Narrative:

I was on a multi-day trip flying a piper J3 cub [when] I experienced some momentary engine roughness. Not thinking much of it as it the RPM drop was very slight with power returning to normal; I continued en route. [Later] the engine began to run noticeably rougher with a loss of power. At that point; I applied carb heat and did a mag check. The engine continued to run rough with a loss of power that resulted in me having to continue to advance the throttle to maintain cruise power. As I was currently getting flight following from [ATC] I informed them of my engine issues. I made a straight in to landing on [a suitable alternate]. Once over the threshold I pulled the power back to idle. At that point; the engine quit just as the wheels touched down for a safe landing. Upon inspection; it was discovered that the carburetor was wet. I made calls to my a&P; the previous owner of the aircraft as well as to a fellow CFI to discuss the event. We all agreed that given current close temperature/dew point spread; how the engine loss power; and the visible moisture seen on the carburetor after landing that carb ice was the likely cause.after about an hour on the ground; I departed. Before continuing on route; I circled over the field for a good 15 minutes to confirm that the engine was generating the proper power during climb; cruise and at idle. After I was confident that the engine and carburetor were operating normally; I continued to my next destination. For the remainder of the trip I operated with partial and full carburetor heat during all phases of flight. The remaining flights continued without incident. Upon reflection I should have applied carburetor heat at the first sign of engine roughness. This would have likely gotten ahead of the carburetor ice. I knew that J3 cubs can be susceptible to carb icing while taxiing and during other low engine RPM conditions. As I was running at cruise power for extend periods; the possibility of carb ice didn't become apparent until there was significant engine roughness. Having left two days prior; operating in warmer and drier weather without incident; I was complacent with regard to the possibility of carburetor icing. While I checked the weather prior to the flight; I did not factor in the close temperature/dew point spread ([destination airport] was reporting a temp/dew point of 08/06 with clear skies) which would have told me to periodically engage carburetor heat as a precautionary action. In addition; as I was making a long; multi-day flight; I feel my attention was skewed toward making my daily checkpoints so that I may make each day's distance goals.

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Original NASA ASRS Text

Title: Piper J3 pilot reported diverting to an alternate airport after his engine began running rough; which was later traced to carburetor ice.

Narrative: I was on a multi-day trip flying a Piper J3 Cub [when] I experienced some momentary engine roughness. Not thinking much of it as it the RPM drop was very slight with power returning to normal; I continued en route. [Later] the engine began to run noticeably rougher with a loss of power. At that point; I applied carb heat and did a mag check. The engine continued to run rough with a loss of power that resulted in me having to continue to advance the throttle to maintain cruise power. As I was currently getting flight following from [ATC] I informed them of my engine issues. I made a straight in to landing on [a suitable alternate]. Once over the threshold I pulled the power back to idle. At that point; the engine quit just as the wheels touched down for a safe landing. Upon inspection; it was discovered that the carburetor was wet. I made calls to my A&P; the previous owner of the aircraft as well as to a fellow CFI to discuss the event. We all agreed that given current close temperature/dew point spread; how the engine loss power; and the visible moisture seen on the carburetor after landing that carb ice was the likely cause.After about an hour on the ground; I departed. Before continuing on route; I circled over the field for a good 15 minutes to confirm that the engine was generating the proper power during climb; cruise and at idle. After I was confident that the engine and carburetor were operating normally; I continued to my next destination. For the remainder of the trip I operated with partial and full carburetor heat during all phases of flight. The remaining flights continued without incident. Upon reflection I should have applied carburetor heat at the first sign of engine roughness. This would have likely gotten ahead of the carburetor ice. I knew that J3 Cubs can be susceptible to carb icing while taxiing and during other low engine RPM conditions. As I was running at cruise power for extend periods; the possibility of carb ice didn't become apparent until there was significant engine roughness. Having left two days prior; operating in warmer and drier weather without incident; I was complacent with regard to the possibility of carburetor icing. While I checked the weather prior to the flight; I did not factor in the close temperature/dew point spread ([destination airport] was reporting a temp/dew point of 08/06 with clear skies) which would have told me to periodically engage carburetor heat as a precautionary action. In addition; as I was making a long; multi-day flight; I feel my attention was skewed toward making my daily checkpoints so that I may make each day's distance goals.

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.