ACFT EQUIP PROB MALFUNCTION ON SMA MANDATES A RETURN LAND. NIGHT OP.

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I HAD BEEN FLYING A TFC RPTING FLT IN THE ATLANTIC CITY REGION FOR THE PREVIOUS 5 HRS. AS DUSK APCHED I WAS PLANNING A FINAL CHK OF THE AREA TO THE S OF ATLANTIC CITY WHEN I OBSERVED AN UNFORECAST SCATTERED LAYER BELOW ME. I TURNED BACK TO RETURN THE PNE (NE PHILADELPHIA) AFTER CHKING THE WX WITH FSS. VFR FORECAST CONDITIONS; HOWEVER; A TSTM WAS APCHING THE PHILADELPHIA REGION. THE SCATTERED LAYER; WHICH WAS QUICKLY BECOMING BROKEN; WAS UNFORECAST. I DECIDED TO LAND AT ACY FOR FUEL; BECAUSE THE POSSIBILITY TO DEVIATE EXISTED; AND TO CHK THE NEW TERMINAL RPTS. WHILE ON THE GND; ACY WENT IFR; 800 BROKEN AND 7 MI VISIBILITY. EXPECTING TO COMPLETE THE REMAINDER OF THE FLT VFR; I FILED IFR TO DEPART ACY; I DID ALSO DECIDE TO WAIT FOR THE TSTM IN THE PHILADELPHIA REGION (MY DEST) TO PASS. AFTER WAITING 1 1/2 HRS THE TSTM HAD BROKEN UP LEAVING PNE 2000 FT BROKEN AND 5 MI VISIBILITY WITH LIGHT RAIN. WHILE GETTING MY IFR CLRNC FROM ACY CLRNC; I DEBATED CONTINUING DUE TO THE POOR CONDITION OF THE INTERIOR LIGHTS. I REASONED THAT THE VFR NATURE OF THE MAJORITY OF THE FLT WOULD BE OK. BY THIS TIME IT WAS XA30 PM; I WAS TIRED; HUNGRY ; AND EAGER TO GET HOME. I HAD TO WORK EARLY THE FOLLOWING DAY ON A NON FLYING JOB. I RECEIVED MY CLRNC AND DEPARTED RWY 22. CLBING ON RWY HDG WAS UNEVENTFUL. I CONTACTED DEP AND RECEIVED A NEW VECTOR. WHILE TURNING R AND JUST PASSING THROUGH THE TOP OF THE BROKEN CLOUDS (AT 1200 FT MSL) THE ENG SPUTTERED; SHOOK AND BEGAN LOSING PWR. I TOLD DEP THAT THE ENG WAS ROUGH AND THAT I NEEDED A 'VECTOR BACK TO THE ARPT;' TO WHICH I WAS QUERIED 'WHICH ARPT.' I DID NOT DECLARE AN EMER. I WAS CONSIDERING DECLARING AN URGENCY BUT WAS SIDE TRACKED TRYING TO SOLVE THE ENG PROB. THE RPM'S WERE DECREASING AS THE ENG SHOOK. THEY OFFERED TO VECTOR ME FOR A VOR APCH; BUT I DID NOT WANT TO BE VECTORED FROM THE ARPT. I EXPECTED THE ENG TO QUIT. I COULD SEE THE GND LIGHTS BELOW; I WAS HDG N PRESUMABLY OVER THE ARPT; SO I DSNDED THROUGH A GAP IN THE BROKEN LAYER. I INFORMED APCH THAT I WAS DSNDING THROUGH A BREAK IN THE CLOUDS. I SAW THE ARPT BELOW US BUT MISTAKENLY CALLED MY POS AS DOWNWIND FOR RWY 22; WHEN IT WAS ACTUALLY RWY 31. I LANDED WITHOUT EVENT. I HAD RECEIVED CLRNC TO LAND ON RWY 31; BUT I REALIZED DURING THE ENG TROUBLE THAT I WAS NOT INST CURRENT; AND THE AIRPLANE WASN'T EITHER. REALIZING THIS CAUSED ME TO HESITATE IN DECLARING AN EMER. THE TWR TREATED IT AS AN EMER SIT ANYHOW. I FELT DSNDING THROUGH THE CLOUDS NEAR THE ARPT WAS SAFER THAN RISKING HAVING THE ENG QUIT ALTOGETHER WHILE I WAS BEING VECTORED FOR AN APCH.