A C172 TRAINING FLT RETURNED TO BASE WHEN THE ENGINE STARTED RUNNING ROUGH.

Narrative

INSTRUCTIONAL FLT (SPECIFICALLY PVT PLT RECURRENT TRAINING). ACFT PREFLT INSPECTION WAS UNREMARKABLE; WITH NO DEV OR ISSUES. ACFT START; TAXI AND ENG RUN-UP WAS UNEXCEPTIONAL AND CONFORMED TO STANDARD TOLERANCES. TKOF SEEMED SOMEWHAT SLUGGISH; BUT THERE WAS SIGNIFICANT DENSITY ALT (1800 APPROX); SO IT IN ITSELF WAS NOT TOTALLY UNEXPECTED. ON CLBOUT THE ACFT SEEMED TO PERFORM BETTER; OP WAS WITHIN GUIDELINES. AS THE ACFT REACHED 1500 FT; A SLIGHT VIBRATION STARTED TO BECOME APPARENT; AND REMAINED AFTER THE RECOMMENDED LEANING PROC WAS FOLLOWED. AFTER LEVELING OFF AT 2000 FT; THE VIBRATION BECAME WORSE AND THE ENG (AT FULL THROTTLE) WOULD NOT OPERATE PAST 2150-2200 RPM AND CRUISE SPDS OF 100-105 KTS. AFTER TROUBLESHOOTING THE PROB I MADE THE DECISION TO RETURN TO ZZZ. I CONTACTED DEP (HAVING NEVER BEEN RELEASED; AND JUST REACHING THE OUTER BOUNDARY OF THE CLASS C AIRSPACE) AND ASKED FOR A RETURN HDG. I WAS OFFERED THE CHANCE TO DECLARE AN EMER; BUT CHOSE NOT TO BECAUSE WHILE OPERATING AT 2000 RPM OR LESS; THEN ENG WAS SIGNIFICANTLY SMOOTHER. I ALLOWED THE STUDENT TO LAND THE ACFT AND TAXIED BACK WITHOUT INCIDENT. IN RETROSPECT I SHOULD HAVE THOUGHT MORE ABOUT RETURNING TO THE ARPT SOONER GIVEN THE INDICATIONS OF A PROB DEVELOPING INSTEAD OF TROUBLESHOOTING FOR THE PROB. FLTS LIKE THIS SERVE AS A GREAT REMINDER OF WHY WE FOCUS SO MUCH ATTN ON EMER OPS.