Narrative:

I was enroute in a mooney M20 with one passenger when my engine began running rough. I had descended to 4;000 feet as my passenger had indicated he needed to use a restroom and we were discussing where to land. I requested with ATC center to deviate and land and they cleared the request and asked the reason. They gave me an altitude of 3;000. When I reduced power to begin my descent is when the first indication of roughness began. My final destination was ZZZ; but we had deviated toward ZZZ1 due to a band of IMC conditions moving across [area]. We were on an IFR flight plan but I planned my route to stay in VMC conditions in order to eliminate any structural icing concerns. When the engine began running rough I performed the emergency checklist which includes items like carb heat on; electric fuel boost on; switch fuel tanks; mixture rich; etc. The checklist showed that if engine roughness did not improve to turn carb heat off after one minute.I followed the checklist and the engine improved momentarily; then began losing power again. I continued to try carb heat and diagnose the issue. I lost some power again and was unsure if I would be able to hold altitude. The nearest airport was ZZZ2. After the second time that the engine lost power I called ATC center and [advised them of situation]. I did so because I was unsure if the aircraft would completely lose power and I was outside of glide distance of an airport. From my training safety comes first and I have read and learned that hesitating to [advise ATC] can lead to increasing risks. I also reviewed the checklists for an emergency landing. The engine continued to run and regained most of its power. It seemed to be running at approximately 40-80 percent power at different times. I was able to make ZZZ2 and had altitude to fly over and then turn and land safely. After landing the engine seemed to lose its roughness. After taxiing to the ramp I called flight service to cancel IFR and advise we were down safely. I did a run up on the ramp and the airplane appeared to run normally again. After shutting down we called a mechanic and conferred on what to do. We also spoke with airport staff and located a heated hangar to place the aircraft in. The weather deteriorated and we waited until evening for it to improve and plan what to do. My passenger and I looked at the airplane and could see through the cowl that the butterfly valve was not moving when the carb heat was applied. I called a mechanic again and they stated it should shut completely when carb heat is on. We moved the control back and forth and were able to free up the butterfly valve and now the carb heat was working properly. Now that the carburetor heat was fully operational and the plane had over 6 hours to warm up; I did several full power run ups and several normal run ups. The carb heat produced more of an RPM drop than earlier in the day. We checked again that it was working properly and noted the position of the control lever when the carb heat was fully on. Conditions were now VFR and we flew home. The airplane flew normally and had no further issues.in hindsight I think staying calm and working through the checklist as well as letting ATC know help calm my passengers worries and helped me focus on the task at hand rather than worry about the situation or let fear take over. During my run up at the beginning of the day I noticed an RPM drop while using carb heat but it was not as much of a drop as after the carb heat was working fully. If I had realized that was an issue I would have had a mechanic inspect the plane prior to departure. I learned from this experience and will be more attentive to the symptoms of carb ice and also to possible mechanical issues that may not be immediately obvious.

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Original NASA ASRS Text

Title: M20 pilot reported engine roughness that did not improve with carburetor heat. After diverting; the carburetor heat was found to not be functioning properly.

Narrative: I was enroute in a Mooney M20 with one passenger when my engine began running rough. I had descended to 4;000 feet as my passenger had indicated he needed to use a restroom and we were discussing where to land. I requested with ATC Center to deviate and land and they cleared the request and asked the reason. They gave me an altitude of 3;000. When I reduced power to begin my descent is when the first indication of roughness began. My final destination was ZZZ; but we had deviated toward ZZZ1 due to a band of IMC conditions moving across [area]. We were on an IFR flight plan but I planned my route to stay in VMC conditions in order to eliminate any structural icing concerns. When the engine began running rough I performed the emergency checklist which includes items like carb heat on; electric fuel boost on; switch fuel tanks; mixture rich; etc. The checklist showed that if engine roughness did not improve to turn carb heat off after one minute.I followed the checklist and the engine improved momentarily; then began losing power again. I continued to try carb heat and diagnose the issue. I lost some power again and was unsure if I would be able to hold altitude. The nearest airport was ZZZ2. After the second time that the engine lost power I called ATC Center and [advised them of situation]. I did so because I was unsure if the aircraft would completely lose power and I was outside of glide distance of an airport. From my training safety comes first and I have read and learned that hesitating to [advise ATC] can lead to increasing risks. I also reviewed the checklists for an emergency landing. The engine continued to run and regained most of its power. It seemed to be running at approximately 40-80 percent power at different times. I was able to make ZZZ2 and had altitude to fly over and then turn and land safely. After landing the engine seemed to lose its roughness. After taxiing to the ramp I called flight service to cancel IFR and advise we were down safely. I did a run up on the ramp and the airplane appeared to run normally again. After shutting down we called a mechanic and conferred on what to do. We also spoke with airport staff and located a heated hangar to place the aircraft in. The weather deteriorated and we waited until evening for it to improve and plan what to do. My passenger and I looked at the airplane and could see through the cowl that the butterfly valve was not moving when the carb heat was applied. I called a mechanic again and they stated it should shut completely when carb heat is on. We moved the control back and forth and were able to free up the butterfly valve and now the carb heat was working properly. Now that the carburetor heat was fully operational and the plane had over 6 hours to warm up; I did several full power run ups and several normal run ups. The carb heat produced more of an RPM drop than earlier in the day. We checked again that it was working properly and noted the position of the control lever when the carb heat was fully on. Conditions were now VFR and we flew home. The airplane flew normally and had no further issues.In hindsight I think staying calm and working through the checklist as well as letting ATC know help calm my passengers worries and helped me focus on the task at hand rather than worry about the situation or let fear take over. During my run up at the beginning of the day I noticed an RPM drop while using carb heat but it was not as much of a drop as after the carb heat was working fully. If I had realized that was an issue I would have had a mechanic inspect the plane prior to departure. I learned from this experience and will be more attentive to the symptoms of carb ice and also to possible mechanical issues that may not be immediately obvious.

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.