Intermittent Voltage/ charging/ idling problem

[1997 Boat Owner]

Took my '97 SR 330 dancer for a shakedown ride. Everything great for 45 min's. Slowed to idle for no-wake zone and port engine shut off. I could restart it, but the only way to keep it running was to goose throttle and keep rpm's above 1600. Cursed. Turned around. Limped on 1 screw. Restarted port motor, put in forward in an rpm dip, ran the boat at 3200 back to dock. Ran smooth and strong (full rpm at full throttle,etc.) Died again once off plane. I believe the problem is voltage related. Voltage at the house and port batteries when idling is either 13.9 or 12.4. Sometimes it gets power from alt, sometimes not. Can't pinpoint conditions that lead to it not working (warm/cold/etc.) Checked wires, connections best I could. New batteries. New carbs. New fuel filters, fuel. New ignition parts. How likely is this the alternator? Does anyone know if the stock merc alternators have internal voltage regulator? Other ideas? Starboard engine perfect (has its own battery and alternator.) Does typical charge setup keep one side from helping the other?

 

[DarkJester]

can you see a big change in the voltage as you vary the engine RPM? Does the Voltage change more when throttling one engine more than the other? I Honestly don't know how your engines are wired, I would have to look at a diagram. But, normally voltage change won't kill a running engine. You mentioned carbs, so no injector solenoids to fire (big current draw), do both engines idle at the same speed? if you shutdown the Starboard engine will the port die? How does it sound when it dies? does it sputter and quit or is it like someone pulled the plug?
Is this an MPI motor? I know you said carbs, but...
Sounds so much like an IAC...just curious.

 

[1997 Boat Owner]

Port engine is not affected by anything I do with starboard. When I apply throttle the gauge dips. But sometimes I lightly goose it a couple of times and it starts to climb to almost 14 within a 30 second period. Starboard side seems just fine - climbs to 14 and is mostly parked there.

 

[1997 Boat Owner]

That's what's giving the most trouble. If it just didn't work I could trouble shoot it better. I think I am going to just put a new alt on. Even if that isn't the problem last wknd. The thing might actually be original.

 

[1997 Boat Owner]

Another clue. I went down a day later. Batteries had been topped with the charger. The port motor idled perfect. Still had intermittent voltage at the battery. I am guessing that with a full charge, the batt can run the ignition, but it gets drawn down a bit the lower voltage makes it fail? But it would still run on plane. Maybe the alt will spit out some juice at the higher engine speed. That's what's driving me nuts.