Problem Solved!

I tried to isolate the problem by disconnecting the magneto p-leads, then
the alternator. After disconnecting, I would fire up the engine to check the
radio and it still would be very garbled at low engine r.p.m. and then clear
itself up at higher r.p.m.

While sitting in the cockpit, I noticed that my antenna, which is connected
to my new TKM MX11 radio,was vibrating back and forth. As I increased
R.P.M's, the antenna would stop vibrating and the garbled sound at low
r.p.m. disappeared. My antenna is a stainless steel whip model that sits in
front of the windshield on the cowling. See attached link to view:
http://home.cogeco.ca/~pl2/p3.html

I quickly switched to my other comm antenna and now have no problems.
There's probably a ground fault at the antenna base - I haven't had a chance
to sort it out. I don't know why this didn't manifest itself with the old
Narco 120.

BTW, the TKM MX-11 radio works great. As a slide in replacement, it's
easier than removing the old tray and installing a new tray for a ICOM A200
or a Bendix/King KY76A. It should be noted however that, barring the
antenna problem, the MX-11 needed some work before it would work. It seems
that if you run your radio through an intercom, which I do, you have to add
a capacitor to the radio's computer board, which I did. This took a little
less that an hour, but after I did this the radio worked right away.
However having said that, apparently, if you run the radio through an audio
panel, you don't have to make this tiny modification. Strange. In my
opinion, I still saved hours of grief with this installation versus a new
tray, etc.

Blue Skies

Eugene Wendland