1996 Chris Craft 454 omc couple of issues possibly

Omc454 said:

My friend has the boat purchased 2 new batteries. however, upon using them it was running rough on the one. Looking at his gauge and even under power the 1 shows 12v.
That would indicate that the alternator is either NOT charging, or that the charge rate is not reaching that battery!
You will want to trace this out.

While not going over 12 sounds to me like a bad battery not taking a charge.
Above, you mention "purchased 2 new batteries".

Thinking for a minute it seemed odd under mild throttle and cruising 2500 rpm it didn't have had enough power to fire the plugs properly.
I would think that even at 12 volts, the ignition system should operate.

There was no radio on or load besides the engine and gauges. It can use either battery or both and using the 2nd battery or both settings run fine and shows 14 volt.
We should avoid using the ALL/BOTH selection, unless for momentary use only.

It last had a tune-up including plugs I believe 2 years ago. He uses the ethanol free at his Marina and told me used the proper sta-bil. We're going to swap out the battery and probably run spray some carb cleaner into it in a couple days. Is there anything else to check such as would plugs need changed this soon?
I would suggest looking at all Positive and Negative battery cable connections.
I'd also suggest testing the MBSS.
If by chance this is an old 90* sweep Perko MBSS, I'd toss it in the recycle bin and replace it with a better unit.

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