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I had the exact same issues. As you mentioned in your first post, there never seemed to be a smoking gun nor a definitive fix. I wish I could point you in the right direction. Alas, the symptoms were all over the map and there appeared to be several different problems.
Pumps - I replaced both the LP and HP pumps. Engines seemed to run well for a while with the new pumps, but not ALL the time. Don't buy YAM pumps, they are insanely priced and you can easily find replacements for MUCH MUCH less.
Filters - All new, no obvious crud noted in any of the ones I removed.
Fuel - Fresh from gas stations, not old marina fuel. Startron Enzyme Fuel Treatment was added to all fill-ups since I only had access to E10 here.
Squeeze bulbs - New. Non YAMs that gave POSITIVE feedback that you were actually moving fuel.
Racor's - Housings removed and the mating surface thoroughly sanded and polished to ensure an airtight connection between the housing and the filter element.
Fuel lines - All old "ends" trimmed to eliminate potential air leak sites. Many of them were replaced.
VST's - Here things get interesting...
My engine fuel lines were sloughing off rubber bits from the RETURN side of the fuel injector loop. The HP pumps send more fuel than the engine needs. Excess fuel is returned to the VST through the loop. That fuel contained tiny bits of rubber and crud. That crud would clog the intake screen on the bottom of the HP pump. Drove me nuts. I was cleaning that screen every 20-30 hours of operation.
There were TWO things that caused my engines to only run up to 3500-3800 RPM. 1) The rubber bits clogging the HP intake screen and 2) The SMALL rubber bits going through the HP pump and then clogging my fuel injectors. If the screens were clean, I changed the injectors. Use aerosol carb cleaner or brake cleaner to clean a dirty screen. Use Mr Injector https://www.mrinjector.us/ to reman your injectors.
Eventually, I replaced most of the engine fuel lines. I did not buy Yam hoses because 1) many are unavailable and 2) YAM wants 700-800 bucks per engine for all those small runs of hose. Go to Advance Auto and buy a spool of high-pressure fuel line. I had to make some of them longer to eliminate sharp turns where the fuel line would kink. Took my time and it came out great. Saved at least 1500 bucks. You pay dearly for those YAM tight bends. After this, the engines were fairly reliable since I eliminated the VST HP Screen clog and the F.I. clogs.
I lastly had an intermittent issue with the engine quitting after it had been cooled down for an hour or so. It would fail at throttle up. Squeeze the bulb, restart and it would run like a string trimmer for the rest of the day. Screwy.
I finally got it running great for several months and sold the boat. I wanted something more offshore capable. Sorry that I can't offer any additional info. When they were right, they were great. But the intermittent issues drove me nuts.
Bought a boat with YAM F300s. So far, they are phenomenal. I've done nothing to them except toss in new oil, water pumps, plugs, zincs, and T-stats. They are much easier to wrench on with the exception of the internal zincs. They have similar fuel burn to the 225s at lower cruise speeds where I spend most of my time. And because they run so well I spend less time searching for answers here. Hope this helped, but I doubt it. Sounds like you've already been beating the horse. Hopefully, it will help the next guy.
Best of luck. Broken boats SUCK. Mine is currently down with SeaStar Optimus EPS issues. Electrons, cheap connectors, and pump control modules living next to saltwater. Gee, what could POSSIBLY go wrong?
This is the wake at 1.9MPG, roughly 17 GPH and 32 knots. That's pretty economical for an 8000-pound boat.
F32458EB-285C-4F3F-BF99-75F1A8E716ED_1_105_c.jpeg
Pumps - I replaced both the LP and HP pumps. Engines seemed to run well for a while with the new pumps, but not ALL the time. Don't buy YAM pumps, they are insanely priced and you can easily find replacements for MUCH MUCH less.
Filters - All new, no obvious crud noted in any of the ones I removed.
Fuel - Fresh from gas stations, not old marina fuel. Startron Enzyme Fuel Treatment was added to all fill-ups since I only had access to E10 here.
Squeeze bulbs - New. Non YAMs that gave POSITIVE feedback that you were actually moving fuel.
Racor's - Housings removed and the mating surface thoroughly sanded and polished to ensure an airtight connection between the housing and the filter element.
Fuel lines - All old "ends" trimmed to eliminate potential air leak sites. Many of them were replaced.
VST's - Here things get interesting...
My engine fuel lines were sloughing off rubber bits from the RETURN side of the fuel injector loop. The HP pumps send more fuel than the engine needs. Excess fuel is returned to the VST through the loop. That fuel contained tiny bits of rubber and crud. That crud would clog the intake screen on the bottom of the HP pump. Drove me nuts. I was cleaning that screen every 20-30 hours of operation.
There were TWO things that caused my engines to only run up to 3500-3800 RPM. 1) The rubber bits clogging the HP intake screen and 2) The SMALL rubber bits going through the HP pump and then clogging my fuel injectors. If the screens were clean, I changed the injectors. Use aerosol carb cleaner or brake cleaner to clean a dirty screen. Use Mr Injector https://www.mrinjector.us/ to reman your injectors.
Eventually, I replaced most of the engine fuel lines. I did not buy Yam hoses because 1) many are unavailable and 2) YAM wants 700-800 bucks per engine for all those small runs of hose. Go to Advance Auto and buy a spool of high-pressure fuel line. I had to make some of them longer to eliminate sharp turns where the fuel line would kink. Took my time and it came out great. Saved at least 1500 bucks. You pay dearly for those YAM tight bends. After this, the engines were fairly reliable since I eliminated the VST HP Screen clog and the F.I. clogs.
I lastly had an intermittent issue with the engine quitting after it had been cooled down for an hour or so. It would fail at throttle up. Squeeze the bulb, restart and it would run like a string trimmer for the rest of the day. Screwy.
I finally got it running great for several months and sold the boat. I wanted something more offshore capable. Sorry that I can't offer any additional info. When they were right, they were great. But the intermittent issues drove me nuts.
Bought a boat with YAM F300s. So far, they are phenomenal. I've done nothing to them except toss in new oil, water pumps, plugs, zincs, and T-stats. They are much easier to wrench on with the exception of the internal zincs. They have similar fuel burn to the 225s at lower cruise speeds where I spend most of my time. And because they run so well I spend less time searching for answers here. Hope this helped, but I doubt it. Sounds like you've already been beating the horse. Hopefully, it will help the next guy.
Best of luck. Broken boats SUCK. Mine is currently down with SeaStar Optimus EPS issues. Electrons, cheap connectors, and pump control modules living next to saltwater. Gee, what could POSSIBLY go wrong?
This is the wake at 1.9MPG, roughly 17 GPH and 32 knots. That's pretty economical for an 8000-pound boat.
F32458EB-285C-4F3F-BF99-75F1A8E716ED_1_105_c.jpeg
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