A PVT SMA PLT LANDED OFF ARPT WITH A ROUGH RUNNING ENG. THERE WAS NO DAMAGE OR INJURY RPTED.

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AT ROUGHLY 1000 AGL (DUE TO FLOOR OF TCA IN THAT AREA) I EXPERIENCED ENG ROUGHNESS. I HAD LEFT THE PROP CTL IN FULL AND THROTTLE FULL WHICH I DON'T NORMALLY DO (IT WAS TO BE A SHORT FLT. HADN'T FLOWN FOR 2 WKS). IT DID NOT SEEM A NORMAL (750 HRS EXPERIENCE) SITUATION. OIL TEMP WAS HIGH; PRESSURE SEEMED LOWER; JUST BELOW GREEN AND ENG WAS ROUGH. I SPOTTED A LARGE; UNINHABITED PASTURE AND INITIATED A PRECAUTIONARY LNDG UNDER PWR (REDUCED; I THINK). AFTER HIKING OUT AND RETURNING WITH HELP; WE DID A RUNUP; A COMPLETE CHK OF ACFT (AFTER ADDING ADDITIONAL FUEL TO ENSURE ADEQUATE VFR RESERVES AND PRECLUDE FUEL CONTAMINATION). WALKED THE AREA; CHARTED TKOF DISTANCE; AND DEPARTED AREA TO NEAREST ARPT. IN RETROSPECT IT MAY HAVE BEEN A COMBINATION OF DENSITY ALT (LACK OF PROPER COOLING AND THINNING OF 20-50 OIL) AND CARB ICE OR A SMALL AMOUNT OF CONTAMINATION IN FUEL. I AM REALLY NOT SURE. UPON OUR RETURN AND SUBSEQUENT RUNUP; EVERYTHING SEEMED NORMAL. SINCE IT WAS PRIVATE PROPERTY AND NEITHER I; THE ACFT; OR ANY PROPERTY WAS DAMAGED; I FEEL THAT THE DECISION WAS THE RIGHT ONE. CONTINUED FLT MIGHT HAVE RESULTED IN REDUCED CHOICES. ONE OF THE PLTS SUGGESTED CARB ICE AND THE SYMPTOMS SEEM TO FIT; BUT I GUESS FUEL CONTAMINATION (WATER?) MIGHT ALSO. I DID DRAIN BOTH WING TANKS AND GASCOLATOR DRAIN. THE ACFT HAS BEEN FINE SINCE. SINCE I HAVE NOT HAD ANY ACCIDENTS OR INCIDENTS IN 8 HRS OF FLYING; I COULD ONLY WISH THAT I HAD BEEN HIGHER AGL SO THAT I WOULD HAVE HAD MORE TIME TO LET CARB HEAT WORK OR THE ENG TO PROCESS THE CONTAMINATION BEFORE 'COMMIT' TIME. I FEEL THE BIGGEST CAUSE OF INJURY ACCIDENTS AND ACFT DAMAGE IS THE FAILURE TO MAKE A DECISION. I DID NOT DECLARE AN EMER BECAUSE I DID NOT FEEL THAT I WAS IN MORTAL DANGER AT ANY TIME.