A C172 IN CRUISE AT 2000 FT DECLARED AN EMER AND LANDED OFF FIELD DUE TO SURGING PWR CAUSED BY A MALFUNCTIONING CARB.

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STUDENT XCOUNTRY DUAL FLT FMY; SEF; SRQ; FMY. FMY TO SEF UNEVENTFUL. STUDENT DID 3 TOUCH-AND-GOES SOLO AT SEF. SEF TO SRQ CRUISING AT 2000 FT; 2500 RPM; OUTSIDE AIR TEMP 37 DEGS C; CLR VMC AT OUR ALT. AT 3000+ FT; SMALL (LESS THAN 1/4 MI WIDE) CUMULUS TYPE CLOUDS WERE STARTING TO FORM. APPROX 10 NM LATER (ALMOST 10 MINS FROM START OF CLB AND 6 OR SO MINS AFTER LEVELING OFF); THE ENG SURGED ABRUPTLY AS IF THROTTLE HAD BEEN BUMPED CLOSED THEN REOPENED RAPIDLY ONCE; THEN CONTINUED TO RUN STRONG AT 2500 RPM. THERE WERE NO ABNORMAL INDICATIONS FROM THE ENG INSTS AND NO PWR LOSS OTHER THAN THE SURGE. IT WAS AT LEAST 1 MIN; PROBABLY MORE LATER; AS WE WERE DISCUSSING THE STUDENT'S PROC FOR ENTERING THE CLASS C AIRSPACE AT SRQ; THE ENG AGAIN ABRUPTLY HAD A PWR LOSS. THIS TIME IT DID NOT RECOVER AND CONTINUE TO RUN IMMEDIATELY LIKE BEFORE. CARB HEAT WAS APPLIED IMMEDIATELY; MIXTURE CHKED; MAGNETOS SWITCHED; PRIMER CHKED; FUEL TANK SELECTOR CHKED ON BOTH; FUEL GAUGES INDICATED 3/4 FULL ON BOTH SIDES. TRIED PUMPING THROTTLE; WITH VERY MINOR CHANGE; A COUPLE OF TIMES THEN NOTHING. WE HAD A NICE LONG STRETCH OF STATE HWY IN FRONT OF US; SO MIXTURE WAS PULLED TO CUT-OFF AND ACFT WAS LANDED ON HWY; THEN PUSHED TO THE SIDE OUT OF THE WAY OF TFC. CLOSED VFR FLT PLAN BY CELL PHONE AND CALLED MECH. AT XQ00 HRS MECH CHKED THE ENG OVER AND FOUND NOTHING; TEST RUNS WERE OK. ABOUT 40 MI INTO THE FLT; WE DIVERTED 20 MI E TO AVOID TSTMS AND RAIN SHOWERS. LNDG AT ARCADIA (X06); AT XS45 HRS WE DEPARTED X06 HDG W AND SW ABOUT 40 MI AND WERE STARTING TO GO THROUGH THE EDGE OF A LIGHT RAIN SHOWER WHEN THE ENG SURGED AGAIN LIKE THE VERY FIRST TIME. CARB HEAT AND FULL THROTTLE WERE APPLIED AND LEFT ON WHILE WE TURNED AROUND AND FLEW TO CHARLOTTE COUNTY (PGD) AND LANDED. AT XW00 HRS WE DEPARTED PGD; APPLIED CARB HEAT RIGHT AFTER TKOF AND LEFT THROTTLE FULL OPEN FOR 15 MIN FLT TO FMY. RAIN HAD SUBSIDED AT THIS TIME. WE FOUND THE CARB HAD BEEN REPLACED THE AFTERNOON BEFORE OUR FLT. THIS ACFT HAS FLOWN REGULARLY 30-50 HRS PER MONTH FOR THE LAST SEVERAL YRS IT HAS BEEN OWNED WITHOUT ANY PROBS OF THIS NATURE. THE OLD CARB THAT WAS TAKEN OFF WAS REPAIRED WITH NEW PARTS AND REINSTALLED ON THE ENG. THE ACFT WAS TEST FLOWN AT VARIOUS PWR SETTINGS IN AND OUT OF LIGHT RAIN SHOWERS WITH NO PROBS. IT NOW HAS 10 OR MORE HRS ON IT WITH NO PROBS.