Narrative:

During cruise flight at 6500 ft; while receiving flight following from TRACON; the engine suddenly ceased producing power; running very roughly. I declared an emergency; requested vectors to the nearest airport; attempted troubleshooting the cause (details below) and ultimately landed uneventfully with partial but sporadic power. The cause of the power interruption is still undetermined at this time. I had 2 passengers on board. We had departed 15 minutes earlier. I have a fuel flow meter and totalizer installed on the aircraft. The last refueling I personally topped the tanks (total 80 gallons) and re-set the totalizer. According to my calculations; I had used approximately 36 gallons of fuel; leaving 44 for the return flight home. The fuel totalizer agreed with this; showing 36 used upon start-up. I had conducted a thorough preflight; sumping both the wing tanks and header tanks on the aircraft; as well as opening the strainer drain in the engine compartment. There was no evidence of water in fuel. Run-up was normal. I departed; climbed to a safe altitude; and turned on course. Over the reservoir; I contacted approach for flight following and got a discrete squawk code. Engine performance was normal during the climb phase. I leaned the mixture as we climbed through 3000 ft MSL. I leveled off in cruise at 6500 ft. When we were over the north end of the reservoir; the engine suddenly sputtered and lost power. I immediately switched fuel tanks (it is a two tank system with no 'both' setting) and put the prop in full fine pitch. The engine continued to sputter. I richened the mixture and activated the auxiliary boost pump. There was a temporary improvement; but then engine continued to run very roughly. I applied high boost setting for the auxiliary fuel pump without any difference in results. I switched fuel tanks again. The engine continued to sputter; running very roughly and not producing power. About 1/2 to 1/4 of the time; the engine would suddenly run smoothly and produce maximum power. I checked the magnetos several times. Running on a single magneto (either left or right) only seemed to make the engine run worse; so I returned the ignition setting to both. I raised the nose and trimmed for 70 KTS; maximum range glide speed. Because the engine continued to make full power for part of the time; I eventually was able to climb to 8500 ft by the time I arrived overhead ZZZ. I found that moving the mixture knob so that it was about 1.5 inches out (lean) and closing the throttle to about 11 inches of mp resulted in low power; but smoother running. I maintained that power setting and circled down over the airport. Power was occasionally interrupted as we descended; but we had landing assured at ZZZ and I so advised approach. I performed a normal landing on and taxied under power to the ramp. After shutdown and taking care of my family; I called TRACON per their request to advise that we were safely on the ground and to thank them for their help. I sumpted the fuel and header tanks again and ran the strainer drain. Again; there was no evidence of water in the fuel. I tasted the fuel on my fingertips. I removed the cowling from the aircraft. There was no fuel stain or odor. All injector lines were in tact. The air filter was unobstructed. In short; there was nothing irregular that would explain the power interruption. I secured the aircraft; loaded the baggage into the rental car and returned home.

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Original NASA ASRS Text

Title: C206 pilot experiences a rough running engine at 6500 feet and is able to land safely at a nearby airport.

Narrative: During cruise flight at 6500 FT; while receiving flight following from TRACON; the engine suddenly ceased producing power; running very roughly. I declared an emergency; requested vectors to the nearest airport; attempted troubleshooting the cause (details below) and ultimately landed uneventfully with partial but sporadic power. The cause of the power interruption is still undetermined at this time. I had 2 passengers on board. We had departed 15 minutes earlier. I have a fuel flow meter and totalizer installed on the aircraft. The last refueling I personally topped the tanks (total 80 gallons) and re-set the totalizer. According to my calculations; I had used approximately 36 gallons of fuel; leaving 44 for the return flight home. The fuel totalizer agreed with this; showing 36 used upon start-up. I had conducted a thorough preflight; sumping both the wing tanks and header tanks on the aircraft; as well as opening the strainer drain in the engine compartment. There was no evidence of water in fuel. Run-up was normal. I departed; climbed to a safe altitude; and turned on course. Over the Reservoir; I contacted Approach for flight following and got a discrete squawk code. Engine performance was normal during the climb phase. I leaned the mixture as we climbed through 3000 FT MSL. I leveled off in cruise at 6500 FT. When we were over the north end of the Reservoir; the engine suddenly sputtered and lost power. I immediately switched fuel tanks (it is a two tank system with no 'both' setting) and put the prop in full fine pitch. The engine continued to sputter. I richened the mixture and activated the auxiliary boost pump. There was a temporary improvement; but then engine continued to run very roughly. I applied high boost setting for the auxiliary fuel pump without any difference in results. I switched fuel tanks again. The engine continued to sputter; running very roughly and not producing power. About 1/2 to 1/4 of the time; the engine would suddenly run smoothly and produce maximum power. I checked the magnetos several times. Running on a single magneto (either left or right) only seemed to make the engine run worse; so I returned the ignition setting to both. I raised the nose and trimmed for 70 KTS; maximum range glide speed. Because the engine continued to make full power for part of the time; I eventually was able to climb to 8500 FT by the time I arrived overhead ZZZ. I found that moving the mixture knob so that it was about 1.5 inches out (lean) and closing the throttle to about 11 inches of MP resulted in low power; but smoother running. I maintained that power setting and circled down over the airport. Power was occasionally interrupted as we descended; but we had landing assured at ZZZ and I so advised Approach. I performed a normal landing on and taxied under power to the ramp. After shutdown and taking care of my family; I called TRACON per their request to advise that we were safely on the ground and to thank them for their help. I sumpted the fuel and header tanks again and ran the strainer drain. Again; there was no evidence of water in the fuel. I tasted the fuel on my fingertips. I removed the cowling from the aircraft. There was no fuel stain or odor. All injector lines were in tact. The air filter was unobstructed. In short; there was nothing irregular that would explain the power interruption. I secured the aircraft; loaded the baggage into the rental car and returned home.

Data retrieved from NASA's ASRS site as of April 2012 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.