Originally Posted by
Big Pistons Forever
First thing I did was point the aircraft at a good field, and then methodically went through the checks. When I switched from both to left on the mags the engine suddenly went back to normal operation, so I flew back to my home airport which was about 15 miles away, arranging my flight path to keep within good gliding distance of suitable areas for a forced approach. I them flew a PFL pattern over the airport to an uneventful landing
Turned out a gear inside the right mag had failed advancing the spark 40 deg. The 2 mags were now fighting each other which totally discombobulated the engine switching off the bad mag corrected the situation
Something similar happened to a mate while getting a Biennial with an instructor in the US - also a refresher after several years not flying. They flew south from Space Coast Airport to do practice manoeuvering and some circuits elsewhere (probably Merritt Island) in a C152; on the way back the engine started running roughly and the instructor diagnosed a faulty mag - one or other worked ok but I recall my mate telling me that the instructor kept his hand on the mags all the way back for some reason while pointing out suitable places to go if it all went quiet.