2014 carburated 454 stumbling

Ok, after this weekend about two hours into our 2.5 hr trip my starboard engine (170 hrs) starts losing power and fluctuating RPM's. Running smooth at about 3000 rpms then starts to drop down to 800-1000 then bounce back up. Did this for approx 15 minutes then shut down. Excessive white smoke (more than normal) out of the exhaust while running. I cut the port engine back and "putted" for about 10 minutes. Cranked up the starboard and fired right up. Ran smooth for twenty minutes at 2500 rpms and little white "smoke" from exhaust as it normally does. Started to fluctuate again when I reached destination. Haven't ran it since- left it at the marina as I didn't want to "chance" it getting back home.
The fuel pump and fuel filter was changed a few weeks ago and had been running stronger than ever since. Now this. Driving there tomorrow to mess with it, bringing a new fuel filter with me. All advice and ideas are appreciated.

Are both engines running off the same tank? Separate tanks? Different filters? Water/fuel separators? Collapsing fuel line? More history on your yacht might help. Old fuel from last year? Gunk in tanks, Water in fuel? when was last tuneup? First run this year? Is this a new issue? Was she running good when you laid her up last winter?

Thanks Johnnygjr.
Both run off same tank, different water/fuel separators. Fuel lines are metal except for approx 12" from the tank.. Fuel is fresh, just topped off. Tank was emptied and cleaned two years ago when new engines installed. previous tune up at 100 hrs. 170 hrs on engines now. Just had all checked by mechanic with new water/fuel seperator and fuel pump installed. Yes it's a new issue and no we use our boats year round down here in South Louisiana. The engine wasn't running warm at all (as per guage) and was running perfectly up to this point with good oil pressure, temp and vacuum. Both engines running the same as per gauges.
Stupid me for dropping over 20k on brand new engines to rid of the problems I was having with the old ones, only to get a different set of problems. May be time for learning to golf....

Are you sure it is "white smoke" in the exhaust??? That's an indicator of coolant being burnt in the combustion chamber....

Assuming the tank is still clean, I'd start with a fuel pressure test and check the suction side with a vacuum gauge as well....I'm thinking gummed up anti-syphon valve....

"Steam" maybe?? I'm thinking your thinking is right about the anti siphon valve. I looked at the configuration coming out of the tank and the lines/valves don't look as though they've ever been dealt with (boat is a '75 31 Hatt).

"Steam" maybe?? I'm thinking your thinking is right about the anti siphon valve. I looked at the configuration coming out of the tank and the lines/valves don't look as though they've ever been dealt with (boat is a '75 31 Hatt).