What was found
PILOT REPORTED LOW FUEL FLOW AND HIGH EGT'S. AFTER EXTENSIVE T/S AND BENCH CHECK OF ENGINE FUEL INJECTION SYSTEM COMPONENTS, DISCOVERED THE AIRFRAME FUEL SELECTOR VALVE SCREEN ASSY IMPROPERLY ASSEMBLED. ACFT EQUIPPED WITH STC'D TIP TANKS WHICH UTILIZED A NON-STANDARD FUEL SELECTOR VALVE. DUE TO DATE OF ISSUANCE OF STC, INSTRUCTIONS FOR CONTINUED AIRWORTHINESS (MANDATED BY PAR 21.50) WERE NOT ISSUED BY THE STC HOLDER. BY NOT HAVING INSTRUCTIONS AVAILABE, IT IS POSSIBLE TO INCORRECTLY ASSEMBLE THE FUEL SCREEN WHICH CAN RESULT IN RESTRICTED FUEL FLOW AT HIGH POWER SETTINGS. THIS CAN RESULT IN EXCESSIVELY LEAN MIXTURES WITH EXCESSIVELY HIGH EGT'S WHICH CAN CAUSE CYLINDER DAMAGE AND POSSIBLE ENGINE FAILURE.