Narrative:

I was pilot at the controls; while my copilot was the pilot in command for this leg. He is an instrument rated pilot; with 1200+ hours; and we had been flying the second leg of our journey. We departed and began our climb to 7;000 feet. At about 6;000 feet we ran into engine roughness and shaking; fairly substantial. We leveled off and began to troubleshoot the issue after notifying ATC. After full rich and adjusting throttles and re-leaning we regained smooth run and continued climb. We reached 7;000 feet and were able to continue on course. Oil pressures and fuel flow were fine. We monitored those closely.approximately 10 miles north of ZZZ the engine issues came back. We began to troubleshoot roughness by swapping tanks; re-leaning; but could see there was no direct relationship between symptoms and inputs on the system (changing inputs did not affect performance). We asked for assistance and vectors and went to ZZZ. Doing the turn-descent we lost all power briefly; and regained it. By now we [advised ATC]. The engine resumed running with inconsistent roughness and performed reasonably smooth in the final stages of arrival. We were able to land with power.during the descent we also lost communications on radio 1 somewhere between 4;000 and 3;000 feet. We were unable to hear ATC and assumed they could not hear us. We had already configured tower at ZZZ into radio 2; and we swapped to that and reestablished communications. I confirmed they were aware we had [advised ATC] and were inbound for runway xx; the most direct runway in sight.engine performance was mixed; producing what appeared like full power; but neither of us was certain whether that was actually the case. The winds were 20+ knots and we were landing with a major crosswind; unreliable engine; wind shear and gusts. Not the best landing; but one that worked and got us on the ground safely.upon landing; we were able to taxi to FBO under our own power and eventually have a mechanic do an initial assessment. At the time of the initial diagnosis the mechanic confirmed poor compression in cylinder 1; and experienced violent shaking at full power that was yet to be isolated or understood. The plane was left at ZZZ for diagnosis and repairs.

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Original NASA ASRS Text

Title: Cessna 172 pilot reported engine problems during cruise dictating a diversion to a nearby alternate airport.

Narrative: I was pilot at the controls; while my copilot was the pilot in command for this leg. He is an instrument rated pilot; with 1200+ hours; and we had been flying the second leg of our journey. We departed and began our climb to 7;000 feet. At about 6;000 feet we ran into engine roughness and shaking; fairly substantial. We leveled off and began to troubleshoot the issue after notifying ATC. After full rich and adjusting throttles and re-leaning we regained smooth run and continued climb. We reached 7;000 feet and were able to continue on course. Oil pressures and fuel flow were fine. We monitored those closely.Approximately 10 miles north of ZZZ the engine issues came back. We began to troubleshoot roughness by swapping tanks; re-leaning; but could see there was no direct relationship between symptoms and inputs on the system (changing inputs did not affect performance). We asked for assistance and vectors and went to ZZZ. Doing the turn-descent we lost all power briefly; and regained it. By now we [advised ATC]. The engine resumed running with inconsistent roughness and performed reasonably smooth in the final stages of arrival. We were able to land with power.During the descent we also lost communications on Radio 1 somewhere between 4;000 and 3;000 feet. We were unable to hear ATC and assumed they could not hear us. We had already configured Tower at ZZZ into Radio 2; and we swapped to that and reestablished communications. I confirmed they were aware we had [advised ATC] and were inbound for Runway XX; the most direct runway in sight.Engine performance was mixed; producing what appeared like full power; but neither of us was certain whether that was actually the case. The winds were 20+ knots and we were landing with a major crosswind; unreliable engine; wind shear and gusts. Not the best landing; but one that worked and got us on the ground safely.Upon landing; we were able to taxi to FBO under our own power and eventually have a mechanic do an initial assessment. At the time of the initial diagnosis the mechanic confirmed poor compression in Cylinder 1; and experienced violent shaking at full power that was yet to be isolated or understood. The plane was left at ZZZ for diagnosis and repairs.

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.