Yesterday I started our 5.7L Merc I/O (circa 1986). I could hear a
"snapping" noise coming from the engine compartment. I popped the
engine hatch and could see the coil shorting from where the coil wire
plugs into the coil. It was arcing to BOTH the positive and negative
terminals on the top of the coil. Needless to say, with 120 gallons
of gas about 3 feet away, I shut the engine down immediately.

I'm not an expert in this area. I know I must use marine-designed
starters, alternators, etc. Can I use a garden-variety coil and coil
wire, or do I need to stick with the Mercury "Thuderbolt Ignition"
components for this situation? The price difference between an
automotive coil & a Merc coil isn't all that much, so it isn't a
question of dollars. It does appear that the new high-performance
automotive coils are a little better quality.

Also, what would cause this? Is the coil wire bad? The coil, itself?
I know they can develop hairline cracks that can cause this sort of
situation.

Thanks for any explanation and advise.

Regards,

Coff