Narrative:

I was conducting an instructional flight 11 NM northeast of roa at 3000 ft MSL when the engine started running extremely rough with moderate vibrations. I checked the fuel valve for the 'on' position, confirmed the mixture control was full rich, applied full carburetor heat and noted further power reduction. I then removed carburetor heat, checked each magneto position, then returned the ignition switch to the 'both' position. I set the transponder to 7700 and declared an emergency with roa departure. I informed roa departure that I intended to land at a private airstrip in fincastle, va. During the next several mins, I considered trying to return to roa but was uncertain if the engine would continue to hold partial power. In the interest of safety, I elected to stay with my original decision and land the airplane on the private airstrip which I had in sight. The approach and landing was without incident. An airplane mechanic turned the propeller through by hand several times and determined that a valve was stuck which shut down 1 of the 4 cylinders.

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Original NASA ASRS Text

Title: THE ENG OF A C150 STARTED RUNNING VERY ROUGH CAUSING THE PLT INSTRUCTOR, ON A TRAINING FLT, TO DECLARE AN EMER AND DIVERT TO A NEARBY PVT AIRSTRIP. STUCK ENG VALVE WAS THE SUSPECTED CAUSE.

Narrative: I WAS CONDUCTING AN INSTRUCTIONAL FLT 11 NM NE OF ROA AT 3000 FT MSL WHEN THE ENG STARTED RUNNING EXTREMELY ROUGH WITH MODERATE VIBRATIONS. I CHKED THE FUEL VALVE FOR THE 'ON' POS, CONFIRMED THE MIXTURE CTL WAS FULL RICH, APPLIED FULL CARB HEAT AND NOTED FURTHER PWR REDUCTION. I THEN REMOVED CARB HEAT, CHKED EACH MAGNETO POS, THEN RETURNED THE IGNITION SWITCH TO THE 'BOTH' POS. I SET THE XPONDER TO 7700 AND DECLARED AN EMER WITH ROA DEP. I INFORMED ROA DEP THAT I INTENDED TO LAND AT A PVT AIRSTRIP IN FINCASTLE, VA. DURING THE NEXT SEVERAL MINS, I CONSIDERED TRYING TO RETURN TO ROA BUT WAS UNCERTAIN IF THE ENG WOULD CONTINUE TO HOLD PARTIAL PWR. IN THE INTEREST OF SAFETY, I ELECTED TO STAY WITH MY ORIGINAL DECISION AND LAND THE AIRPLANE ON THE PVT AIRSTRIP WHICH I HAD IN SIGHT. THE APCH AND LNDG WAS WITHOUT INCIDENT. AN AIRPLANE MECH TURNED THE PROP THROUGH BY HAND SEVERAL TIMES AND DETERMINED THAT A VALVE WAS STUCK WHICH SHUT DOWN 1 OF THE 4 CYLINDERS.

Data retrieved from NASA's ASRS site as of July 2007 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.