As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE PILOTS PERFORMED A SATISFACTORY PRE TAKEOFF ENGINE RUN AND WERE AWAITING TAKEOFF CLEARANCE WHEN THE ENGINE DROPPED FROM 1400 RPM TO 200 RPM SEVERAL TIMES. AS THEY ATTEMPTED TO RETURN TO THE APRON, THE ENGINE AGAIN DROPPED TO 200 RPM REPEATEDLY. MAINTENANCE DUPLICATED THE ISSUE AND DETERMINED THE CARBURETOR WAS SUFFERING FROM AN UNKNOWN INTERNAL FAILURE AND WAS REPLACED. THE CARB HAD BEEN INSTALLED ON THE AIRCRAFT FOR 4 MONTHS.