I have a complete MSD marine ignition for my 540 BBC engines. I was test firing them before putting them back in the boat. I had them on a mercrusier platic pallet, hooked to fuel and a battery. The MSD box was loose, on a table, and the coil is mounted to the engine.
I hooked water to it and fired it up. When I reached for the throttle, I about goit laid out from getting shocked. I thought I touched a bad plug wire, so I began investigating, and when I touched the exhaust riser WHAM !!!!! I got it again. I used a volt meter between me, and the engine and read 260 volts AC. I shut the engine down and went to engine number two. Same set up, same MSD box. I started the engine and used the volt meter - nothing. I touched the carb, no shock. MSD says it was a poor ground....not sure I buy that one. Two identical engines, same battery, same cables, same set up as far as I can tell.
Question is ....what the heck gave me a jolt. Obviously something is leaking voltage somewhere. Coil ??? Distributor ??? All the plug wires are in great shape, dist and coil are only two seasons old. The engine with the loose sparks did sound a little hesitant on throttle reponse, but it may be my imagination. No miss, no backfire, just a tiny bit sluggish.
Any one have anyy ideas ????

May be firing thru the top of your rotor into the dist shaft , seen it before . deep

It's impossible to get 260 Volts AC out of any MSD or any boat 12V DC system. AC Leakage to ground could be present if a shore power AC system, wiring or inverter/charger system is defective. What probably happened is a leaking to ground MSD box or improper grounds in the system. MSD current is low with high voltages. The system is capacitive discharge DC. On another note, most sure power systems are only 120_volt AC. It's much safer than 208 or 240-volt system. get your box checked along with your grounding system

This is what I was thinking... DC system will not supply 260V. If you got hit with 260V, you'd have more than just a jolt. Hope you figure out whats going on.

It sounds like you may have a short in the coil primary or the ground to the ignition module is questionable. The output from CD boxes is around 500 volts dc depending on model. Because of the multispark output it would read 260VAC.

1) can not possibly shore power. Engines are not in the boat, no external power other than battery. The engine is on a pallet, no alternators (which by the way ARE AC, but rectified.

2) makes some sense that I could be leaking through the rotor - will check

3) Makes a lot of sense to see an AC reading due to the multiple spark, or from getting a spark 8 times per 2 revolutions. The DC amperage may be too low for the meter to pick up. I will make sure the "box" is totally grounded.

4) as far as getting a jolt from 260 volts ..... Holy Crap !!!!!!!!! However, it is amperage that actually kills. Garb a loose plug wire sometime, 40Kv, but low amps (generally not lethal), 120 volts and 300 amps (dangerous at a minimum)