A C180 PILOT REPORTS A ROUGH RUNNING ENG AFTER AN IDLE DESCENT FROM 11500 FT TO 5000 FT. A MAG CHECK LEADS TO SUSPICION OF FOULED PLUGS. AFTER POWER ADJUSTMENTS THE ENG SMOOTHED OUT.

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I WAS INVITED TO LAND BY THE OWNER OF A PVT AIRSTRIP. I MADE ALL THE REQUIRED PREFLT PLANNING; AS WELL AS A BRIEFING FROM THE OWNER ABOUT THE AIRSTRIP. I MADE A SLOW DSCNT FROM 11500 FT MSL TO THE STRIP AT ABOUT 5000 FT MSL. I WAS THE SOLE OCCUPANT WITH A LIGHT FUEL LOAD; THUS THE ACFT WAS ABOUT 600 LBS BELOW GROSS WT AND THE AIR WAS ALREADY RISING SOMEWHAT AS THE SUN BEGAN TO WARM THE GND. A FAIRLY LOW PWR SETTING WAS NEEDED TO GET DOWN. I'D NOT BEEN TO THIS PVT AIRSTRIP BEFORE SO WITH THE ENG CTLS SET FOR APCH/LNDG (RICH MIXTURE AND CARB HEAT 'ON'); I MADE TWO SLOW; LOW PWR; L HAND PATTERNS TO INSPECT THE LNDG AREA; THE ENG SEEMED TO BE RUNNING SMOOTHLY. WHEN ON SHORT FINAL; THE SIGHT PICTURE OF THE RWY CHANGED SOMEWHAT DUE TO SOME TALLER WEEDS BLOCKING MY VIEW OF THE SURFACE TEXTURE; I DECIDED TO OVERFLY THE TOUCHDOWN AREA THEN DO A GAR. INCREASING ENG ROUGHNESS DEVELOPED AS PWR WAS ADVANCED CAUSING ME TO CHOOSE A L TURN TOWARD AN OPEN FIELD RATHER THAN PROCEEDING STRAIGHT AHEAD OVER AN OBSTACLE. I STOPPED ADVANCING THE THROTTLE WHEN I HAD ENOUGH PWR TO GET BACK TO FINAL AS IT SEEMED TO GET ROUGHER THE MORE IT WAS ADVANCED. THIS L TURN INSURED A CROP-FIELD AREA FOR IMMEDIATE FORCED LNDG IF PWR FAILED. IT ALSO CAUSED ME TO BASICALLY MAKE A LOW; CLOSE-IN L TFC PATTERN BACK TO FINAL. BACK ON FINAL AGAIN AND IN POS FOR NORMAL LNDG; I ADVANCED THE THROTTLE TO FULL. THE ENG CONTINUED TO BE ROUGH FOR A MOMENT BUT THEN GAVE FULL AND SMOOTH PWR. I DID A GAR STRAIGHT AHEAD THIS TIME UP TO A SAFE ALT TO DO A MAGNETO CHK; WHERE I FOUND THE ENG WAS ROUGH ON 1 MAGNETO BUT NOT BOTH. I ADJUSTED A LOW CRUISE PWR SETTING AND LEANED FOR PEAK EGT. AFTER A FEW MINS THE ENG WAS SMOOTH ON EITHER MAGNETO AS WELL AS BOTH. I RETURNED TO THE LNDG STRIP AND DID A NORMAL L PATTERN TO A NORMAL LNDG. LATER I DID A NORMAL RUN-UP AND L TFC DEP WITHOUT FURTHER PROB. I'VE SURMISED THAT SPARK PLUGS MUST HAVE FOULED DURING THE LONG LOW PWR DSCNT/APCH; THE PRE-LNDG MIXTURE SETTING ALONG WITH THE CARB HEAT 'ON' LIKELY CAUSED AN OVERLY RICH FUEL/AIR MIXTURE CONDITION.