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Last summer after I replaced the fuel pump on my starboard engine (454) in an attempt to address a stalling problem the engine would not start. Investigation showed that fuel was not getting to the carburetor. We replaced the fuel pump with another one, but the problem remained.

The mechanic at my marina eventually fixed my stalling problem by cleaning out the carburetor and replacing springs for the metering needles. The only way he could get the engine to start, however was by pouring filling the fuel filter with gasoline an pouring some into the carburetor. Once started the engine ran properly and would restart again a few days later as long as the fuel had not drained back into the tank or evaporated. The problem went away shortly after Labor Day but reappeared about two weeks ago, after my boat had been in the water for about a month.

I am using the same kind of fuel pump on my port engine and it does not have a starting problem. We swapped the fuel lines at the tank to eliminated the possibility of a problem with the fuel intake pipe.

Any idea what's going on here?

Unless the fuel lines have been altered, a single pick up feeds both engines. So unless one of the lines were clogged,, swapping lines does not prove anything as far as the pick up goes.

Did you check the automatic choke to make sure it is working properly? Could a line have an air leak?

Sorry that is sell i have for you.

Do you have anti syphon valves in the system?

I installed anti siphon valves after I started having this problem.

Automatic choke is working.

I know we dont like to do extra work BUT you could swap the carburetors then if the problem follows the carb...you know it is the carb. If the prob stays with the engine then you know the prob is in the plumbing

What Frank said!! It is the good thing about having twin engines!

Just had a rebuilt carburetor installed. Still not getting gas to the carburetor.

check the line at the suction side of the pump. Maybe run a temporary line to feed the fuel pump.

If your not getting fuel to the carburetor and you have a brand new fuel pump then it sounds like you have a bad fitting or hose possibly that is feeding the system.
If you get air then it will not create suction properly. When the mechanic was priming it last year and it would last for a while then happen again could be a result of the line very slowly losing its prime. BUT when it is completely dry the prime ca not even begin.

Run a temporary line feeding it and by-pass everything BEFORE the mechanical pump. See if the fuel pump is able to prime from that.

Mechanical fuel pumps are pretty simple

to follow onto Scenario's train of thought - walk back the fuel line from the fuel pump back to the tank. I'm guessing you've got at least a filter and a shut off somewhere in there, plus one or more fuel pickups in your tank(s) check all of them.

to bypass all of it, a short length of fuel hose coming from the input side of the fuel pump and have it go directly into a 1 gallon gas tank.

another thing to check and I am not sure if it would be in your 454 would be a small fuel line inlet filter being clogged. My 3.0L carbs had one where the fuel line entered the carb.

I've run into a few fuel problems on my boat as well as friends, I'm going overkill on my fuel system with 2 fuel pickups in the tank each with their own separate fuel line plumbed into this makeshift manifold https://photos.app.goo.gl/5n3U1n2cMDeYtwqx6 which will allow me to shift between pickups and plumb in a temporary tank for testing if required. The output of the manifold will go into my fuel filter and out to the motor.

Can also cap one end and put a vacuum pump with gauge on it and look at loss. (Short of running all new suction side plumbing first).

It sure does sound like an air leak, and if you switched the fuel lines at the tank with no change it has to be between the end of the stbd fuel line and the pickup. Here's some thoughts:
Take the anti-syphon valve out of the stbd line and re-connect without it. If pump primes the valve is the problem. If not, remove and inspect the pick-up tube--I had one develop a hairline crack inside the tank, just under the fitting.
Disconnect stbd line at the tank before the anti-syphon, hook temp line from there to 1 gal can--pump should prime with no problem. If it does not, maybe it's not an air leak. Hook a fuel pressure gauge to the output side of the pump--it should be, I believe, between 5-7 psi. My thought is that mechanical pumps are pretty old hat, and even a new pump off the shelf might have been made years ago, before they began using ethanol proof materials. That poison will destroy a regular diaphragm in short order, so that it moves just enough fuel to keep the engine running, but so soft it is not able to pull a prime once the fuel drains.

The mechanic found a pinhole leak in the steel line from the fuel filter the fuel pump. I have not yet done a bay test, but both engines start now.

air in the line. Surprised it did not impact the other engine since the tank from the factory works off a common pickup for both sides.

The leak was in the pipe between the filter and the fuel pump. Also, my take has separate pickups for port and starboard.

I am sure that was the problem. Makes sense that air in the system originates from the vacuum side. Hopefully a good seatest will prove it. Good luck!

Took a short run in the bay today and both engines started easily and ran well. With minimal chop, I was getting 23 knots at 3000 rpm.

In addition to fixing the leak, the mechanic removed the intake manifold to replace a broken stud, installed a rebuilt carburetor, and replaced the valve cover gaskets. Up to this point, I had been doing almost all of the maintenance and repair work myself. After spending the previous two summers trying to diagnose intermittent problems, however, it was worth it to pay a knowledgeable mechanic.

"When you can spare the boat for about week", he wants to put a rebuilt carburetor on the port engine, replace the fuel lines, and fix an exhaust leak. The latter will probably require an new exhaust manifold. Depending on how the boat runs this summer, I will wait until after Labor Day for those items.

Watch that exhaust leak. Fumes kill.

Peter,
Glad you sorted that out. Like a P0442
(Very small leak) in an evap system in the automotive field these things can be difficult to trace down. Speaking of which, a smoke machine in those lines may have shortened leak hunting time some too! Good job finding that!