STINSON PLT ON A VFR SIGHTSEEING FLT EXPERIENCES ROUGH RUNNING ENGINE AND MAKES PRECAUTIONARY LNDG ON GRASS STRIP.

Narrative

IT WAS A CAVU DAY (CEILING AND VISIBILITY UNLIMITED). TOOK OFF FOR A SIGHTSEEING FLT. SUDDENLY THE ENG STARTED RUNNING ROUGH WITH PWR REDUCED AND FLUCTUATING (A WAKE-UP CALL) 200-300 RPM. CHKED MIXTURE; GAS; CARB HEAT; AND MAGNETOS. THEY WERE ALL WORKING. I HEADED DIRECT FOR ZZZ WATCHING FOR SUITABLE LNDG AREAS AS I CAME UPON A SOD RWY AND RECOGNIZED THE LNDG STRIP THAT I USED QUITE OFTEN WHEN I WAS IN THE MIL. THE FACILITY IS DEACTIVATED BUT STILL USED ON A LIMITED BASIS. AT THIS TIME I DECIDED TO MAKE A PRECAUTIONARY LNDG. AS IT WAS SATURDAY THERE WAS NO ACTIVITY AND NO 'X' ON THE RWY. I MADE AN UNEVENTFUL LNDG; SHUT DOWN THE ENG; AND MY PAX AND I GOT OUT TO INVESTIGATE AND FIND THE PROB. MY PAX WAS MY A&P MECH WITH INSPECTOR AUTH; FORTUNATELY. THE PROB WAS EASILY FOUND; THE CARB FOAM AIR FILTER WAS PARTIALLY DISPLACED AND FOULED THE INLET. THE FOAM WAS REMOVED; AN ENG RUN-UP AND CHK-OUT WERE MADE; STATIC RPM 2200 (LIMITS 2175-2250). ENG MAGNETO CHK; R MAGNETO 50 RPM DROP; L MAGNETO ALSO 50-60 RPM DROP. WITH THE A&P'S OK; I MADE A NORMAL TKOF AND RETURNED HOME SAFELY. WE SUBSEQUENTLY INSPECTED THE INLET AIR DUCT AND CARB THROAT FOR FOREIGN OBJECTS (FOAM) -- NONE FOUND. INSTALLED A NEW FILTER. INTERESTINGLY; NO ONE SHOWED UP AT THE FACILITY. JUST A QUIET DAY FOR 2 OLDIES.