What was found
(CAN) ON APPROACH PILOT NOTICED ROUGHNESS FROM LEFT ENGINE AND UPON LANDING REPORTED THAT THE LEFT ENGINE WOULD NOT IDLE UNLESS MIXTURE PULLED BACK HALFWAY. INSPECTION REVEALED THAT THE COTTER PIN WHICH HOLD THE MIXTURE INTERCONNECT ROD TO THE THROTTLE VALVE HAD BROKEN AND THE ARM FELL OFF ALLOWING NO MIXTURE ADJUSTMENT WITH THROTTLE MOVEMENT AS REQUIRED. NEW COTTER PIN INSTALLED, AIRCRAFT GROUND RUN AND CHECKED SERVICEABLE. ADDITIONAL CHECK CARRIED OUT TO ALL CYLINDERS, COMPRESSION AND BORESCOPE REVEALED NO APPARENT DAMAGE CAUSED AIRCRAFT RETURNED TO SERVICE.