1947 Chrysler

[caddy60s]

I (as Usual ) am confused with the use of an electric fuel pump to fill the carb.  I have an electric fuel pump added in line but  unless its plumbed correctly it only brings it up to the Mechanical fuel pump---- Am I correct ?

 

Bruce

[m-mman]

Fuel pumps operate through the use of one way check valves.

Whether mechanical or electric (I am skipping vane and rotary pumps) They draw down a vacuum which sucks fuel into a chamber through a one way check valve. Then it pushes out, compressing the fuel and the entry valve is automatically closed but the exit valve is opened by the pressure and the fuel moves along it's way. 

 

If you have a second (electric) pump (mounted near the tank) it sends pressurized fuel to the mechanical pump (on the engine) and the flow opens both the entry and exit valves and heads for the carb. This happens whether the engine is turning or not. 

 

Back when our cars were driven daily and fuel did not evaporate so quickly there was always fuel in the carb that could prime the engine with a push of the accelerator and give an easy start. 

 

Modern fuels that quickly evaporate and cars that sit need substantial cranking to operate the mechanical pump long enough to fill the carb for starting. Having an accessory electric pump that can be turned on before the car is cranked will fill the carb (passing through the mechanical pump) to give a quick start as if it was parked just a few hours ago. 

 

After starting, the electric pump is turned off and the mechanical pump draws fuel through the check valves in the electric pump. You never know that it is there. 

[alsancle]

That was a great description.     Typically the electric pump is too strong for the needle valve and must be turned off or it will blow gas everywhere.  It also needs to be a draw through pump so that doesn't restrict the mechanical pump.

 

Either a switch, pressure reducer inline or a return line are required to make it work correctly.

[caddy60s]

M-MMAN,

 In your second paragraph you mention the electric pump sends fuel to the mechanical pump & I have to assume from there the Mechanical pump takes over & pumps the fuel to the carb.  So the electric pump in essence does not fill the carb bowl is that correct?

 

Bruce 

 

[Rusty_OToole]

The electric pump pushes fuel THROUGH the mechanical pump to the carburetor. The pump has 2 one way valves, the electric pump easily pushes the fuel through them.

In the same way, the mechanical pump will draw fuel through the electric pump when it is shut off. You do not want both running all the time, that would give too much pressure. Your carburetor only needs 2 -4 lbs pressure. If both pumps are going and both put out 4 pound that would be 8 pounds, enough pressure to flood the carburetor.

[caddy60s]

Rusty, checking the fuel pressure at the carb it shows less than 2 Lbs when only the electric pump is on.  The mechanical pump shows around the same just slightly less than 2 lbs but the car wont start. I have tried priming the carb but it will only start for a second until the gas is used up that was used to prime it.  The only filter on the line from the gas tank is on the electric fuel pump which I took apart this am & it is as clean as could be.  If the diaphragm in the mechanical pump has failed because of the ethanol additive in the gas, would that also not allow the electric pump to push through? I don't want to replace the pump if I am missing some other reason for the lack of fuel delivery.

 

Thanks for your interest

 

Bruce Watson

[Rusty_OToole]

As long as the carb is getting fuel the pump is not the problem. Your stock pump could have failed, I have no way of knowing that, but it isn't preventing fuel from getting to the carb.

Look down the carb and pump the throttle. Do you see a squirt of gas from the acceleration pump? If so the carb is full of gas. You could have dirt in the idle passage or, maybe the problem has nothing to do with the carb, it could be dirty points or a million things. Are you getting spark? Is gas getting to the carb? Does the engine turn over and does it have compression?

[m-mman]

4 hours ago, caddy60s said:

Rusty, checking the fuel pressure at the carb it shows less than 2 Lbs when only the electric pump is on. 

 

Pressure is discussed and focused on many times but FLOW is just as important, if not more important.  

 

Rusty's information above is good for a full diagnostic. But to eliminate each pump as the source of the problem, disconnect the suction side of a pump. (either pump) Rig up a hose to suck fuel from a can. Rig up another hose to squirt the output from that pump into a can. Operate the pump. How much FLOW do you get? A good flow even at low pressure will run the car. Especially at idle. 

 

Operating the mechanical pump might be more difficult but you said the the engine runs for a while with a prime. Again test the mechanical pump for FLOW during the time the engine is running.  

NOTE: if you use a squirt can or are careful, you can add fuel directly to the float bowl by putting it into to the vent hole/tube at the carb. A full float bowl should operate an engine for about 30 seconds, enough to check the mechanical pump. 

 

Checking for flow from & to a loose can will demonstrate with certainty that the pump (each pump individually) is either good or bad.

 

You said that 'the filter' is clean BUT is it the ONLY filter? is there one inside the tank? A good old fashioned blow back into the tank to check that the line is clear is also needed. Remember that any bad rubber hoses can also collapse and stop all flow. 

 

It is always possible that both pumps have failed. . . . . . 

[c49er]

There is a "oilite" tank filter.

Not accessible. Some run a stiff wire thru it to bust it open.

Others tank the tank to a pro to have the tank cleaned and the filter removed.

[28 Chrysler]

I have had diaphragms go bad and the fuel can leak out a vent hole on the pump or can leak into the crankcase. Another problem could be a stuck check valve.