As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT RUN UP FOR TAKEOFF AND ENGINE BEGAN RUNNING VERY ROUGH. AIRCRAFT RETURNED TO RAMP. PILOT COMMENTED THAT MIXTURE WAS NOT CORRECT AT IDLE OR AT POWER AND THAT THROTTLE CONTROL FELT SPRING LOADED. MAINTENANCE FOUND THROTTLE CONTROL VALVE, ON FUEL CONTROL, STIFF AND BUSHING MOVING. REMOVED AND REPLACED UNIT.