A B747-200 WAS DISPATCHED IN NON COMPLIANCE WITH #2 ENG N1 INDICATOR DEFERRED INOP; BUT MEL WT LIMITS NOT OBSERVED.

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UPON DEP FROM SVMI THE N1 TACHOMETER INST FROM ENG #2 WAS NOT INDICATING THE EXPECTED READING ON IDLE THRUST. CIRCUIT BREAKER WAS RECYCLED AND INST CONTINUED ERRATIC. NO N1 ROTATION ALTERNATE PROC WAS CONDUCTED AND GND CREW CONFIRMED N1 ROTATION FROM #2 ENG OK. SINCE WE WERE STILL AT THE TARMAC; MEL WAS CONSULTED AND MAINT WAS CALLED UPON THE ACFT. THE ITEM WAS DEFERRED FOR ZZZ; OUR NEXT DEST; AND THE WT PENALTIES DESCRIBED ON THE MEL WERE APPLIED FOR OUR TKOF PERFORMANCE OUT OF SVMI. THE PAYLOAD WAS NOT AFFECTED; AS THE ACFT WAS BASICALLY FERRIED WITH 3500 KGS ONLY TO PICK-UP THE CARGO IN ZZZ. THE N1 DURING TKOF ROLL GOT BACK TO NORMAL READING; AND WE DISCOVERED THEN; THAT THE INST WAS FAULTY ONLY ON IDLE THRUST. THE FLT FROM SVMI TO ZZZ WAS UNEVENTFUL AND CAUTION WAS TAKEN IN USING OTHER ENG PARAMETERS TO PROTECT THE ENG FOR POSSIBLE EXCEEDANCES SHOULD THE N1 INST GO INOP. IN ZZZ MAINT LIAISED WITH XXX MAINT OFFICE TO REPLACE THE N1 INST AND THE ACFT SPARES CABINET WAS OPENED FOR THAT CONTINGENCY. THE N1 INST WAS REPLACED AND THEN SWAPPED; AND THE ENG #2 WAS CRANKED TO VERIFY THE N1 INST READING. THE N1 INST ON IDLE THRUST WAS AGAIN UNRELIABLE AND WAS AGAIN DEFERRED (DMI) TO MAINT STATION MIA. DURING THE TKOF ROLL OUT OF ZZZ THE N1 INST RETURNED TO NORMAL AND WAS WRITTEN ON THE TECH-LOG ONCE AGAIN FOR PROPER INSPECTION IN MIA. AFTER REVIEW OF PROCS INFLT; THE FE AND I REALIZED THAT THE TKOF PENALTIES LISTED ON THE MEL WERE NOT APPLIED FOR THE TKOF PERFORMANCE OUT OF ZZZ. WE BASICALLY OVERLOOKED THE N1 INST WT PENALTY LIMITATIONS ON THE PREMISE THAT THE INST WAS NOT INOP BUT RATHER ERRATIC. I FEEL THE N1 INST PROB AND THE MAINT ACTION TAKEN IN ZZZ LEAD TO AMBIGUITY. ON THE ONE HAND THE COMPANY DISPATCH DID NOT MAKE ANY MENTION ON THE CFP (COMPUTER FLT PLAN ZZZ-MIA) AND THE RATIONALE BEHIND IT WAS THAT BY MAINT CHANGING THE INST IN ZZZ; WE ALL THOUGHT THE PROB WAS OVER. THE N1 INST ON IDLE THRUST WAS ALWAYS ERRATIC AND UNRELIABLE. OTHER ENG PARAMETERS WERE XCHKED AND GND CREW WAS ALWAYS CONSULTED FOR CONFIRMATION OF N1 ROTATION. HAVING SAID THE ABOVE; I FELT I SHOULD HAVE TAKEN THE CONSERVATIVE APCH OF DECLARING THE N1 INST INOP; AND THEREFORE APPLYING THE WT PENALTIES DISCUSSED FROM THE MEL SECTION UNDER N1. THE TKOF PERFORMANCE FROM ZZZ TO MIA DID NOT TAKE INTO CONSIDERATION THIS WT PENALTY OF AROUND 6000 KGS FROM THE MRTOGW.