Originally Posted by
treadigraph
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.
I know the mags are independent of one another but possibly he was getting ready to flip between L/ R/ BOTH alternately at the slightest hint of a cough.