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.