This test should be part of the run-up pre-flight check.

You are verifying that the P-lead correctly grounds the magneto (by
observing the drop)
and that the firing side is operational. You are also, of course, verifying
that the engine will run on the ungrounded side (and that it is firing that
entire bank of spark plugs by scrutinizing how well it runs).

I am sure you know all of this (grin), I just chimed in becuase its a check
best done on the run-up pad.

Todd

"Peter Dohm" wrote in message
...
"a" wrote in message
...
Next time you're at altitude and in cruise mode, switch from 'both' to
a single bank of spark plugs. It will remind you what happens should
you have one side decide to go in/op when in flight. I had the engine
(IO 360) go rough a couple of weeks ago, but rough was a lot better
than the very smooth and silent effects the left mag only gave me.

Sounds like at least two problems--unless the fault was in the switch.

Peter
Just a thought