Narrative:

An exhaust manifold of PA28-180 broke off over open water causing me to declare an emergency. I took off for a cross country flight. When I was 50 NM from my destination airport I noticed a loss of RPM so I started switching tanks; leaning and enriching the mixture. Turning on the fuel pump and turning on carburetor heat. Etc. The plane sounded like it was coming apart. It was backfiring and sputtering like crazy and any change in throttle or mixture made it worse. I immediately declared an emergency and set transponder to 7700 as I was not in glide range of an airport and was loosing power and altitude. I was over water and swamp with nothing to land on. I declared to ATC that I was flying through a cloud as I was holding the altitude I had while engine was still running. They advised me against it but I was at the cloud already it was really a non issue as once I got to it I skipped right under it so it all worked out. When my a&P came to see what happened he said the cylinder was running overly lean from the exhaust leek and also it was blowing all the exhaust gas right in the carburetor which is probably what caused all the backfire. In hindsight I guess the engine wasn't totally stopped maybe I shouldn't have declared an emergency as I made it to the airport but I was loosing altitude and the engine could have stopped at any time so if I had to do it again I would have probably done the same thing.

Related reports:

Google
 

Original NASA ASRS Text

Title: PA28 pilot reports a very noisy and rough running engine with reduced power during a cross country flight. Reporter is able to return to the departure airport safely.

Narrative: An exhaust manifold of PA28-180 broke off over open water causing me to declare an emergency. I took off for a cross country flight. When I was 50 NM from my destination airport I noticed a loss of RPM so I started switching tanks; leaning and enriching the mixture. Turning on the fuel pump and turning on carburetor heat. Etc. the plane sounded like it was coming apart. It was backfiring and sputtering like crazy and any change in throttle or mixture made it worse. I immediately declared an emergency and set transponder to 7700 as I was not in glide range of an airport and was loosing power and altitude. I was over water and swamp with nothing to land on. I declared to ATC that I was flying through a cloud as I was holding the altitude I had while engine was still running. They advised me against it but I was at the cloud already it was really a non issue as once I got to it I skipped right under it so it all worked out. When my A&P came to see what happened he said the cylinder was running overly lean from the exhaust leek and also it was blowing all the exhaust gas right in the carburetor which is probably what caused all the backfire. In hindsight I guess the engine wasn't totally stopped maybe I shouldn't have declared an emergency as I made it to the airport but I was loosing altitude and the engine could have stopped at any time so if I had to do it again I would have probably done the same thing.

Data retrieved from NASA's ASRS site as of July 2013 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.