Alrighty. I've replaced three mechanical fuel pumps in my 73 240z in the past five months. [Because I initially thought this was the root of all of my problems]

It seems as though the pump loses it's prime every time she sits for a week or two. 

For example, today: oem-spec fuel filter is clean and new. The filter is full of gas. Car will not start with a solid 2 minutes of cranking. No fuel is making it to carbs. What gives? It's like the pump is sitting there doing nothing.

I can blow gas through and suck gas in from the supply line easily. Gas tank is fairly clean--fuel filters have never been dirty in the past 6 years of ownership.

SU floats have been set 3 times, mixture nuts 2.5 turns down. Losing my mind here!

 

My two next steps:

Remove valve cover to inspect eccentric gear and activate pump by hand.

Remove OEM fuel rail and check for debris (is this even a likely culprit?).

 

I am already planning an electric fuel pump install because I'm so tired of this.

 

Here are the threads I looked into:

 

 

Look for cracks in the soft fuel hoses. That can introduce air into the line when you're trying to pump fuel.

If you have a hand-held vacuum pump, try seeing if you can pull fuel up to the hose for the mechanical pump with it. If you have difficulty doing that, so will the mechanical pump.

Edited May 21, 20177 yr by SteveJ

4 hours ago, SteveJ said:

Look for cracks in the soft fuel hoses. That can introduce air into the line when you're trying to pump fuel.

If you have a hand-held vacuum pump, try seeing if you can pull fuel up to the hose for the mechanical pump with it. If you have difficulty doing that, so will the mechanical pump.

I've replaced all fuel hoses in the engine bay within the past year, most within the past few months.

I didn't think of the vacuum pump Steve. Are you saying connect it between the pump and the oem fuel filter?

I can't help but think if my fuel system is sucking air somewhere, it must also be leaking fuel somewhere? I did find a slight crack/drip of a fuel line about a month ago and replaced it immediately. I thought it was the culprit, allowing the filter to slowly drain, but I'm back where I started.

Connect the vacuum pump to the hose where the mechanical pump would go. See if you can pull fuel through it.

A hairline crack above the fuel level would may not show much/any fuel leaking out, but it might be enough to let air in. I have seen threads in the past where people found this was the problem with pumping fuel through the fuel lines.

Thinking too hard here guys. Just take a hose off one carb, and put it in a cup, and try cranking the motor and see what you get for volume. If you get a decent stream, try putting your finger over the end of the hose and see if you can build up pressure. You should not be able to hold the pressure back without making a squirty mess. It's not about pressure, or flow, but both. 

 

I have also seen the small disc in the pump that act as valves stick closed which negates the pumping action

10 hours ago, zKars said:

Thinking too hard here guys. Just take a hose off one carb, and put it in a cup, and try cranking the motor and see what you get for volume. If you get a decent stream, try putting your finger over the end of the hose and see if you can build up pressure. You should not be able to hold the pressure back without making a squirty mess. It's not about pressure, or flow, but both. 

 

I've disconnected the hoses from the carbs plenty of times. Not even a trickle.

I know this isn't helpful at all but I thought I would mention it. 

When I was a kid (long ago) I had a similar problem with my '73.  At times, I struggled to get it started.  I resorted to pulling the fuel line at the filter.  Then I put the hose in a glass jar of gas.  Sometimes it pulled fuel and sometimes it didn't.  So I replaced the fuel pump and the behavior was the same.   I was a dumb kid at the time.  So perhaps I was thinking more of my girlfriend than my car?  Regardless, I never did figure it out.  I am sure that car went to the scrap yard 30 years ago. 

You can pull the inlet of the pump and stick it in a gas can to see if it pulls fuel.  You could have a crudded up gas tank?