The main reason for PGM's to die is misadjusted or seized power valves. This kills that part of the PGM but the will continue to run but without the RC valve working.

I've now seen a couple of PGM where a lack of spark was due to burn pins into side the multi pin connectors going into the PGM, these can be cleaned with contact cleaner.
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If I have to take the carbs off once more...