1979/1980 Merc Fuel pump problems

[frankenmerc]

Hello
I have this 80 hp in line 4 cyl that is giving me some problems.
Here is what she is doing.
Crank her up and let her run for a while and eventually runs out of gas and checked carb drain plugs and not much fuel come out at all.
its like i am loosing fuel pressure.It will be running for a while at idle and it will start to hesitate and die and then i can rev it up and looks like the pump will start pumping again but if i let it she will die, it ran for as much as 20 min at idle and eventually stop.I have a filter that is white and i can watch it and it dont look like much fuel is coming in unless i rev her up.
This is what i have done.
1)changed diaphram and put new kit.
2)Moved filter between outlet of pump and carb inlet.
3)adjusted idle
4)Changed fitting on hose end and fitting that connects to motor
5)blew out pulse holes
6)Changed check valves

Running out of ideas anyone have any sugestions would be greatly appreciated.
Thanks

 

[Chris1956]

If you can pump the primer bulb and keep the motor running, you have a fuel delivery issue. Check for air leaks in the fuel line, especially at the fittings.

 

[kmarine]

What fuel filter are you using, do you see bubbles in the fuel? Did you check the anti siphon valve? Do you have a cloged pickup screen in your tank? This is wher I like to start before replacing the pump.

 

[frankenmerc]

Thanks for the input,on the primer bulb yes if i pump fuel manuely it will continue to run. I am using a small white marine outboard filter.Antisiphon valve not sure if you refering to the primer bulb but thats ok and pumps up stiff to start.
Pick up screen is ok and no obstructions and i changed the hose in the tank.
not sure what to do now runs good as long as i can keep the fuel going to the carbs.

 

[racerone]

Did you check to see if the check valves in the fuel pump are operating ?----The fuel pump is driven by crankcase pulses.---A positive pressure pulse and a slight negative pulse.-----Have you checked for those pulses ?-----A cylinder compression test MAY give you a clue on crankcase compression .----Crankcase compression can not be directly determined with a compression test.

 

[frankenmerc]

Thanks

I have replaced them with new gaskets, I did blow through the inlet line and was able to blow through and suck on the line and the valves close and wouldnt let air back through the pump.
i didnt have a way to check fuel pressure.
I did pull the line off that goes to the carb and noticed a small pulse but weak and incosistant as i spinned it over.
compression test revealed 115lbs per cyl. --Hmmm.
not sure if i follow you on the last part.
Thanks again

 

[racerone]

???--Do you have pressure pulses coming from the port / s of the crankcase when you spin it over ?

 

[frankenmerc]

I do have some but seems to me its weak looking at the manuel it shows it to not have much 2 1/2 to 3 1/2 psi. I may have to try to get a guage and check the pressure.