I think it sounds electrical too.
The fuel/separator change seems coincidental, it simply allows time for whatever electrical part or ground/connection that's on the way out to temporarily "get happy again" (cool down ECU module, let some ground wire/power wire make a few corrosion blocked electrons pass through again, jiggle some wires nearby that you move to change the filter etc)

An instant "key off" shutdown is electrical, until proven otherwise IMO.

Fuel related would usually sputter, a few cylinders miss, run lean, shut down after some of that behavior.
It also would not tend to "fire right up and run great instantly" because WHATEVER water or bad gas IN the fuel lines so bad to cause "instant shutdown" is STILL in all those fuel lines and would take some time to crank, cycle the bad fuel through to good fuel (sputtering and acting weird awhile) til "good fuel supply" was there. If the fuel situation was SO bad to cause "instant shutdown" hard to see it fixing "instantly"...

(Fuel injected a bad high pressure fuel pump with sudden 0 PSI if pump is going complete shutdown suddenly could act that way...)
You have carbs so won't "instantly" shut down, NOR REFIRE, from fuel, takes some TIME to sputter the bowls empty, THEN some time to refill them if they WERE empty or filled with water!
Find all possible electricals to coil, ECU, grounds, battery, get out sand paper/file and redo the connections.

Changing ECU sounds reasonable if nothing else found. Parasail941's advice sounds on "If it doesnt show any signs of stumbling or poor performance until it quits or slows down,it's most likely the module.Usually,once the module gets hot, the problem will occur at much shorter intervals than the initial shutdown of the day."

I agree with most of you it does seem electrical, however, this weekend to start as a baseline I simply put a new fuel water filter on and ran the boat alot. Ran like a champ with no proplems at all. It should be noted it was about 10 degrees cooler than the other days so electrical with heat might still be the issue. I have no ECU on the boat, but I followed all wires for ignition (key, starter, plug wires, distributor, and coil. Everything appeared normal and nothing was ever hot. My next step will be to dump the filter contents into a jar and see how much water there actually is.

Any thought on anything else to work on?

So here is what I have found. Electrical is out.....because it happened twice today on the river and each time the fuel pressure gauge was hardly off 0 psi. I would dump the filter and then start the boat back up and the pressure would start at zero and then moments later it would jump to 5 psi and hold firm. The motor would idle perfectly at just a hair above 0 psi and when throttle was added it would backfire. Is this water in the fuel causing the separator to fill with water and block the flow of fuel. Another though is the one fuel line to the gauge from the filter is laying on the water circulator hose and could this be causing vapor lock on a 160 degree consistent temperature motor?

Please help motor heads.

Jed

If engine dies, the fuel psi will instantly become 0. Don't get fooled by that.

Water does not block flow. It flows like fuel...edit in: removed improper info. LOL.

Yes ! Get your fuel line off of any heat source ! Farther away even better. Atleast a few inches minimum.

Is your fuel water seperator near full or near empty when you take it off ?

Breaking the seal of the seperator and then putting back on to make it run fine for a bit tells me one of two things:

1) You have vapor lock before the pump
and/or
2) the antisiphon valve is getting stuck and/or restricted. The quick psi change of pulling seperator makes it move.

Sounds like you have had tons of chances to do that....with all the seperator removing you've done. LOL.

Should have done this earlier.....so, yes....do this ! Get this diagnostic out of the way.

Actually, water is heavier than fuel, hence the reason why it sinks to the bottom of the fuel/water separator.

The motor didn't shut totally down these two times it just dropped from cruising RPM to idle and at idle it was zero psi. I will absolutely move the fuel line off the water hose. The separator is always full when dumped and I did dump in a jar and there was some water settled in the bottom, it nothing crazy. I think with these last tests electrical can be ruled out correct.

Ps-the check valve on the tank was removed years ago thinking this was the issue.

Sunday AM ! hahahahahahahahaha

More coffee.

LOL.

Yes, fuel floats to top of water. Thank goodness.Thus why seperators easy to design / make.

I'll say 100 times -" I'm such an idiot. Don't post/say/diagnose things until you are awake."

hahahahahahaha

I'm actually on my second cup right now, the only reason why I am marginally thinking.