MOMENTS AFTER THE DC-3 TOOK OFF, THE TWR CTLR NOTED THE L ENG WAS TORCHING & ASKED THE FLT IF IT WAS 'HAVING A PROBLEM.'THE CREW REPLIED 'AFFIRMATIVE,' THEN THE CTLR CLRD THE FLT TO LND ON ANY RWY. THE ACFT BEGAN TURNING LEFT AT THE END OF THE RWY & CONTD TURNING AT LOW ALT UNTIL IT STRUCK UTILITY POLES & CRASHED THRU A WAREHOUSE ROOF ABOUT 2 MI FM THE ARPT.A WITNESS RPRTD THE ACFT WAS AT AN ALT OF APRX 100 TO 150 FT AGL, THE L PROP WAS FEATHERED & THE R ENG WAS AT FULL PWR. AN EXAM OF THE L ENG DISCLOSED THE #14 CYLINDER WAS MISSING A SPARK PLUG FM THE FRONT SPARK PLUG HOLE. THE SPARK PLUG LEAD & 'B' NUT WERE ATTACHED TO THE IGNITION HARNESS, BUT NO PLUG WAS ATTACHED TO THE LEAD. THE SPARK PLUG HOLE & B-NUT THREADS WERE UNDAMAGED. A WHITE DEPOSIT WAS FND INSIDE THE CYLINDER SPARK PLUG THREADS & OUTSIDE THE CYLINDER, ADJACENT TO THE HOLE. THE DEPOSIT WAS IDENTIFIED TO CONTAIN HIGH CONCENTRATIONS OF LEAD & BROMINE, MOST LIKELY FORMED FROM EXHAUST GASES & FUEL DURING CYLINDER OPERATION. A COMPANY MECHANIC RPRTDLY WORKED ON THE PLUGS ON THE PREVIOUS DAY.