Cessna 172 pilot reported an engine malfunction during cruise on a day flight in VMC conditions. The pilot performed checklist procedures; troubleshooting; contacted ATC; and diverted to a non-towered airport and landed safety.

Narrative

Using the checklist for pre-flight; but deferred fuel check to the end so I could do all sumps at the same time. Upon sampling the RH fuel tank; there were approximately 2-3 drops of water. I emptied the container and then filled it 50-75% full two additional times and no water was observed. Only the RH sump showed water during the checks. Run-up checks were all within normal tolerances for both carb heat and both L/R mags.Level flight at 4500 ft. MSL @ 2300 RPM leaned. At approximately 1.3 hrs into my flight; there was a large reduction in power to approximately 1500-1600 RPM. I suspected carb ice and turned carb heat on and put the mixture full rich. The engine then went back to 2300 RPM and was steady for about 3-4 min; so I thought that the issue must have been carb ice and had been resolved so I decided to continue my flight. I then experienced another large reduction in power and applied full throttle. The engine then began to fluctuate between 1500-2000 RPM. I identified the nearest airport 10nm to the SW and diverted. During this time; carb heat was still on; mixture was full rich; and throttle was full forward. I then tried both left and right mags and had no improvement. I then switched to left tank only due to the fact that there had been a small amount of water in my first of three sumps. When I switched to left tank; there was an even lower reduction in engine power; so I went back to both. At this point; using ForeFlight glide advisor; I determined if the engine quit; I would not have sufficient altitude to make the runway; so I advised ATC. Center gave me the appropriate center frequency and I switched over. Center had an individual confirm that all steps had been completed. Once I was assured that I could reach the runway if the engine were to quit; I informed Center and switched to CTAF. The engine continued to sputter during the landing and approach; but I was able to safely land on runway XX. I then switched back to Center to inform them I was down and safe. I proceed to taxi to the nearest ramp where I shut down the aircraft and called maintenance to arrange for someone to come and recover the aircraft.