Art wrote:
Dave,
You might try measuring the mag drop using a device like the Proptach
Digital Optical Tachometer.
www.proptach.com
Mechanical Tachometers do not have sufficient resolution to measure a small
RPM drop.
The Proptach has a resolution of 1 RPM and an update rate of 1 second and no
hook up required.
There should always be a drop on each mag if the engine stops in the OFF
position of the switch.
Another possible failure mode could be a short between the Right and Left P
leads but not to ground.
That failure could be in the wiring harness or the switch and would be like
having only one mag system.
Art Sundeen

There should be a visible (with a regular tach) drop when on one mag.
If not, there is a either a problem with the P-lead (the mag is not getting
grounded) or the mags are mistimed.

Never believe that the behavior of the mags in the L or R position tell
you anything about what will happen when you put the switch in OFF.
Believe me, my ignition switch (before it was replaced) could be set to
what appears to be off (you could even remove the key) and it not ground
the mags (but the normal L-R positions give you mag drops).

Of course, then there's always the old Rod Machodo click-click-click-BOOM!
mag check his students do. If the engine dies for whatever reason when you
turn the mag switch, close the throttle before you turn it back.