THE ENGINE LOST POWER WHILE RETURNING FROM A BANNER TOWING FLIGHT DUE TO FUEL EXHAUSTION. THE PILOT RELEASED HIS BANNER AND MADE A FORCED LANDING. THE SUBSEQUENT 'FLAT' LANDING STRUCTURALLY DAMAGED THE WING SPARS. THE PILOT LATER LOCATED A FUEL LEAK NEAR THE CARBURETOR AT A FUEL LINE CONNECTING JOINT.