ATTN, Troubleshooting gurus...head scratcher...
 

Friends boat, 1996 Searay 330 sundancer, twin 7.4 V drives, port engine will run 15-20 minutes at cruise, 3200, then sputter and die. hard starting but will start after cranking and holding throttle full open. on restart will not idle, runs very rough then bam all cylinders fire and you have to throttle back...it settles out, you can go back go to cruise...then 20 minutes later it conks put again,


-replaced coil
-swapped ignition module port to starboard, no change,
-replaced distributor cap, rotor, sensor assembly
-replaced fuel filter (spin on)
-blew out fuel tank vent lines...
-fresh gas

about to change plugs and plug wires

I'm at wits end

Any ideas?

Plugged tank vents? Back flow preventers clogged up? Fuel filters, probably more than one feeding each engine?

changed filters, blew out vents...

I'm thinking degraded fuel lines, and they are collapsing, at higher RPM...

I would check voltage and fuel pressure when it is running rough to determine if it is a spark or a fuel issue

Thats my guess.

FYI these engines have those small flat fuel filters in addition to the normal looking filters? It's a screen that has to be removed and cleaned, and often overlooked as a filter. File your fuel lines to make sure you don't have these. If you do, take apart and clean.

Sounds like IAC or pressure regulator…

Put a gauge on it and see what fuel pressure is when problem occurs

Nevermind I thought it was EFI

That could be an issue although I've never actually ran into one with that problem as of yet...seen a ton of PWC with that issue though.
Since you say its a carb engine then the only rubber line would be from the fuel tank to the fuel filter assembly....the rest should be hard line stainless or aluminum bent tubing all the way to the carb unless someone has messed with it after the fact....so changing that line should be an easy thing to do and fairly cheap to try and rule that part out. The way you describe things happening it almost sounds like vapor lock...have you tried just running the boat with the hatches off or cracked open to give it maximum cool air and see how it runs? And what are the temps of the motors when this happens?

there's about 10 feet of 5/8 marine fuel line from tank to filter, then hardlines from there to carb...engine temps are 170 indicated, high for what i'm used to but i'm told they have always run at that temp...I can run with hatch cracked, but would be hard on it in anything but flat water, its a one piece hatch probably 5 x 10...with only one ram holding it up, I did run with blowers on to try to get air into engine bay...