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THE INSTALLATION OF AN UNAPPROVED WING TIP FUEL TANK SYSTEM; AN INADEQUATE ANNUAL INSPECTION WHICH DID NOT NOTE, OR CORRECT, THE UNAPPROVED FUEL SYSTEM ADDITION; A WING TANK FUEL SELECTOR VALVE INSTALLATION WITHOUT ADEQUATE PLACARDS TO SHOW POSITION OR OPERATION; AND INCORRECT INSTRUCTIONS TO THE PILOTS BY THE OWNER AND MECHANIC ON POSITIONING FUEL SELECTOR VALVES WHICH LED TO INADVERTENT MISPOSITIONING BY THE PILOTS AND SUBSEQUENT FUEL STARVATION. IN ADDITION, THE PILOT AT THE CONTROLS FAILED TO MAINTAIN ADEQUATE AIRSPEED FOLLOWING THE POWER LOSS WHICH RESULTED IN AN INADVERTENT STALL/SPIN. A FACTOR WHICH CONTRIBUTED TO THE LOSS OF CONTROL WAS THE DARK NIGHT LIGHT CONDITION.