Fuel Line Pressure & Suction

[CRW-FK5]

I was checking the fuel line suction (from inlet of tank sending unit to carb) and pressure (from tank sending unit to outlet of fuel filter) and have a few questions. 

With sending unit pulled out of car and a vacuum gauge connected to the inlet of the sending unit and clamped, I barely get 1" Hg when cranking engine over.  When I do this test from inlet side of fuel pump I get around 16" Hg, a huge difference.  I have the fuel filter that has the return line running back to the tank and when I pulled that off, put my finger over the inlet, and suck through the outlet (after blowing the excess fuel out, or course) I see that there is flow through the return line.  With this return line open, it seems like that is where my pressure (or suction) loss is.  How then can the pump ever pull enough fuel through the main line with a system like this.  What am I missing?

[cdr]

that filter goes after the pump.

[cdr]

the one by itself  hooks to the pressure OUT of the pump, the others ,one in the center goes to carb & the other goes to return

[CRW-FK5]

Not following.  My sending unit has 2 ports, one a 5/16" line running to the center inlet of the fuel filter and the other around 3/16" running to the smaller side port on the filter.  The outlet side runs to the pump. 

[CRW-FK5]

Is a 5/16" line too small for mild 440?

[cdr]

5/16 is ok!! now listen,1.  the 5/16 line on the sender goes straight to the suction side of the fuel pump NO FILTER IN BETWEEN .
2. now the pressure side of the pump gets the filter, the nipple that is alone on one end gets hooked to this line.
3. on the other end of the filter has 2 nipples,, the one in the middle goes to the carburetor.
4. the other nipple hooks to the line 1/4 that goes all the way back to the sending unit & hooks to the 1/4 nipple. 

[CRW-FK5]

Got it.  Thanks.

[green69rt]

The thing you show in the pic looks like a vapor separator and goes in exactly like CDR says.   It is supposed to have a restriction orifice in the return line that keeps all the gas from being returned to the tank (and starving the carb.)  Some on this site have found that repro vapor separators are missing the orifice.  Easy to check, just run a piece of gas line from the return nipple to a bucket with the engine running, flow should be small.  

[CRW-FK5]

The thing you show in the pic looks like a vapor separator and goes in exactly like CDR says.   It is supposed to have a restriction orifice in the return line that keeps all the gas from being returned to the tank (and starving the carb.)  Some on this site have found that repro vapor separators are missing the orifice.  Easy to check, just run a piece of gas line from the return nipple to a bucket with the engine running, flow should be small.  

Good idea.  Thanks.

[CRW-FK5]

I checked my fuel flow and looks like I'm around 37gph.  Pressure is between 4-5 psi.  Is this enough to satisfy requirements for my 440?

[cdr]

should be,  did you reroute the fuel line ?

[troy.70R/T]

What is the part # on that fuel filter? I can't read the whole #. Thanks

[CRW-FK5]

What is the part # on that fuel filter? I can't read the whole #. Thanks

G-3587  (FRAM P/N)

[troy.70R/T]

Thanks CRW-FK5.

[CRW-FK5]

Thanks CRW-FK5.