Inconsistent Fuel Pressure

[Road Demon]

I got a 74 Duster with a 360ci in it and the fuel pressure is erratic at best. Little over a year ago the stock fuel pump went out so I upgraded to a holley pump. Long story short worked well till 40+ year worth of gunk clogged the fuel line and also developed a leak after a long trip. Put a new tank, sending unit, 3/8 stainless lines, and hoses under it got it back on the road. Started noticing the fuel pressure would start at 7-8 psi then at different stops be at 5, 3, 0, 9, or some other number when I checked it. Finally the car was giving problems starting and running so I bought a new fuel pump. Now when the motor's first started starts out at 7-8psi but after driving awhile pressure goes to between 0-3. Funny thing about it at 0-3psi you can floor it and be pend to the sit. Kinda afraid to drive it right now till I figure out what's going on. Anyone got any ideas because I've run out?:banghead:

 

[Bulldozer]

are you using a regulator?

i have a 5 - 7 psi carter electric pump and regulator at the fuel cell which feeds a mechanical pump. my gauge is right at the carb and my regulator is set at 5.5 psi..... my gauge reads a rock solid 5.5 at all rpms across the board.

 

[Road Demon]

My apology, I am using a holley mechanical pump not electric. I do have an electric if all else fails but I'm trying to keep everything as mechanical as possible to limit the rat nest of wires. Why are you running both electric and mechanical? Is it to help cut down on vapor lock?

 

[Bulldozer]

My apology, I am using a holley mechanical pump not electric. I do have an electric if all else fails but I'm trying to keep everything as mechanical as possible to limit the rat nest of wires. Why are you running both electric and mechanical? Is it to help cut down on vapor lock?

i use the electric to start the car after its been sitting. the electric is there to maintain fuel pressure when im working the engine. its also my backup if my mechanical fails. i dont use it all the time.

 

[crackedback]

bad fuel pump is my guess

Or something is plugging up somewhere

 

[Road Demon]

I don't believe it to be the pump, been having this problem with two different mechanical pump both of which where new when I installed them. Still have the erratic pressure at idle. The first one lasted about a year then start giving problems, cleaned up the fuel system before trying the second pump.

 

[65TerrorCuda]

See this is a bit late but...
Does your car have a fuel tank vent line?
If that collapses, it could cause a low pressure problem. Try pulling the gas cap off and see if it goes away. Don't drive it like that though