1971 Mercury 1350 fuel pressure?

[Jhoppy]

1971 Mercury 1350 fuel pressure?


After some trial and error I have figured out I am loosing fuel pressure under certain times and conditions with this motor, it will hold at two to three pounds of fuel pressure at idle and at wide open

But going from idle to mid range it will lose pressure sometimes not always it has relatively new fuel pump diaphragms in it and the check valves look ok and hold pressure when blowing though them and are seated properly

Wile initially testing I had the fuel line running from the tank to a water separator though the primer ball to motor thought the diaphragm fuel pumps then to the carbs and my fuel pressure gauge.

What kind of fuel pressure should this have wile under way?

What size of fuel line should I be running from the fuel tank to the motor is 5/16 large enough or should I be running 3/8

I am relatively sure the add in water separator is not causing too much restriction as it will loose fuel pressure even with it by passed


I have now dummied in a electric fuel pump so now the fuel comes from the tank into the electric fuel pump then to a water separator then though the primer ball to the motor though the diaphragm fuel pumps then the carbs and my fuel pressure gauge and it runs good with four to six pounds of fuel pressure at all speeds .

Also with the electric fuel pump running there are no leaks so that leads me to believe the diaphragm fuel pumps are not sucking air

My plan at this point is to bypass the diaghram fuel pumps and run the fuel from the tank to the electric fuel pump wired into ignition power then to the water separator and to the motor to the carbs bypassing the built in fuel pumps all together unless I figure out something to fix them,

When I bought this motor originally a couple years back the built in fuel pumps were bypassed and an electric pump was installed and I had replaced the diaphragms and removed the electric fuel pump, thinking then an add on electric pump was not necessary but now I am at wits end with this thing

Thanks

Hoppy

[Texasmark]

Info here comes from personal automotive and marine experiences.

Any obstruction preceding a vacuum diaphragm pump, or probably any fuel pump can cause the pump to pull harder than the fuel can respond. Result is a boiling of the fuel due to the lower pressure of the vacuum allowing the boiling point of the fuel to be lower than at 1 atmosphere where it doesn't boil. Engine stalls; lack of fuel. Fuel pump pressure of 3-6 lbs is what my electric fuel pump specified and you could dial in what you wanted. I think I ran my tower around 4 psig.

I think you are perfectly warranted in your idea to just bypass your engine pump and run off the electric that you already have placed correctly....pushing fuel, not pulling fuel through your separator and trying to "pull it up the tower" to feed the upper carb.

My 2c,
Mark