First to all the LML friends who offered many and very thoughtful
insights:  thank you so much. It was very helpful.

Condition:  In cruise at FL230-250 an occasional "backfire" would occur.
. . 4-5 times in the previous 6 hours.  The afterfiring occurred at
2400rpm, 29.5"mp and 70 degrees LOP burning 15.5 gph with GAMI
injectors.  There would only be a single pop.

Doing a mag check on the ground everything was normal with about 30 rpm
drop.  At 8,000, the plane was set up for cruise as above and the mag
check performed again.  The left mag was smooth.  The right mag ran with
slight roughness and no backfiring.

Findings:  Removing the cap from the right mag showed carbon deposits on
the yellow cable shields and all 3 screws were corroded.  Inspection
showed no evidence of water, dirt or debris.    The left mag also had
carbon deposits on the yellow cable shields, although to a lesser degree
than on the right mag.  All else appeared normal.

The upper spark plugs were gapped correctly. The lower plugs were
slightly wide and regapped to spec.  All ignition wires appeared normal.
The timing was correct.  The pressurizing hose to the mags appeared
normal with no obstructions or debris.  The inline filter was replaced.
There is no evidence of any moisture or debris in the filter.

Action:  Both pressurized Slick mags were replaced.  Slick offered to do
this under warranty and overnighted them to me.  Slick people were very
helpful, available and cooperative.

Results:  The 5 flights (2 at FL230 and 240) since replacement had no
afterfiring.  The engine runs smoother although I would have described
it as smooth before also.  I gained about 5-7 knots IAS in cruise (is
this from bugs I cleaned off the leading edges or new mags?)

My Conclusion:  There had been cross arcing in the mags resulting in the
carbon deposits inside the cap.  The corrosion on the cap screws
probably came from ozone from the arcing as there was no evidence of
water or debris in the mags.  (If it had been water, I would also expect
to see corroded screws in the left mag.)  Why there was crossfiring in
the mag with what appears to be an intact pressurization line is
something on which I hope Slick can shed some light.

Carl Cadwell, N25CL, IVP with 250 hours.