F50 VST issue or something else?

[amaranth43]

I have a pair of F50's on my commercial fishing boat. The port engine (2021, approx 2000 hours) has started losing revs when operating at 4000-4500 (normal cruising revs for me). The engine won't stall, it drops revs to 1000-2000 and then jumps back up again. If I don't alter the throttle it would continue in this yo-yo fashion, but if I bring the throttle down it will happily run at 2500 rpm, any higher and the issue will happen again

If I pump the fuel bulb, this solves the problem for maybe 20-30 seconds. Also, it doesn't happen for the first minute or two of running at 400-4500.

I drained the VST tank yesterday and sprayed some carb cleaner up the drain tube, but this hasn't helped.

Before I take the VST tank off is there anything else I should be checking first? I wondered about air leaks in the fuel line connectors, or maybe the fuel lift pump is failing?

Thanks in advance.

[99yam40]

if pumping the primer bulb helps then I would think the VST is not staying full.
air leak or pump problem is what I would be looking for

[boscoe99]

Pump problem would be my first guess. Mechanical and electric.

Fuel leaks in hoses should be obvious. A gauge can be installed to test for air leaks.

[99yam40]

Pump problem would be my first guess. Mechanical and electric.

Fuel leaks in hoses should be obvious. A gauge can be installed to test for air leaks.

yes a leak after the primer bulb should show up with a fuel leak when you pump the primer bulb but not always. might just leak under a vacuum not pressure filter housings and tank fittings

[amaranth43]

Thanks everyone,
I'll have a good look at all the fuel lines tomorrow, and make sure nothing is loose/split. And also look at the lift pump too.
I have a spare engine ashore so can swap parts out with that one to try and figure out what the cause may be.
Will try and report back if I get a fix.

[DennisG01]

What's happening inside your VST tank is that it's not staying full enough to keep with the higher RPM fuel demand. By far, the most common reason is failing low pressure lift pumps. They're pretty easy to test - testing for holding vacuum/etc - a pretty straight forward test is listed in the service manual, though. The rubber diaphragms simply get old over time. You can rebuild - but new ones aren't far off from that price.

[Bluefish]

On my 150 there is one more fuel fitter before the VST that I forgot about my mechanic called it a C filter could be clogged

[yamaha1988]

It sounds like you might be dealing with a fuel delivery issue, but there are a few things worth checking before you pull the VST apart. Air leaks in the fuel line are a common cause of the symptoms you’re describing, especially since pumping the bulb temporarily resolves the problem. Take a close look at your fuel lines, connectors, and the primer bulb. If there’s any sign of cracks, wear, or loose fittings, those could be letting air into the system.

The fuel lift pump is another thing to consider. If it’s starting to fail, it might not be able to keep up with demand at higher RPMs, causing the VST to run low on fuel. Testing or replacing the pump could save you some time and hassle later.

Also, don’t overlook the basics. A clogged fuel filter or water separator could easily restrict flow at cruising speeds. If those haven’t been changed recently, it’s a good idea to swap them out.

If all that checks out, pulling the VST might be necessary. When you do, pay close attention to the internal filter and the high-pressure pump—those can get partially clogged and cause similar issues.

Let us know what you find! It’s definitely frustrating, but with a process of elimination, you’ll get it sorted.

[amaranth43]

So an update on this.

I checked all fuel lines first, all seemed ok. Swapped out the lift pump with a spare I had, no change.
Today I put the boat on the beach as it was due oil changes and a scrub anyway, and I did the VST. I'm never sure with the VST filter how to tell if it is clogged, it was all yellow and I would say the mesh was darker too. Also a bit of crud in the bottom of the tank, and the injectors didn't look that great either. Fitted new filter, gave everything a good blast with carb cleaner and fresh fuel, and back together. Also replaced inline filter and the primary filter in the engine, and cleaned the water separator and put a new element in it.

Went for a spin once the boat refloated, and everything seems ok. Ran at 4400 for at least 5 mins and didn't skip a beat. I'll be putting more hours on the engine later in the week so will see what happens.

A side note, I have battled with what I have heard others refer to as the 'demon bolt' (the bottom bolt on the intake manifold) and after spending considerable time removing and replacing it today with a 12mm spanner, I realised as I finished replacing the manifold that there is a special hole in the cowling which lines up perfectly with said bolt, allowing access with a socket! DOH! So much time wasted! At least I'll know for next time.....