C172 ON SPI TRAINING FLT. ENG BEGAN RUNNING ROUGH. INSTRUCTOR ATTEMPTED TO RESTORE NORMAL PWR AND UNABLE. ADVISED HHI ON 121 PT 5 HE WAS LOSING PWR AND LNDG ON CROP DUSTER STRIP. CTLR DECLARED AN EMER AND ADVISED WOULD SEND EMER EQUIP. LANDED WITHOUT INCIDENT. FOUND BROKEN ROCKER ARM VALVE STUD. MAINT REPAIRED AND FLEW THE ACFT OUT.

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WHILE A STUDENT PLT WAS PRACTICING S-TURNS ACROSS A ROAD AT 1400 FT MSL (800 FT AGL) THE ENG STARTED RUNNING VERY ROUGH. (LATER IT WAS DETERMINED TO BE A BROKEN ROCKER ARM VALVE STUD.) AS A CFI; I TOOK THE ACFT CTLS AND ATTEMPTED TO DETERMINE WHAT WAS WRONG WITH THE ENG AND SEE IF I COULD GET IT RUNNING SMOOTHER. WHILE REVIEWING THE EMER PROCS FOR A LOSS OF PWR DURING FLT; I FLEW THE ACFT TOWARD AN EMER LNDG SITE NEARBY. THIS LNDG SITE IS A DUSTER STRIP LOCATION IN THE MIDDLE OF A SUGARCANE FIELD. I WAS UNABLE TO FIX THE ENG AND WAS UNABLE TO MAINTAIN ALT AT FULL THROTTLE. AS A PRECAUTIONARY MEASURE I DECIDED TO LAND THE ACFT ON THE DUSTER STRIP RATHER THAN RISK FURTHER FLT BACK TO HNL ARPT. I DECLARED AN ENG PWR LOSS EMER ON FREQ 121.5 AND SQUAWKED 7700 AND WAS ABLE TO TELL THE CTLR MY SIT AND WHERE I WAS LOCATED. HE SAID TO CALL BACK WHEN I REACHED THE FIELD AND THAT HE WOULD SEND FIRE AND AMBULANCE VEHICLES JUST IN CASE. I CIRCLED THE LNDG SITE ONCE AND LANDED IT. AFTER LNDG ON THE DUSTER STRIP WITH NO COMPLICATIONS I RPTED BACK ON 121.5 THAT WE WERE ON THE GND WITH NO FIRE AND NO INJURIES. LATER THE FLT SCHOOL FIXED THE CYLINDER AND FLEW THE ACFT BACK TO HNL.