PLT OF SMA SEL ACFT FAILED TO MAINTAIN A 'LEAN ENG FUEL MIXTURE' DURING DSCNT INTO A HIGH ALT ARPT RESULTING IN A ROUGH RUNNING ENG AND DECLARING AN EMER. NON TWR ARPT.

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FLT FROM MARCH AFB IN AERO CLUB ACFT TO BIG BEAR ARPT (L58) APPROX 30 NM AWAY. BIG BEAR ARPT IS AT 6750 FT. MARCH AFB (RIV) 1538 FT. BIG BEAR IS SITUATED IN MOUNTAINOUS AREA WITH TOPS OF MOUNTAINS FROM 8400 TO 11500 FT. CLBED ACFT TO 10500 FT TO OVERFLY TOPS. DURING DSCNT TO ENTER TFC PATTERN CALLED FOR LNDG ADVISORY ON UNICOM AND WAS TOLD R TFC FOR RWY 7. WHILE DSNDING TO ENTER ON 45 AN SMT CALLED BASE FOR RWY 7. I OBSERVED THIS SMT ON BASE FOR RWY 25 AND CALLED A WARNING ON THE RWY DIRECTION. SMT RESPONDED THAT THEY WERE GOING AROUND. THE SMT THEN FLEW DOWN THE RWY CTRLINE; DID A 180 DEG TURN APPROX 1 1/2 MI PAST THE THRESHOLD OF RWY 7; AND THEN MADE STRAIGHT-IN APCH. AS SMT WAS ON 1/4 MI FINAL; I WAS ON DOWNWIND JUST ABOUT TO TURN BASE. AT THAT TIME MY ENG BACKFIRED AND BEGAN TO RUN VERY ROUGH. I SWITCHED TANKS; FUEL PUMP ON AND THEN MADE A RADIO CALL DECLARING AN EMER AND ASKING THE SMT TO GAR. HE COMPLIED. WHILE TURNING FINAL DURING MY BEFORE LNDG CHK I RECOGNIZED I HAD THE MIXTURE FULL FORWARD. WHEN I PULLED IT BACK THE ENG SMOOTHED OUT. I BELIEVE MY PREOCCUPATION WITH KEEPING AN EYE OUT FOR THE SMT AND OTHER TFC RESULTED IN MY AUTOMATIC ENTERING THE PATTERN PROC; I.E.; MIXTURE FULL RICH; PROP FULL FORWARD; LOWER GEAR; 10 DEGS FLAPS. I WAS SO PREOCCUPIED I FORGOT TO LEAVE THE MIXTURE LEAN FOR THE DENSITY ALT. ALTHOUGH I LANDED SUCCESSFULLY IT WAS A VERY TENSE (REAL SCARY) TIME.