GA SMA DEVELOPED ENG PROBLEMS AT CRUISE ALT OF 6000 FT. PLT UNABLE TO RESOLVE PROBLEM; DECLARED AN EMER FOR A LNDG AT DTW.

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PERFORMED THOROUGH PREFLT. DID RUNUP PER CHKLIST -- NORMAL. DEPARTED OWOSSO NORMAL. 20 MINS INTO FLT NOTICED SLIGHT ENG MISS AT 6000 FT. TRIED CARB HEAT; MIXTURE ADJUSTMENT -- NO CHANGE. DSNDING FROM 6000 TO 3000 ENG RAN WORSE. PERFORMED EMER PROCS FOR ROUGH RUNNING ENG/ENG FAILURE -- SWITCH FUEL TANKS; MIXTURE FULL RICH THEN DIFFERENT SETTING VARIOUS THROTTLE MIXTURE; CARB HEAT SETTINGS/COMBINATIONS. CHKED BOTH MAGNETOS -- NO IMPROVEMENT. TURNED FUEL BOOST PUMP ON -- NO CHANGE. CHKED PRIMER -- OK. COULD ONLY GET 20 INCHES OF MANIFOLD PRESSURE. WAS MAINTAINING 3000 MSL. AIRSPD WAS DOWN TO 85 KIAS. AT 9 DME FROM ARPT TOLD APCH HAD ROUGH RUNNING ENG. REQUESTED PRIORITY. DID NOT HAVE RWY IN SIGHT. RECEIVED VECTOR -- L TURN TO HDG 140. I SAW THE ARPT JUST ABOUT SAME TIME AS RECEIVING VECTOR. AT 6 DME ENG CONTINUALLY GOT WORSE. I FELT IF I DIDN'T HEAD STRAIGHT FOR THE ARPT AN OFF FIELD LNDG WOULD HAVE TO BE MADE AS A COMPLETE ENG FAILURE WAS COMING. I DECLARED AN EMER AND WAS CLRED TO LAND ON RWY 21L. LANDED WITHOUT ACCIDENT; INCIDENT OR INJURY. TAXIED TO FBO. TALKED TO MECH THAT IS CHKING AIRPLANE. SO FAR THE MECH HAS FOUND THE R MAGNETO COMPLETELY DEAD AND THE L MAGNETO VERY BAD. I HAVE NO IDEA AS TO WHY THE PROBLEM OCCURRED OR HOW TO PREVENT IT AS THE RUNUP WENT NORMAL. (I HAVE APPROX 200 HRS IN THIS SPECIFIC AIRPLANE.) I HAVE NO IDEA HOW TO PREVENT IT AS THE PROBLEM OCCURRED APPROX 20-25 MINS INTO THE FLT. THE REASON I WENT FROM REQUESTING PRIORITY TO DECLARING AN EMER WAS TO LET ATC KNOW THE SEVERITY OF THE PROBLEM AND TO PREVENT POSSIBLY GETTING ANY VECTORS THAT WOULD TAKE MY ANY FURTHER FROM THE ARPT. I'M NOT BLAMING ATC FOR ANYTHING; JUST THAT WHEN THE VECTOR WAS GIVEN TO ME THE ENG WAS ALMOST COMPLETELY GONE. THAT'S WHY I DECLARED AN EMER. I DON'T BELIEVE ANY FARS WERE VIOLATED BY THIS ACTION.