5.7 fuel issue ( I think)

[ukstu]

Hi all, I have a Mercruiser 5.7, Electronic fuel pump, Rochester 4BLL carb, Thunderbolt ignition, Alpha one gen 1 drive leg.

This season i have had trouble with the carb draining down during long gaps between use (even though i thought i had sealed the well plugs last winter during maintenance). It will take a fair while to start on the first attempt but once started will start at the turn of the key for the rest of the day, and the next day. ANything longer than a few days and its slow again.

Now, i am not sure if the above is connected to the problem i have experienced or not but i would value your advice / input.

Went out on Monday last week, short trip approx 8 miles, sat at beach then went to return about 2 hours later.
About 15 minutes into run at about 28kts, it started lunging / lurching as if it was being starved of fuel. Dropped down from 25 knots to about 10 and it kept going all the way back.

Since, i have been down and checked the fuel separator, fuel filter and disconnected fuel line to carb and it is pumping as expected. Ran up against jetty and all seemed fune.

Went out today, slowly and steady to start with, decided all was ok, managed to run about 35 miles at approx 20-25kts with no problem at all.

Mid way back experienced same lunging. Stopped had a check round, started again and it ran normally for about 20mins then same.
Stopped, started again and same again, short while running fine then lunging.

I also found that if i reduced revs right down, without actually stopping, it would then quite happily get back up on plane for a while.
On one occasion, and only once, i did hear a slight cough of a backfire. This literally only happened once in the whole time. Not sure if that's relevant.

Below about 2000rpm it would run and keep going.

20 kts = approx 2700rpm
25kts = approx 2900rpm

I hope this isn't too much info or hard to follow but i'd really appreciate some advice, i'm wondering if the float bowl is losing fuel or something along those lines..

Thanks in advance

 

[ukstu]

View attachment WhatsApp Video 2021-09-12 at 21.50.57.mp4
I've tried adding a clip that my wife took while underway, i think the sound describes it better than i could..
Hopefully it will work.

 

[zellerj]

Sometimes a piece of crude is in the float bowl, and gets sucked into the main jet and causes this symptom. It is erratic and depending on where the crude is in the carb, can run fine for a while or cause issues. I would take the carb apart and look in the bowl for crap. Sometimes this crap can be caused by stuff breaking free from the gasoline supply system after the filters.

Another source of issues is a plugged vent in the gas tank - loosen the cap and see if it improves.

Another source of issues is sucking air - either in the fuel lines, or pick up tube, that causes the pump to not feed enough fuel to the carb.

 

[alldodge]

Agree, think your running out of fuel.
Fuel pressure, weak pump, antisiphone valve, other fuel line restriction or tank vent restriction.

As zeller mention start with the easy part, remove the gas cap and see if it comes back to doing ok. If not, then get a gauge on the fuel line between filter and carb

 

[ukstu]

Thanks for the replies. I had another look around today, and noticed what looks to be a fuel leak from the carb / manifold gasket, same both sides,

could this be a possible cause, I’m guessing if it is leaking fuel out then it will be getting starved of fuel under load?

 

[alldodge]

Might have or have "had" a leak but the pump can keep up with most leaks. Unless your seeing a smelling fuel flowing out of the carb, that isn't the issue

 

[nola mike]

Might have or have "had" a leak but the pump can keep up with most leaks. Unless your seeing a smelling fuel flowing out of the carb, that isn't the issue

Unless that gasket is leaking unmetered air into the engine under load.