C172 PLT HAD ENG FAILURE AND FORCED LNDG ON TKOF.

Narrative

ACFT RECEIVED A THOROUGH PREFLT AND PRETKOF CHKLIST. TKOF INITIATED ON RWY 10 AT GENESEE COUNTY ARPT. AT 200 FT AGL AND 800 FT FROM DEP END OF RWY 10; THE ENG FAILED. IT ACTED AS IF SOMEONE HAD CLOSED THE THROTTLE TO IDLE. THE BOUNDARY FENCE AND OTHER OBSTACLES WERE APCHING. I ELECTED TO CLOSE THE THROTTLE TO AVOID THE POSSIBILITY OF A PWR SURGE. I MADE A FULL FLAP LNDG ON THE GRASS WITH NO DAMAGE TO THE ACFT OR OBJECTS ON GND. I WAS NOT HURT. IT WAS A NORMAL GRASS FIELD LNDG. THE PRE TKOF MAGNETO CHK WAS TOTALLY NORMAL AND THE ENG HAD BEEN OPERATING NORMALLY ON PREVIOUS FLTS UP UNTIL FAILURE. ENG RAN FINE AFTER RESTART ON GND. SUBSEQUENT REVIEW BY AN A&P AND IA FOUND ALL FUEL AND ACFT SYS NORMAL. REASON FOR THE ENG FAILURE WAS 'BUTTER NUTS' IN THE CARB AIR BOX. APPARENTLY AN ANIMAL HAD STORED THEM IN THE SCAT TUBING AND AIR BOX FOR THE CARB HEAT SYS. ONCE THE 'BUTTER NUTS' WERE REMOVED; THE ACFT WAS INSPECTED; TEST FLOWN AND RETURNED TO SVC BY THE A&P AND IA. SUBSEQUENT OPS HAVE BEEN TOTALLY NORMAL.