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...

Id pull the pickups and see if they have those sock thingies on them, and replace the 29 yr old fuel lines

^^^^very well could be that stuff.
Have worked on 2 boats that it was the carb’s. 1 was about 5 min’s cruise speed. Good cleaning, full rebuild kit with needle and seats, wallah…,I thought ignition but nope…..

psst: not saying this is your problem. Just throwing more on the table.

if it has quadrajet carburator, check the stone filter in the carburator fuel inlet

The re-start symptoms sounds like a flooded eng; "gotta hold throttle wide open, runs rough at first then clears up". Do you see black smoke out the exhaust when restarting after one of these episodes? If you have a fuel press gauge, see if it holds press for a while after eng shuts off and look down the throat of the carb if fuel press falls quickly. Could also look for liquid in the hose that connects the diaphragm cavity of the fuel pump to a port on the carb (if equipped).

Ive had my fair share of problems just like this. Ill repeate what others have said

• check fuel filter in carb if Q-jet
• make sure fiklter is installed correctly in carb. One way it will starve the carb for fuel the other way is good
• when it gets restarted see if the exhaust is black- Qjets were know for having leaking primary jet wells
• Check that the choke is fully open
• if you can check that the fuel pickup isnt partianlly clogged. The screen in the end are pretty small
• vapor lock- check temperature of fuel pump with a infra red temp gun
• bad fuel pump- if you ever used auto gas, the diaphram can get hard and wont pump as well
• If possible fuel a fuel line from a gas can to fuel pump to bypass rubber line.
• Check grounds for distributor
• When it does shut down, take spark arrestor off both engines, compare accelerator pump shot between both engines

It sounds like something temperature related. since this is the port engine the fuel pump is stuck between two engine and may not get enough air flow.
I had a similar problem, thought it was bad fuel pump, installed a electric pump, still had problem, replaced lines, problem seemed to go away, but it had a stumble once, pulled fuel pump apart and found the diaphram was hard, rebuilt fuel pump, pulled tank pickups, screens were 1/3 plugged but fuel outlet was 1/4 npt (WTF) replaced pickup with custom 3/8 stainless with no screens and outlets are -8an now.

I pulled the rubber fuel line, it looks ok, ran a wire through its length no issues, cut it in two places, looks fine, pliable...I put a fuel bulb inline, at the filter inlet. at 3200 rpm the bulb is stable no contraction, I can't pull the inlet out of the tank, but blew into it, no restriction. the clear tube on the pump is empty,

I'm leaning to an issue with the float flooding the engine, because of the having to hold it wide open to clear and run...it has a weber 9780S 4 barrel, I'm not familiar with it, will put up another post for advice on it....

If it have raw water pump integrated fuel pump, check it if cam or follower are worn out. Then it won’t pump enough fuel at higher load.

pulled carb off, going to clean and rebuild,

Found it, took carb apart noticed floats were set way too high, put pressure gauge on fuel line at carb, 9 psi at idle over 15 at 2500, off scale at 3300, checked gauge on my 7.4 litre, 6 psi all day...pulled (mechanical) pump, housing full of fuel not gear oil, replaced fuel pump, runs perfect. fuel way over pressure, gets by needle, flooded it....