Strange running

[ukstu]

Hi all, I’d like some advice and thoughts if possible please.
I have a new ( 5 hours) 5.7 Mercruiser Alpha one with gen 1 drive.
I’m having some strange problems at what seems to be completely random stages.
Engine starts and runs fine with good power but occasionally, it will lose power, it feels like it is surging, kind of lunging on and off power.
It feels to me like a lack of fuel supply issue but can’t find anything wrong.
I’ve changed the water separator and fuel filter, it’s regularly serviced and the fuel is new and I’ve checked all plug caps are correctly fitted and distributor cap is clean and dry.
I went out last week, put the power on and about 5 minutes later it did it, and it lasted for about half an hour.
The next day, it ran perfectly, 25 knots at about 2800rpm for about 40 minutes before doing it. (this time I could hear it coughing and back firing a bit when trying to put throttle back on / more throttle to see if it would push through it)
Any ideas please?

 

[alldodge]

It's a 5.7 what, serial numbers are best?

 

[ukstu]

Engine number is OC772014 I believe..

 

[alldodge]

Coming up as a 1987 carb TB ignition and mechanical fuel pump

Do you know if it does it quicker if it has a full tank of gas as opposed to half tank?

Can you dump the contents of the fuel filter in a clear container to see if there is anything but clean clear gas?

 

[ukstu]

Thanks alldodge. Yes correct engine with the exception that it now has an electronic fuel pump.
Tank of gas is an interesting question. On way out when happened quickly, it was about 2/3 full, on way back when it ran perfect for about 40 mins it was full when leaving.

I’ve checked the fuel that came out of the separator and filter in a glass jar and no problems with it at all.

 

[alldodge]

Might be a clogged vent line
If it does it, stop and open the gas cap and listen for air being sucked in. If you do then tank vent line is clogged.

The fuller the tank the quicker it takes the fuel pump to draw a vacuum.

The more empty the tank is the more time it takes to draw the a vacuum

 

[ukstu]

That makes sense, but, it ran longer without doing it when it had a full tank.
I’ll certainly check the cap next time though and listen.

 

[alldodge]

Might be fuel pump is loosing power, weak or oil pressure safety switch.

Check to see if fuel pump runs when cranking and could bypass switch "for testing" and see if issue goes away, otherwise might need to check fuel pressure