Stray sparking from coil to distributor

During warm up I noticed the tachometer needle was bouncing around and the starboard engine was starting to idle rough. I noticed there was arcing between the positive Pertronix wire from coil to side of the cast iron engine block half way to the Pertronix in the distributor. There wasn't any sign of bare wire or insulation on the two wires from coil to distributor. I had installed the Pertronix in both twin engines many years ago and have never had any issues! The engines are twin 1981 BB140A's. Does Pertronix wear out over time? I thought there is nothing to wear in a Pertronix system? Could it be a bad distributor cap, rotor, or coil? Probably the crimp splice has developed corrosion in it because I didn't cover it with a heat shrink tube wrap years ago when I installed it! Should I shield the wires? The arcing did not come from area around the crimp splice.

Arcing is typically caused by HIGH voltage, not the primary voltage....so I'd say you have a 'leak' from inside the cap/coil wire.

If the cap(inside) and rotor look okay, make sure all of the high tension wires are seated and use a smear of silicone dielectric grease around the boots...

you can also use a screwdriver - grounded with a jumper - to probe around all of the cap openings to 'find' the high voltage leak...

While there is nothing to wear out in a Petronix system, caps and rotors still do.... My OEM HV ignition system in my boat never gets more than 2 (reliable) seasons out of a cap/rotor.

HV wires have a finite life... Even the best OEM/Aftermarket ones start to degrade at about 10 seasons.

I covered the two wires together with a black wire shield tube between the coil and distributor. It fixed the problem of the spark jumping to the engine block. I also just received new distributor caps and rotors today via Amazon. I'll install them and see if it also helps with the rough idle after warm up. As always the new spark plugs get black quickly! Older Sea Loc manual recommends .028 on the spark plug gap. I've been gapping them @.030 for the Pertronix. Could I be wrong? Does the wider gap produce a colder spark? Should I go back to .028? I have the timing set at 6 degrees BTDC and idle after warm up is around 900 rpm. If this doesn't fix the problem of engine dying on idle after warming up then I try using fresh fuel or blame the brand fricken new Weber/Redline K861kit I recently installed!