72 datsun 521 fuel problems

[Aqualung]

Hello my truck was running fine then all of a sudden it cranked and would not start.  I checked the carb and realized there was no fuel although there was fuel visible in the filter under the hood. I tested the mechanical fuel pump by putting the line connected to the carb in a bucket and cranked it but no fuel was coming out. I replaced the pump with a new one along with new fuel filter.  The new pump drew fuel up into the new filter under the hood and filled it but still no fuel coming out of the outlet end of the new pump no matter how much I cranked the engine.  What do I check at this point as to the reason the pump draws fuel into the filter but will not pump it out of the inlet?

[thisismatt]

Hole in the line between pump & filter?

[Aqualung]

7 minutes ago, thisismatt said:

Hole in the line between pump & filter?

All the fuel lines under the hood are brand new.

[thisismatt]

I'd pull the line from behind the filter and pipe straight into a can to test. You may have picked up something from the tank clogging the line.

[datzenmike]

Well pull the hose off the metal line from under the truck from the pump and direct into a suitable container. Turn ignition on so pump is running. Does gas come out?

[datzenmike]

You're so fast Matt. I think there is the tank, a fuel filter, the pump (which also has a filter, metal line to the engine compartment, fuel hose to carburetor. Don't think there is a filter in the engine compartment on the 720.

[Aqualung]

8 minutes ago, datzenmike said:

Well pull the hose off the metal line from under the truck from the pump and direct into a suitable container. Turn ignition on so pump is running. Does gas come out?

Not sure exactly what you mean?  The metal fuel line comes in behind the pass side headlight and connects to flexible fuel line then fuel filter then fuel pump.  

[thisismatt]

5 minutes ago, datzenmike said:

You're so fast Matt. I think there is the tank, a fuel filter, the pump (which also has a filter, metal line to the engine compartment, fuel hose to carburetor. Don't think there is a filter in the engine compartment on the 720.

Aren't we talking about a 521? 😅

[Aqualung]

4 minutes ago, datzenmike said:

You're so fast Matt. I think there is the tank, a fuel filter, the pump (which also has a filter, metal line to the engine compartment, fuel hose to carburetor. Don't think there is a filter in the engine compartment on the 720.

It’s a Datsun 521 with an l16 engine, mechanical fuel pump attached to the engine and a fuel filter under the hood.  I believe there is another fuel filter under the truck between the tank and under the hood filter.

[thisismatt]

4 minutes ago, Aqualung said:

It’s a Datsun 521 with an l16 engine, mechanical fuel pump attached to the engine and a fuel filter under the hood.  I believe there is another fuel filter under the truck between the tank and under the hood filter.

There isn't unless someone added it

[Aqualung]

20 minutes ago, thisismatt said:

I'd pull the line from behind the filter and pipe straight into a can to test. You may have picked up something from the tank clogging the line.

 

2 minutes ago, thisismatt said:

There isn't unless someone added it

There’s a metal one on the driver side rear which I haven’t gotten around to changing yet because it’s in such a precarious spot,  gonna have to take the wheel off to get to it.  Possibly a really clogged fuel filter could be restricting fuel flow?

[bilzbobaggins]

6 minutes ago, Aqualung said:

 

There’s a metal one on the driver side rear which I haven’t gotten around to changing yet because it’s in such a precarious spot,  gonna have to take the wheel off to get to it.  Possibly a really clogged fuel filter could be restricting fuel flow?

That sounds like possible the fuel vapor recovery tank.  Not a factory filter back there.

 

The access door in the bed is a blessing and a curse on these trucks.  It allows water to run directly on top of the tank and set on the lock ring.  As the lock ring is recessed, it gives water every chance it can to get in the tank and start rusting up the tank.  It almost sounds like your pickup might be plugged up.  

 

When you say the new pump will not push out fuel, does this mean you had the line off of the carb?  If not, maybe your needle and seat on the carb it stuck shut.

[datzenmike]

oh man. Is it 4:20 yet??? Yeah 521. The electric pump threw me.

[Aqualung]

6 minutes ago, bilzbobaggins said:

That sounds like possible the fuel vapor recovery tank.  Not a factory filter back there.

 

The access door in the bed is a blessing and a curse on these trucks.  It allows water to run directly on top of the tank and set on the lock ring.  As the lock ring is recessed, it gives water every chance it can to get in the tank and start rusting up the tank.  It almost sounds like your pickup might be plugged up.  

 

When you say the new pump will not push out fuel, does this mean you had the line off of the carb?  If not, maybe your needle and seat on the carb it stuck shut.

Jut looked up what a fuel vapor recovery tank looks like and that’s not it, it’s def a silver colored metal fuel filter someone added in the rear.  I changed the fuel filter under the hood and the fuel pump,  hooked everything back up and cranked it over.  The fuel filter filled with gas but nothing was coming out of the pump when I had the fuel line pulled off the carb only a tiny trickle.

[Crashtd420]

3 minutes ago, Aqualung said:

Jut looked up what a fuel vapor recovery tank looks like and that’s not it, it’s def a silver colored metal fuel filter someone added in the rear.  I changed the fuel filter under the hood and the fuel pump,  hooked everything back up and cranked it over.  The fuel filter filled with gas but nothing was coming out of the pump when I had the fuel line pulled off the carb only a tiny trickle.

You could try again with a little starter fluid.... get the engine spinning a bit faster....

 

Even disconnect the output of the pump at the pump....

 

Do you a metal line between the pump and the carburetor or just a hose?

Make sure it's clear too....

[mainer311]

Maybe the eccentric came loose on the end of the camshaft? Is the pump actually pumping? I would pull the rocker cover and just check that the eccentric is still in place and the pump arm is installed correctly. Also make sure the little phenolic pump spacer is still installed. It needs to be there for both mechanical and thermal reasons.

 

Also: if the fuel filter fills completely full, there’s usually a blockage or related issue. The fuel filter doesn’t need to fill completely full for flow to happen, and if it does, that usually means the filter is dirty or there’s a clogged line.

Edited January 24, 2023 by mainer311

[banzai510(hainz)]

thats weird it can suck gas into the stock clear filter but not out the pump!!!!!!!! so to me it has emough vaccum to pull the gas from the tank.

 

you have the spacer still from the pump to the head still?

 

Maybe pull the pump off and pump the pump arm by hand.. IM 99% this is not happening but maybe the diaprame is bad and leaking into the engine Since it can suck to fill the Filter but not go out the out put line .

Edited January 25, 2023 by banzai510(hainz)