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I've had similar issues in the past, but my motors are running pretty sweet right now. I attacked pretty much everything you did but I also replaced both pumps on the Port engine. I think that the HI psi pump died because it kept cooking when the VST ran empty. At least, that's my belief. But my engine was dying after prolonged idle in the no wake zone. It would always respond to squeezing the bulb until firm. Clearly, the VST was running out of fuel.
I started at the tank and cut an inch off of every hose at every connector in an attempt to eliminate any air from entering the system.
Where I could, I eliminated short runs of hose and barbs.
I added new fuel lines if any were less than new in appearance.
I pulled the racors and lightly sanded their mating surface to the filter canister. I checked the surface with a straight edge. I added new filters and gaskets just in case.
I swapped the check valves from P to S. No change, which is good as that thing is expensive.
I put in a new first stage engine filter and gasket, carefully installing it to ensure that it was snug and not leaking.
I opened the VST and checked the screen. I replaced the VST gasket.
I put in a new F filter.
I monitored the LO PSI pump operation at idle with the engine cowl and air silencer off. Ops normal.
I replaced all the hose clamps.
etc etc etc.
The supply side of the system must be flawless in integrity lest the LO PSI pump suck in air instead of fuel. At least, that's how it seems to me.
The engine is pulling fuel the entire time it's running, and then when you shut it down, the suction on the supply side of the LO PSI pump is still present. If there's a leak, air will enter and there goes the prime for the LO PSI pump, right?? You fire it up using the fuel in the VST and then it quits due to starvation, as Rodbolt says.
Finally, my problem went away. I cannot pinpoint exactly what the issue was. That is frustrating. That was roughly 150 hours ago.
Now, my STBD motor behaves as yours does, but only occasionally. I can leave it in the water for a few days, start it without squeezing the bulb and then run it without an issue. Then, two days later I run it for an hour or two and shut it down for a long drift. It will then quit at throttle up after sitting off while WARM.
I wonder if the temp has something to do with it. Consider that the temp under that cowling is HOT after you shut it down. No water cooling flow to a hot engine. I would not be surprised if the heat plays a role in allowing the delivery side to bleed off pressure as it cools. PERHAPS the increased temps increase the suction differential in the supply line, and when no fuel is flowing, any breach in supply integrity will allow air in?
I don't know, just thinking out loud.
In your video, how long does it take to see fuel flowing back into the first stage (engine) fuel filter? I am surprised to see it empty for so long As the fuel flows to the LO PSI pump I would expect to see an increase in suction due to the filter draining (to the LO pump). Would that not hold the fuel in the supply line? If your other engine is running normally, I'd attempt to recreate that situation with another piece of clear line. If that line does not behave the same exact way, you may be on your way to a solution.
I started at the tank and cut an inch off of every hose at every connector in an attempt to eliminate any air from entering the system.
Where I could, I eliminated short runs of hose and barbs.
I added new fuel lines if any were less than new in appearance.
I pulled the racors and lightly sanded their mating surface to the filter canister. I checked the surface with a straight edge. I added new filters and gaskets just in case.
I swapped the check valves from P to S. No change, which is good as that thing is expensive.
I put in a new first stage engine filter and gasket, carefully installing it to ensure that it was snug and not leaking.
I opened the VST and checked the screen. I replaced the VST gasket.
I put in a new F filter.
I monitored the LO PSI pump operation at idle with the engine cowl and air silencer off. Ops normal.
I replaced all the hose clamps.
etc etc etc.
The supply side of the system must be flawless in integrity lest the LO PSI pump suck in air instead of fuel. At least, that's how it seems to me.
The engine is pulling fuel the entire time it's running, and then when you shut it down, the suction on the supply side of the LO PSI pump is still present. If there's a leak, air will enter and there goes the prime for the LO PSI pump, right?? You fire it up using the fuel in the VST and then it quits due to starvation, as Rodbolt says.
Finally, my problem went away. I cannot pinpoint exactly what the issue was. That is frustrating. That was roughly 150 hours ago.
Now, my STBD motor behaves as yours does, but only occasionally. I can leave it in the water for a few days, start it without squeezing the bulb and then run it without an issue. Then, two days later I run it for an hour or two and shut it down for a long drift. It will then quit at throttle up after sitting off while WARM.
I wonder if the temp has something to do with it. Consider that the temp under that cowling is HOT after you shut it down. No water cooling flow to a hot engine. I would not be surprised if the heat plays a role in allowing the delivery side to bleed off pressure as it cools. PERHAPS the increased temps increase the suction differential in the supply line, and when no fuel is flowing, any breach in supply integrity will allow air in?
I don't know, just thinking out loud.
In your video, how long does it take to see fuel flowing back into the first stage (engine) fuel filter? I am surprised to see it empty for so long As the fuel flows to the LO PSI pump I would expect to see an increase in suction due to the filter draining (to the LO pump). Would that not hold the fuel in the supply line? If your other engine is running normally, I'd attempt to recreate that situation with another piece of clear line. If that line does not behave the same exact way, you may be on your way to a solution.
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