Jay Honeck wrote:
Lately I had noticed just a smidge of roughness in the engine. Not a
roughness, really, but a slight vibration, maybe a little, tiny, occasional
miss? It was not detectable by anyone except someone who flew the plane as
often as I do (and, in fact, Mary often couldn't sense it), but it was
definitely there.

***Saga of changing oil, cleaning and gapping plugs deleted***

Jay, I'll wager that your roughness will still be there when you next
fly. I just went through this on an O320. I cleaned/gapped the plugs,
and this did not cure the roughness.

It turned out that my Slick Mags have 370 hours since new, and the
roughness resulted from the point gap in the Right Mag being too small.
The plastic block that opens the points had worn down to the point
where the point gap was reduced to ~0.006", instead of the specified
0.018".

This manifested as a slight roughness in flight, made more noticable by
shutting down the Left mag. The roughness was not noticable during a
run up (nor was there an asymetrical drop) .

Per the Slick SB, the mags are normally opened up at 500 hours, and the
point gap is set. Mine just happened to need this early...

Mike M.