C172 PLT SUFFERS ROUGH RUNNING ENGINE AND REQUESTS LNDG AT THREE AREA ARPTS. EMERGENCY IS DECLARED AND A SAFE APCH AND LNDG RESULT.

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I WAS IN A CESSNA 172E AT 6000 FT MSL WHEN I ENCOUNTERED ENG ROUGHNESS. MY INITIAL REACTION WAS TO PUT THE MIXTURE RICH BECAUSE THE FUEL FLOW GAUGE KEPT FLUCTUATING SHOWING 4-6 GPH. I WAS RUNNING ON THE R FUEL TANK AT THE TIME AND SWITCHED TO BOTH TANKS. I KEPT TROUBLESHOOTING TO AVOID THE ENG FROM RUNNING ROUGH. I INTERMITTENTLY SWITCHED THE ELECTRIC FUEL PUMP ON AND OFF WHICH SMOOTHED OUT THE ENG FOR A SHORT TIME. I EVENTUALLY TURNED THE FUEL PUMP OFF BECAUSE I DIDN'T WANT TO BURN THE PUMP OUT. ATC KNEW I WAS HAVING ENG ROUGHNESS PRIOR TO APCHING THE VOR. AFTER ABOUT 30 MINS OF TRYING TO KEEP THE ENG FROM RUNNING ROUGH; I REQUESTED TO BE DIVERTED TO ZZZ2. A FEW MINS PASSED BY AND NO ANSWER FROM ATC. I THEN REQUESTED TO BE DIVERTED TO ZZZ3 SINCE IT WAS THE NEXT CLOSEST TO ME AT THE TIME. A FEW MINS PASSED AND STILL NO ANSWER. I KEPT ON WITH MY REQUEST TO BE DIVERTED UNTIL ATC TOLD ME TO TURN AROUND TOWARDS ZZZ AND LAND. THEY (ATC) WERE GOING TO DECLARE AN EMER. I COMPLIED AND WAS VECTORED TO LNDG. DURING THE DSCNT; THE ENG KEPT RUNNING ROUGH UNTIL TOUCHDOWN. AFTER LNDG AND DURING TAXI; THE ENG OPERATED NORMALLY. I WAS TAXIED TO AND SHUT DOWN AT THE GA TERMINAL AT ZZZ. I FLEW HOME VIA COMMERCIAL CARRIER AND THE ACFT WAS PICKED UP BY A MECH AND ANOTHER PLT SEVERAL DAYS LATER. CAUSE OF ENG ROUGHNESS IS STILL UNKNOWN. AN INVESTIGATION IS STILL UNDERWAY.