Narrative:

I was in contact with phoenix approach for flight following. I was about 25 miles southwest of sky harbor and had reached my cruising altitude of 10;500 ft; and had leaned the mixture for this new altitude. About 30 seconds later; the engine began losing power. At first; I believed this to be due to over-leaning the engine; so I richened the mixture. This did not clear the issue; so I added full mixture and full power and still had intermittent power from the engine. I contacted phoenix approach to announce the engine issue and turned back to mobile airport; which was just a couple of miles behind me. While troubleshooting the engine; I circled the airport as we descended to around 5;000 ft. At that point; full power returned to the engine. I continued to circle over mobile airport until I was able to climb back up to 10;500 ft. After the engine power remained stable at full power for about 20 minutes; I requested a return to my departure airport and was cleared to go directly there and chose to remain at 10;500 ft for as long as possible; because while I believed the engine issue to be resolved; I wanted to have sufficient altitude to reach any airport just in case. I was cleared to descend through bravo airspace and then handed off to my departure airport's tower; and was able to land without incident. The engine made full power and appeared to have no problems after the point where power was restored. The fuel tanks had been topped off the night prior to the flight; and no water was seen when sampling the fuel on the day of the flight just prior to departure. It was a hot dry day; so water and carb ice were not suspected. I have since contacted my mechanic; and he is currently checking the engine and fuel system to see if any issue is seen.

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Original NASA ASRS Text

Title: C172 pilot experiences a rough running engine upon leveling off at 10;500 FT and leaning the engine for cruise. The reporter elects to spiral down over a nearby airport but when the engine returns to normal he decides to climb back up to 10;500 and return to the departure airport. This is accomplished successfully with no further engine troubles.

Narrative: I was in contact with Phoenix Approach for flight following. I was about 25 miles southwest of Sky Harbor and had reached my cruising altitude of 10;500 FT; and had leaned the mixture for this new altitude. About 30 seconds later; the engine began losing power. At first; I believed this to be due to over-leaning the engine; so I richened the mixture. This did not clear the issue; so I added full mixture and full power and still had intermittent power from the engine. I contacted Phoenix Approach to announce the engine issue and turned back to Mobile airport; which was just a couple of miles behind me. While troubleshooting the engine; I circled the airport as we descended to around 5;000 FT. At that point; full power returned to the engine. I continued to circle over Mobile airport until I was able to climb back up to 10;500 FT. After the engine power remained stable at full power for about 20 minutes; I requested a return to my departure airport and was cleared to go directly there and chose to remain at 10;500 FT for as long as possible; because while I believed the engine issue to be resolved; I wanted to have sufficient altitude to reach any airport just in case. I was cleared to descend through Bravo airspace and then handed off to my departure airport's Tower; and was able to land without incident. The engine made full power and appeared to have no problems after the point where power was restored. The fuel tanks had been topped off the night prior to the flight; and no water was seen when sampling the fuel on the day of the flight just prior to departure. It was a hot dry day; so water and carb ice were not suspected. I have since contacted my mechanic; and he is currently checking the engine and fuel system to see if any issue is seen.

Data retrieved from NASA's ASRS site as of July 2013 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.