ENG RUNS ROUGH; EMER DECLARED AND AN EMER LNDG WAS MADE.

Narrative

AT 10 MINS AFTER LEVELING AT 6000 FT MSL; DIRECT LANSING VOR; MY STUDENT AND I EXPERIENCED A QUICKLY DEVELOPING - ROUGH RUNNING ENG IN OUR PIPER PATHFINDER. I TRIED TO SMOOTH THE ROUGH RUNNING ENG BY CHANGING FUEL TANKS FROM R TIP TO R MAIN TANK. I THEN TURNED THE FUEL PUMP (ELECTRIC) ON AND CHKED THE FUEL PRESSURE WHICH INDICATED NORMAL. THE ENG CONTINUED RUNNING ROUGH SO I INSTRUCTED MY STUDENT TO ALERT SAGINAW APCH OF OUR PROB AND ASKED FOR A VECTOR TO THE NEAREST ILS. AFTER 2 OR 3 BROKEN XMISSIONS I TOOK THE MIKE AND DECLARED AN EMER. I THEN FOLLOWED A VECTOR TOWARDS THE LOC FOR RWY 5 AT SAGINAW. AS I WAS TURNING FROM 210 DEGS TO 030 DEGS I PULLED ON CARB HEAT WHICH MADE THE ENG RUN EVEN MORE ROUGH. I THAN MOVED THE MIXTURE LEVER OFF TO LEAN THE ENG BUT IT CONTINUED TO RUN ROUGH. AFTER ROLLING OUT ON THE 030 DEGS ASSIGNED HDG I ASKED MY STUDENT TO VERIFY OUR GND SPD AS WE WERE DSNDING TOWARDS 5000 FT MSL. HE CALLED OUT 160 KTS GND SPD FROM THE GPS (HAND HELD). WE INTERCEPTED THE LOC AND PASSED OVER THE FAF 1200 FT HIGHER THAN NORMAL. I WAS ANTICIPATING THE ENG QUITTING SO I WANTED TO HAVE EXTRA ALT AND AIRSPD IN ORDER TO MAKE THE FIELD (RWY 05 OR 23 AT MBS). AT 2 1/2 MI FROM RWY 05 AND 12000 FT AGL WE BROKE OUT OF THE CLOUDS I IMMEDIATELY BEGAN ON OUR TURN IN ORDER TO SLOW TO FLAP SPD. I EXECUTED THIS DIRECTLY OVER THE RWY ENVIRONMENT TO ASSURE OBSTACLE AVOIDANCE; AS THE VISIBILITY WAS VARYING BECAUSE OF BLOWING SNOW. WE ALSO EXPERIENCED PLUS OR MINUS 5 KTS FLUCTUATION ON THE AIRSPD AND LIGHT TURB. RWY 5 IS 8002 FT BY 150 FT; THE WIND WAS 210 DEGS AT 25 KTS. WE TOUCHED DOWN AT THE 6000 FT MARKER (ESTIMATED); DROPPED (RETRACTED) OUR 20 DEGS FLAPS TO DESTROY LIFT AND GENTLY TESTED THE BRAKES. BRAKING ACTION WAS FAIR - SOME SLIDING DUE TO PATCHY SNOW AND ICE. WE EXITED THE RWY AT THE LAST TXWY AND REMOVED CARB HEAT; TURNED OFF UNNECESSARY ELECTRICALS - IE; PITOT HEAT; LNDG LIGHT; ELECTRIC FUEL PUMP; THEN OBSERVED THE GAUGES - ALL INDICATED NORMAL. I ALSO KEPT A CLOSE WATCH ON THE COWLING FOR SMOKE AS CARB ICE WAS ONLY A SUSPECTED NOT PROVEN 100% OF THE CAUSE OF THE ROUGH RUNNING ENG. AFTER REACHING THE RAMP I SHUT DOWN AND WE EGRESSED THE ACFT. I WILL HAVE AN ACFT MECH THOROUGHLY CHK THE ACFT BEFORE I FLY IT BACK TO GOSHEN GSH WHEN THE WX IMPROVES TO GOOD VFR - DAY; FOR THE ENTIRE RTE. I ATTRIBUTE THE SUCCESSFUL OUTCOME TO GOOD CREW COORD; AND THE FACT THAT I WAS WELL RESTED; ALERT AND HAD JUST EATEN LUNCH BEFORE THE FLT.