY BLOCKED ENGINE INDUCTION AIR FLOW AND RESULTED IN A PARTIAL LOSS OF POWER. MS20470AD3 RIVETS (3EA) THAT SECURE DOOR TO AIR BOX ASSY HAD FAILED. IMPROPERLY BUCKED RIVETS DUE TO EXCESSIVE SPRING TENSION DURING ASSY AND/OR TOO MUCH SPRING TENSION FOR THE RIVETS TO BEAR.