1956 Packard won't run on mechanical fuel pump alone.

[packrob]

Before the pro restoration the fuel system worked just fine. I had an electric fuel pump installed as an auxiliary (separate fuel line).  When operating on the mechanical fuel pump alone, the fuel filter bowl is almost empty and at speed the engine starves until the electric pump is engaged.  When electric FP is engaged the bowl if full.  I had the mechanical pump rebuilt by Terrill so I am confident it is working properly but I have not checked the pressure.  Any ideas why the engine starves when using the mechanical pump only?

[Ken_P]

Hard to say for certain without seeing your setup, but is it plumbed in such a manner that the mechanical pump is unable to get a good suction? Have you tried bypassing the electrical pump entirely?

I assume your fuel tank is clean?

I would suggest checking pressure, and if that checks out, run the input of the mechanical fuel pump to a gas jug to half-split the problem to either the mechanical pump and downstream, or the tank, electric pump, and fuel line feeding the mechanical pump. Good luck!

[chistech]

Or the pump lobe on the cam if it runs off the cam. Not sure how the mechanical pump is driven on your car. I’ve seen lots of lobes worn to the point the a new pump won’t move enough fuel.

[37_Roadmaster_C]

I would look closely to the plumbing of the two fuel systems! At the very least I would make sure there is a check valve in BOTH pump discharge lines. It is very possible that the electric pump line is either bypassing fuel from the mechanical pump or causing a problem on the suction side of the mechanical pump. A check valve in the electric discharge line will fix both conditions. One in the mechanical pump discharge is actually overkill as there is almost 100% chance there is one in the pump itself, but for safety it will not hurt!!!

[Owen_Dyneto]

Is your electric pump rotary or pulse style?  If rotary, chances are the mechanical pump cannot pull thru it when it is not in operation.  Depending on how your system is plumbed, this may or may not be an issue.  Mechanical pumps pull easily thru pulse or solenoid-type electric fuel pumps, but not rotary types.