C-172 EXPERIENCED ROUGH RUNNING ENG WITH ASSOCIATED VIBRATION DURING CLIMBOUT. PLT RETURNED FOR PRECAUTIONARY LNDG.

Narrative

ON AUG/XA/07; MY STUDENT AND I WERE ON AN INSTRUCTIONAL FLT ORIGINATING IN DAB WITH A DEST OF CDK. THE FLT TO CDK WAS UNEVENTFUL. AT CDK WE SHUT THE ACFT DOWN FOR APPROX 1 HR. UPON RETURN TO THE ACFT AND BEFORE DEP; I PERFORMED A PREFLT INSPECTION. I NOTED NO VISIBLE DEFECTS WITH THE ACFT. WE STARTED AND TAXIED TO THE RWY FOR OUR RUN-UP. RUN-UP WAS NORMAL AND NO ENG ABNORMALITIES WERE OBSERVED. TKOF AND INITIAL CLBOUT WERE NORMAL. WHEN TURNING XWIND; WE HEARD A LOUD 'POP' AND THE ENG BEGAN TO RUN ROUGH WITH MODERATE VIBRATION. I LOOKED AT THE ENG GAUGES AND NOTED: OIL PRESSURE; AND OIL TEMP WERE BOTH STABLE AT APPROPRIATE NUMBERS. EGT GAUGE SHOWED A MAX FULL SCALE DEFLECTION; AND FUEL FLOW WAS FLUCTUATING BTWN 10 GPH AND 15 GPH. I WAS THE PF AT THE TIME; SO I KEPT CTL OF THE ACFT. SINCE WE WERE IN THE TFC PATTERN I ELECTED TO RETURN FOR A LNDG. WHILE FLYING THE DOWNWIND I ADJUSTED THE PWR TO 1500 RPM. THE VIBRATIONS DECREASED AND THE ENG RAN SMOOTHER. I EXECUTED A CAUTIONARY LNDG WITH PWR; THE REST OF APCH AND LNDG WERE UNEVENTFUL. WE TAXIED AND SHUT THE ACFT DOWN. UPON INSPECTION OF THE ACFT I NOTED THAT THE FORWARD L CYLINDER (WHEN REFING THE PLT'S SEAT) HAD A BROKEN FUEL INJECTOR LINE. THE LINE HAD SEPARATED ABOUT AN INCH ABOVE WHERE THE LINE CONNECTED TO THE CYLINDER. I ALSO NOTED THAT FUEL WAS DRIPPING OUT OF THE LINE ONTO THE ENG. DURING THE EVENT I ELECTED TO KEEP THE ENG RUNNING. I NOTICED THAT THE OIL TEMP AND PRESSURE WERE AT NORMAL LEVELS; AND WHEN I DECREASED THE RPM THE EGT CAME TO A NORMAL LEVEL. HAD I OBSERVED UNSAFE LEVELS IN ALL 3 GAUGES I WOULD HAVE SHUT AND SECURED THE ENG WHILE SETTING UP FOR A LNDG AT CDK.