Larry,

The BEST place to get this info would be here:
http://www.aeroelectric.com/consulting.html

It's free and it's the best electrical info you are going to find.

I would suggest making sure your grounnding system is robust. Perhaps try
grounding the radio all the way back to the battery.

Your P-leads are shielded, right? The shielding should be grounded at the
magneto and not at the instrument panel.

Just a couple of things to check.

Sam

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Sam Hoskins
www.MistakeProofing.Net
618-967-0016 ph.
312-212-4086 fax