What was found
AFTER INSTALLATION AND ADJUSTMENT OF THE CARBURETOR, ACFT WAS TEST FLOWN APPROX. 0.3 HOURS (NORMAL START, RUN-UP, ONCE AROUND THE PATTERN INCLUDING A SIMULATED AUTO-ROTATION). ACFT RETURNED TO RAMP AND PERFORMED NORMAL SHUTDOWN. ENGINE TACH NEEDLE STARTED TO "BOUNCE" WHEN PILOT MOVED MIXTURE TO ICO. ACFT WAS SECURED AND PILOT NOTIFIED ME OF THE ABNORMAL CONDITION. ASKED HIM TO GROUND RUN THE ACFT AGAIN TO TRY TO DUPLICATE THE PROBLEM. PILOT HAD DIFFICULTY GETTING THE ENGINE TO OPERATE SMOOTHLY AT IDLE AND ENGINE STOPPED ON ITS OWN. HE INSPECTED THE ENGINE AREA AND FOUND THE IDLE ADJUSTMENT SCREW AND SPRING LYING ON THE GROUND BELOW THE ENGINE. CARBURETOR TOTAL TIME UNKNOWN. COMPONENT WAS PURCHASED AS AN OVERHAUL/EXCHANGE ITEM.