THE CREW STATED THAT DURING AN ATTEMPTED START THE NO. 1 ENG FAILED TO GAIN IGNITION DURING THE PRESCRIBED 25 SEC LIMIT AND THAT THEY ABORTED THE START. SHORTLY THEREAFTER, GRND PERS REPORTED THAT THE ENG HAD DEVELOPED A TAIL PIPE FIRE. FOLLOWING EMERG PROCEDURES, THE FIRE PERSISTED AND THE CAPT ORDERED THE ACFT EVACUATED. FOUR PAX RECEIVED MINOR INJURIES. INVESTIGATION REVEALED THAT THE 'B' IGN SYS WAS INOP AND PLACARDED AS SUCH AND THAT THE 'A' SYS WAS FUNCTIONING NORMALLY. EXAMINATION REVEALED A SHORT IN THE 'B' SYS HARNESS BETWEEN THE EXCITER AND THE ING PLUG. FOLLOWING REPLACEMENT OF THE 'B' HARNESS, BOTH IGN SYS AND THE ENGINE PERFORMED NORMALLY. THE TAIL PIPE FIRE COULD BE LIT OFF FOLLOWING AN ATTEMPTED START ON THE 'B' SYS, WITH A SUBSEQUENT SECOND START OR SWITCH OVER TO THE 'A' SYS WITHOUT PURGING THE ENG OF THE INITIAL START FUEL. EITHER SEQUENCE WOULD RESULT IN TORCHING.