Hope this is right place for this...

1996 Wellcraft Excalibur with twin 502s. These are pre-"cool fuel" 502s - I least I think so. To my knowledge they are original to the boat and have never been out. Engines likely have about 400 hrs on them.

Background -
I purchased this boat from the second owner in the Annapolis area in 2006. He was a knowledgeable, open wallet owner and I never really had any engine issues for the 3 years I owned it there.
Life changed in 2009, and I moved to CA. I replaced both outdrives with Merc Factory new BIIIs just before I left, because one was starting to clatter a bit. At the same time, the engines were winterized, fogged, and the boat was shrink wrapped on the hard stand. The plan was to get established in SoCal, then have the boat shipped west.
Never did.
10 years later, I'm living in Dallas and brought the boat here last year to use on the local lake.
When the boat arrived, I told the marina mechanic - whatever it needs, or might need...
There was much to do - but the engines both were fine. Both started instantly, ran cool, used zero oil, no leaks...

Boat went in the water, and I cautiously took it out a couple of times for 30-45 minutes, to just check things over - all with zero engine issues. I didn't hammer on it (I never do), but had it running for a few sustained runs up 4000 RPM. On the third such run as I was returning, the Stbd engine required more throttle to match the port... After that, it would never go above 2000-2200 RPMs.
That was last year. The mechanic here replaced all fuel filters, and cleaned the FI pump. Said there was some residue and gunk, but nothing too bad. Note that the engine always starts instantly. Sometimes when returning after a test run, the engine would die when pulled to idle. Always idles fine cold.

One noteworthy item is the boat never had ethanol gas before coming to TX. The marinas here have ethanol in the gas. Before shipping to TX, I had a company come and drain (suck) the 10 year old gas out before shipping.

Since then, I've read everything I can and this year I'd like to actually use my boat. The Marine mechanics here are, shall we say, problematic... I would gladly pay someone to do this work, but .... Looks like I'm going to have to DIY...

I bought a fuel pressure (thx to some of the tips / info here) gauge and checked the pressure on battery power and with engines running. On the key, pressure is low 20s. At idle to 1500 RPM, it was 30.5 psi. I think the pressure is supposed to be around 43psi.
Here are the engine numbers:
OF802268
OF264092
Thinking I might have a restriction from the tank, I used a 5 gallon can to remove that possibility. Pressures were exactly the same - both engines.
So, next I changed both fuel filters and Racor water separators - carefully inspecting the gas that came out of them. The filters and gas were perfectly clean. Ran the engines for a bit in the slip to make sure there were no bubbles. Hooked up the pressure gauge and battery pressure was a littler lower, but idle - 1500 RPM was still exactly 30.5 psi.

Note that I always pinch the line from the Fuel pressure regulator during idle testing to see if the pressure moved or spiked and it never moved.

I have never been a fan of "replacing parts in hopes of it fixing a problem" methodology of troubleshooting, but I'm getting desperate.
At this point I'm ready to think my problem is the 20 year old high pressure FI fuel pumps have worn, perhaps exacerbated by the introduction of ethanol and sitting for 10 years. They aren't cheap or easy to change, but if the smart guys here advise that to be the next step.... I will.

TIA
M

Clean and flow test the injectors. Not saying that will eliminate the problem but is surely due.

Likely pump or injectors

0F802268 comes up as a 454 MPI and has a VST.
Cool fuel came out starting with serial number 0K147350 and up
The other serial number is a Bravo drive

https://www.mercruiserparts.com/7-4l...m-454-v-8-19-1

The motor uses a mechanical fuel pump on the raw water pump which feeds the VST. The VST fuel pressure should be 37 psi and mechanical pump should be 3-7 psi.

What brand gauge did you get?

The VST has a screen in the bottom and can get clogged, and can be damaged very easy if not careful.

Since both are doing the same and only one has the issue, I would reach down on the starboard side of the block under the manifold and disconnect the wire off the knock sensor. Try to run again and if it comes back up, slow back down and reconnect. You don't want to have it disconnected if you have a knock.

If the knock wire runs along next to in line with the plug wires, the wires can emit enough of a pulse for the module to pick up[.

Have the wires been changed?

Your tank is likely full of crap. Common in boats that sit. The crystallized fuel can be plugging pickups.

If your gauge is correct your fuel pressure is low. Since both motors are the same and low (in 20s running is unacceptable) , Id be suspicious of gauge. Id run clean fuel, cleaner etc thru fuel system on hose , change filters then pull injectors and have cleaned and flowed. Another OSO member did pretty much what your doing except he skipped the part about pulling the injectors and subsequently AFTER he melted a piston he had me flow and clean them and there were several that were unacceptably low. Even though you had all the fuel "pumped out" , I wouldn't be surprised if there wasn't 10 gallons left floating around in there that pickup wont suck out. 10years later would be some vile ****!

Another thing to add... replace all of the rubber lines going from the tank to the inlet of the fuel filter/separator. I had a similar problem with low pressure and it was because of a split in the supply line. It was not leaking fuel but was sucking air.

Had the same issue, cracked line sucking air, but no fuel leak

Thats what I’m thinking. 10 year layup, something dryrotted and cracked, hoses, gaskets, o-rings.

BUP is a mechanic in Texas, reach out to him.

BUP is in Arlington, TX. In your back yard.