Narrative:

After receiving a briefing from duats; planning the flight; performing a preflight inspection; fueling the airplane; and waiting 10 minutes before sampling the fuel from the 3 sumps; I departed to the east then I turned to a magnetic heading of 180 degrees and nosed down to begin an enroute descent. I leveled off at 2;000 ft. At this point in the flight; no problems had been detected. Shortly after leveling off; the RPM very suddenly began alternating between the cruise setting I had set and an idle throttle setting. I immediately turned towards ZZZ and used the surges of power to climb as much as possible. I attempted to troubleshoot the problem by activating carb heat (and leaving it applied full for the duration of the flight); applying full throttle; selecting left and right mags and enriching the mixture. None of these resulted in a significant improvement. At this time I advised approach of this emergency. The maximum altitude was 2;900 ft. Approach requested I squawk a discrete code (in the 7700 range). I complied but I was too low for radar contact. I was instructed to switch frequencies and speak to center. I advised approach that I was unable to comply; since by this time I was using my free hand to pump the primer to maintain altitude. I asked approach to call ahead to ZZZ since I was unable to change the frequency to advise traffic of my approach. I was advised that they would do this and that the ground crew would be standing by. Maintaining altitude became impossible. However; sufficient altitude remained such that landing at ZZZ was successful. The cause of the mechanical problem has yet to be determined. While carb icing may be a possibility; no decrease in performance during cruise was evident prior to the very significant RPM fluctuations. Also applying carb hear did not cause any noticeable change; such as rough running or backfire.

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Original NASA ASRS Text

Title: C150 pilot experiences surging engine at 2;000 FT and is able to divert to a nearby airport before the engine quits completely.

Narrative: After receiving a briefing from DUATS; planning the flight; performing a preflight inspection; fueling the airplane; and waiting 10 minutes before sampling the fuel from the 3 sumps; I departed to the east then I turned to a magnetic heading of 180 degrees and nosed down to begin an enroute descent. I leveled off at 2;000 FT. At this point in the flight; no problems had been detected. Shortly after leveling off; the RPM very suddenly began alternating between the cruise setting I had set and an idle throttle setting. I immediately turned towards ZZZ and used the surges of power to climb as much as possible. I attempted to troubleshoot the problem by activating carb heat (and leaving it applied full for the duration of the flight); applying full throttle; selecting left and right mags and enriching the mixture. None of these resulted in a significant improvement. At this time I advised Approach of this emergency. The maximum altitude was 2;900 FT. Approach requested I squawk a discrete code (in the 7700 range). I complied but I was too low for radar contact. I was instructed to switch frequencies and speak to Center. I advised Approach that I was unable to comply; since by this time I was using my free hand to pump the primer to maintain altitude. I asked Approach to call ahead to ZZZ since I was unable to change the frequency to advise traffic of my approach. I was advised that they would do this and that the ground crew would be standing by. Maintaining altitude became impossible. However; sufficient altitude remained such that landing at ZZZ was successful. The cause of the mechanical problem has yet to be determined. While carb icing may be a possibility; no decrease in performance during cruise was evident prior to the very significant RPM fluctuations. Also applying carb hear did not cause any noticeable change; such as rough running or backfire.

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.