Narrative:

This aircraft has a history of plug fouling due to under-leaning. Left and right mag checks resulted in a 75 RPM drop; well within limits. I decided to lean for best power prior to departure. Winds were calm with a temperature of 24 degrees C. On departure the aircraft would only climb out at about 150 feet per minute at full power and vx. I requested a left turn out to avoid the downtown area. I was unable to reach 3;000 feet until clear of the class B airspace. I elected to climb out at 70 KIAS to reduce drag and potentially gain performance. The aircraft was still having trouble gaining altitude at 70 KIAS. I adjusted the mixture in the climb to ensure it was not over- or under-rich. The engine was rumbling a little bit so I applied carb heat for approximately 90 seconds with no effect on performance. At this point I accelerated to 80 KIAS and attempted to hold altitude at 4;400 feet. I enriched the mixture at this point and noticed a slight drop in RPM; so I pulled it back out. At 80 KIAS at 4;400 feet; full power (2;400 RPM; normally that would be 2;700 RPM) and 5 degrees nose up the airplane was showing a 300 FPM descent rate. At this point I notified [center] that I was diverting to [a nearby airport] with partial engine power loss. The controller was very helpful in getting me NOTAMS; weather and airfield descriptions as I was busy flying the aircraft. I made a steep approach to landing. The aircraft never fully lost power but I think it would have.I took away from this event that you should always have a suitable landing area within glide distance of your aircraft. Proper flight planning on a cross country flight could prevent you from landing off of an improved surface.

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Original NASA ASRS Text

Title: AMD Alarus 2000 pilot reported diverting to a nearby airport after experiencing partial power loss.

Narrative: This aircraft has a history of plug fouling due to under-leaning. Left and right mag checks resulted in a 75 RPM drop; well within limits. I decided to lean for best power prior to departure. Winds were calm with a temperature of 24 degrees C. On departure the aircraft would only climb out at about 150 feet per minute at full power and Vx. I requested a left turn out to avoid the downtown area. I was unable to reach 3;000 feet until clear of the Class B airspace. I elected to climb out at 70 KIAS to reduce drag and potentially gain performance. The aircraft was still having trouble gaining altitude at 70 KIAS. I adjusted the mixture in the climb to ensure it was not over- or under-rich. The engine was rumbling a little bit so I applied carb heat for approximately 90 seconds with no effect on performance. At this point I accelerated to 80 KIAS and attempted to hold altitude at 4;400 feet. I enriched the mixture at this point and noticed a slight drop in RPM; so I pulled it back out. At 80 KIAS at 4;400 feet; full power (2;400 RPM; normally that would be 2;700 RPM) and 5 degrees nose up the airplane was showing a 300 FPM descent rate. At this point I notified [Center] that I was diverting to [a nearby airport] with partial engine power loss. The Controller was very helpful in getting me NOTAMS; weather and airfield descriptions as I was busy flying the aircraft. I made a steep approach to landing. The aircraft never fully lost power but I think it would have.I took away from this event that you should always have a suitable landing area within glide distance of your aircraft. Proper flight planning on a cross country flight could prevent you from landing off of an improved surface.

Data retrieved from NASA's ASRS site 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.