Tiara 3100, 454TBI , 1996 backfires & stops

Fairly new boat to me. 970 Hours and they seem to run pretty well. Coming in the other day, however the port motor backfired and turned off. Restarted ok and but it repeated two more times. I’m laid up at the moment, so looking for advise. It sounds to me like it’s loosing ignition power from open circuit at ignition . Maybe replace ignition switch!?

I would suggest troubleshooting it before attempting to throw parts at the problem....because its intermittent, it may take some time to figure it out.

I'd be inclined to start by putting a pressure gauge on the fuel line....and then trying to replicate the problem. If the pressure drops, it could be the fuel pump failing or (based on your description) a restriction in the fuel pickup plumbing due to crud buildup....

Any idea when last tune-up was?

are there separate tanks for each engine or a single shared tank?

Thanks for the suggestions. I have read many threads with your name on them. I like the forums and will post info and hope to benefit in troubleshooting my engines. I will check fuel pressure, each engine has its own gas tank. I have replaced all fuel filters, new plugs, and wires, after I bought it 6/18. I adjusted timing on my problem motor two weeks ago. Went from 10 to 12 degrees before TDC. Seems better. I noticed the timing / run plug had a post corroded off. It looks like a pin to locate plug only.no wire to it, unless it’s in Moulding of plug. I use the boat for fishing and a typical day is 50-80 miles . Up to 178 hours added since I bought it. At 80 I have to wait for the old man days,

thanks again Capfish !

Couple things can make it sputter like you describe. I will assume the thing conked out when coming off plane/decelerating/cutting throttle. I'm not saying these are your issues but the symptoms you describe could be:
Poor fuel delivery...low pressure, clogged or faulty injectors, tank not venting, vapor lock, etc
Vacuum leak...loose, cracked or missing hose, vent or PCV valve; something around the intake manifold. Idle air bypass not working.
Cable adjustment.
Fortunately these are not expensive to remedy, the costly part is paying someone to narrow it down.

Thanks for the info, I do try to do most maintenance on this old gal. Sometimes I have to wait for my old body to come along. I will have lots of things to look at next week. And will keep posting.

For switches/sensors and relays that are easy to remove/replace, it's sometimes better to swap them with the good motor, to see if it rectifies the issue.