PARTIAL ENG FAILURE; OFF ARPT LNDG.

Narrative

WHILE I WAS FLYING MY 1941 J-3 CUB EBOUND OVER THE CITY OF BOYNTON BEACH A STRONG VIBRATION DEVELOPED IN THE ENG COMPARTMENT. PRIOR TO THE VIBRATION DEVELOPING; I HEARD A BANG COME FROM THE AREA OF THE ENG COMPARTMENT. ONCE THE VIBRATION DEVELOPED THE ENG STARTED TO LOSE PWR AND THE ACFT STARTED TO DSND. SEEING THAT THE ACFT WAS LOSING PWR AND ALT; I TURNED TO LANTANA ARPT SO THAT I COULD LAND THERE AND FIND OUT WHAT WAS CAUSING THE VIBRATION. ENRTE TO LANTANA ARPT; I DETERMINED THAT THE ACFT DID NOT HAVE ENOUGH PWR OR ALT TO MAKE LANTANA ARPT. AT THIS TIME; I CIRCLED OVER A FIELD JUST S OF A BUILDING AND ATTEMPTED TO GAIN ALT WITH NEGATIVE RESULTS. NOT BEING ABLE TO GAIN ALT; AND IN FACT LOSING ALT; I LANDED IN THE FIELD S OF THE BUILDING WITHOUT DAMAGING THE ACFT OR CAUSING PERSONAL INJURY. ONCE I HAD SAFELY LANDED THE ACFT; I SHUT DOWN THE ENG AND GOT OUT OF THE AIRPLANE. I THEN WENT TO THE ENG COMPARTMENT AND SAW THAT A SPARK PLUG WAS HANGING BY ITS PLUG WIRE FOR THE L SIDE TOP FRONT CYLINDER. SEEING THIS I PUT THE SPARK PLUG BACK INTO IT'S RESPECTIVE HOLE AND THEN STARTED THE ENG. UPON STARTING THE ENG AND RUNNING IT; I SAW THAT THERE WAS NO LONGER A VIBRATION AND THAT THE ENG WAS DEVELOPING NORMAL PWR. THE ACFT HAD TO BE TRUCKED OUT ON A FLAT BED TRUCK THOUGH BECAUSE OF LCL ORDINANCE WHICH PROHIBITED ACFT FROM TAKING OFF OR LNDG IN THE CITY LIMITS. THE ONLY REASON THAT I CAN DETERMINE THAT THE SPARK PLUG CAME OUT OF THE CYLINDER IS THAT I DID NOT TORQUE IT ENOUGH WHEN I HAD PUT IT BACK INTO THE CYLINDER. I HAD JUST REPLACED ALL THE OLD SPARK PLUGS WITH NEW ONES.