Originally Posted by wwest (Post 5521245)

The "burn through" on the coil tower in conjunction with the "scratch" marks inside the distributor tells the tale.

Your distributor is MIS-TIMED internally, with the rotor tip no where near a spark plug connection at the time of the spark/ignition generation the high voltage spark must/will find another jump/arc point.

Matters made worse by the aftermarket HIGHER voltage output MSD system.

Been a LONG time ago but I did once run across a rotor that was too short to have close enough "connection" with the distributor cap spark plug contact.

Strange as it may seem I have also seen a case where random intermittent shorting inside the alternator seemingly resulted in enough electromagnetic crosstalk coupling to the NEARBY distributor's electromagnetic pickup coil that random CDI firing resulted. At least I think that was what was happening, the problem went away with installation of a new alternator. Disassembly of the old alternator indicated multiple shorting points, burned wiring insulation internally under the metal wiring supports.

You are not using an aftermarket high voltage output ignition coil with an OEM or factory standard CDI, right...??

THAT would be the problem...!!!!

And....

Denver...5000 ft elevation....low atmospheric pressure...sparks "jump" easier.

And one more point, as if the above isn't enough.

The output of the factory CDI ignition is not regulated in any way. So if the battery is being overcharged, the 12 volts rising to, say, 16 volts, the nominal 300 volt CDI output rises accordingly, ~400 volts, and the SCR (Silicon Controlled Rectifier) fires prematurely due to voltage breakdown, breakover. Granted, that's an unusually low breakover voltage for an SCR, but not a late 80's SCR. Which is why the typical failure mode of these older alternators, over-charging the battery, oftentimes leads to a CDI failure.