ACR MLG TAILPIPE FIRE ON START UP.

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HAD A TAILPIPE FIRE DURING SECOND START ATTEMPT OF #2 ENG. I WAS UNABLE TO PUT OUT FIRE BY MOTORING ENG. MAINT USED GND FIRE BOTTLE OF PURPLE K THROUGH FRONT OF ENG TO EXTINGUISH FIRE. THE FOLLOWING IS A SEQUENCE OF EVENTS: THE F/O AND I SHOWED AT APPROX XA35L TO PERFORM ORIGINATING PREFLT. WE PULLED OUT THE HANDBOOK AND USED BOOK TO PERFORM ALL COCKPIT/FLT STATION CHKS. DURING PUSHBACK ATTEMPTED START OF #2 ENG--NO LIGHT OFF. I PUT FUEL LEVER TO CUT OFF AND MOTORED ENG FOR 60 SECS. BOTH F/O AND I THOUGHT MAYBE ONE OF THE IGNITORS WAS NOT WORKING; SO I SELECTED BOTH. ATTEMPTED START OF #1 ENG. NO LIGHT OFF. FOLLOWED SAME PROC. ASKED GND MECH ON HEADSET IF HE WAS AWARE OF ANY PROBS OR LOCATION OF CB'S. HE STATED THAT MAINT HAD WORKED ON ACFT IGNITORS ON RON AND THAT CB'S MIGHT BE OPEN. SURE ENOUGH; ALL 4 CB'S WERE OUT. THERE WAS NO WRITE-UP IN FORMS TO ADVISE CREW OF MAINT WORK. BY NOW AT LEAST 3 MINS HAD GONE BY SINCE FIRST ATTEMPTED START OF #2 ENG. INITIATED START OF #2 ENG AFTER CB'S WERE RESET. HAD A NORMAL START INDICATION FROM THE FLT STATION INDICATIONS. GND MECH ON HEADSET ADVISED THAT WE HAD A TAILPIPE FIRE--FLAMES WERE SHOOTING 15' OUT TAIL OF ENG--ALSO KNOWN AS TORCHING. I CONTINUED START FOR APPROX 15 SECS AND STILL ADVISED FIRE. I PUT FUEL LEVER TO CUT OFF. FLAME REDUCED TO 4'; BUT STILL FIRE. CONTINUED TO MOTOR ENG. CALLED FOR FIRE DEP. PULLED FIRE HANDLE AND DISCHARGED AGENT. STILL HAD FIRE IN TAILPIPE. ANOTHER MECH CAME TO ACFT WITH GND FIRE BOTTLE AND EXTINGUISHED FIRE BY USING AGENT INTO FRONT OF ENG. WE WERE STILL MOTORING ENG UNTIL FIRE WAS OUT. PAX WERE TAKEN OFF ACFT NORMALLY. SUGGESTIONS: CB'S PULLED BY MAINT SHOULD BE TAGGED AND RESET; A NONTOXIC EXTINGUISHER SHOULD BE IN GND BOTTLES; WHY DIDN'T MOTORING ENG CLEAR FUEL?; AND CHANGE START ABNORMAL PROCS TO INDICATE CHK AND DRAIN FUEL BEFORE SECOND ATTEMPT START.