A C150 IN CRUISE AT 4000 FT EXPERIENCES LOSS OF PWR AND DIVERTS DUE TO 2 TOTALLY FOULED SPARK PLUGS.

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I WAS PIC AND AN INST INSTRUCTOR WITH STUDENT ON AN IFR TRAINING FLT. IMMEDIATELY AFTER BEING VECTORED AND CLRED DIRECT ON NAV TO SAX VOR; THE ENG LOST PWR. CARB HEAT RESTORED PWR FOR ABOUT 1 MIN. THEN THE RPM DROPPED TO ABOUT 1500; INSUFFICIENT TO MAINTAIN ALT. IT WAS DARK; I LOOKED OUT THE R SIDE OF THE PLANE AND RWY LIGHTS WENT ON DIRECTLY BELOW. LOSING ALT AND UNABLE TO RESTORE RPM; WE IMMEDIATELY HEADED TOWARD THE ARPT. APCH ADVISED US OF TFC AT 3000 FT; WHICH WE SAW SINCE IT WAS LIGHTED LIKE A CHRISTMAS TREE. ADVISED APCH OF ENG TROUBLE; RWY BENEATH US; AND CANCELED IFR TO SWITCH TO UNICOM TO INSURE LIGHTS ON AND ADVISE ANY TFC OF OUR DIFFICULTY. CTLR GAVE US PHONE NUMBER TO CALL ON LNDG WHICH DELAYED OUR UNICOM ANNOUNCEMENT. ARPT HAD LIGHTS ON REQUEST ONLY; SO WE WERE FORTUNATE. SUBSEQUENT TO LNDG; 2 TOTALLY LEAD FOULED PLUGS WERE FOUND. ONLY 60 HRS SINCE 100 HR INSPECTION AND IN SPITE OF AGGRESSIVE LEANING. HAD WE NOT SEEN RWY LIGHTS AND BEEN OVER AN ARPT WOULD HAVE DECLARED AN EMER AND HOPED.