Narrative:

It was a cavu day (ceiling and visibility unlimited). Took off for a sightseeing flight. Suddenly the engine started running rough with power reduced and fluctuating (a wake-up call) 200-300 RPM. Checked mixture; gas; carburetor heat; and magnetos. They were all working. I headed direct for ZZZ watching for suitable landing areas as I came upon a sod runway and recognized the landing strip that I used quite often when I was in the military. The facility is deactivated but still used on a limited basis. At this time I decided to make a precautionary landing. As it was saturday there was no activity and no 'X' on the runway. I made an uneventful landing; shut down the engine; and my passenger and I got out to investigate and find the problem. My passenger was my a&P mechanic with inspector authority/authorized; fortunately. The problem was easily found; the carburetor foam air filter was partially displaced and fouled the inlet. The foam was removed; an engine run-up and check-out were made; static RPM 2200 (limits 2175-2250). Engine magneto check; right magneto 50 RPM drop; left magneto also 50-60 RPM drop. With the a&P's ok; I made a normal takeoff and returned home safely. We subsequently inspected the inlet air duct and carburetor 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.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.