[...] Recall your own advice "test them in the dark"?
Oh, absolutely, any owner of a CB500/550 is advised to do this simple test from time to time. I've seen quite a few that had either the HT leads or the plugcaps arch to the engine block. Realise this can happen intermittently. In riders reports published in Holland back then, it was reported the #1 issue people had with these bikes. I myself am still on the look out for the thin black sheaths (?) that Honda had slided over the HT leads. Actually these
sheaths on mine were old and damaged, the leads themselves came out like new after removal of the sheaths. I'd like to have them again but they are hard to find, so I may have to follow the suggestion a Ducati/Guzzi mechanic did: just mount them without. Most Guzzis didn't have them anyway. Silicone spray and WD-40 can help fight issues.