Just took my 10 month 2 engine project out for a water test yesterday. Starboard is fine but the Port seems to have an ignition / electrical problem.

Anyone know what could cause the power to cut completely out on the engine...Gages, starter, everything on the engine. It is intermittent, I can turn the key on and everything powers up, turn to start and everything dies like a dead battery...Batterys are fine. It will start and run, but going thru an idle zone it cut out. Started back up and made it home, but it was missing and running rough at an idle...The power on plane was fine.

I figured if it was the shift interrupt the gages would still work. Can the circuit breaker cause this?

Wiing harness plug loose?
Otherwise, the only thing is loose wire/connector somewhere in the wiring from the starter (red wire to key switch) or keyed ignition circuit (purple wire) that supplies power to guages, ignition, etc.
Check the key switch itself too.

Start with the battery cables, may need cleaning at the terminals. Also,make sure all the connections are tight.

Thanks for the input. I will check the harness and all the connections today. I did not know if a Mercruiser would run off the alternator or die if the battery was disconnected. With the gages going out too, it must be a main power cable to the engine.

If it isn't in the engine breaker near the wiring harness then it is probalby in the purple ignition wire. I had a fuse box issue where I lost connection on the starting circuit for one engine and It was intermittent as well. A little solder and I was back in business.

Looks like the rough idle was a bad/weak coil...Replaced it and it has run fine since Friday. The power issue looks like the harness. Checked all wiring on the engine, started it and it quit idling at the dock the first minute. Unplugged / Plugged the harness and ran for several hours. It quit once for a split second on plane Saturday AM but was fine the rest of Sat and Sun. I'm guessing its the harness connection.