Originally Posted by
AdamFrisch
Magnetos can wreak havoc in the BOTH config, if one starts misbehaving. Can fire completely randomly. So, it can be better to run on one than use the false redundancy/safety blanket of both.
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As a general rule engines should not be run at in flight power settings on only one mag for extended periods. If the problem is a fouled/failed plug in one cylinder then switching to the mag driving that plug will isolate the problem as that cylinder will have no plug firing and the engine will be extremely rough. So now your choice is to leave it on the other mag or go to both. My vote is go to both so that all the other cylinders get the benefit of having both plugs firing. In practice there will not be any significant difference in roughness with either the good mag selected or both. If the engine is much rougher on both then just the one then running on the one mag would be appropriate.
I once had a mag suffer an internal failure which massively advanced the spark. The engine instantly lost a lot of power and was extremely rough. As soon as I deselected the bad mag the engine ran normally so I diverted to the nearest airport and landed.