Electric fuel pump to prime carbs

[Frederick]

What is wrong with a single inline electric pump? I have a VDO pump in my tank that feeds my carburetor directly.

Like Mike said:

That is the setup on my 2002 with single 38 weber manual choke: Pierburg pump inline above gas tank near rear firewall direct wired to ignition. Been working flawlessly for 8 years.
Can sit for months during winter storage: get in, turn key, (you hear the pump running) pump pedal once, and it starts instantly.
Maybe auto chokes are different?

 

[Mike Goble]

Like Mike said:

That is the setup on my 2002 with single 38 weber manual choke: Pierburg pump inline above gas tank near rear firewall direct wired to ignition. Been working flawlessly for 8 years.
Can sit for months during winter storage: get in, turn key, (you hear the pump running) pump pedal once, and it starts instantly.
Maybe auto chokes are different?

I have a low oil pressure cut-off switch in mine.

 

[Ohmess]

I have a low oil pressure cut-off switch in mine.

My cut off is tied to an ignition coil signal.

 

[bavbob]

I should add that my car is a Euro with a non-venting gas cap. My only vent is a 6mm line from the tank neck that travels to a location under the rear bumper. This may be an issue when using my pump. I now wait a few seconds after priming before I start the car. No issues.

 

[Dan Wood]

I should add that my car is a Euro with a non-venting gas cap. My only vent is a 6mm line from the tank neck that travels to a location under the rear bumper. This may be an issue when using my pump. I now wait a few seconds after priming before I start the car. No issues.

Rob
If you have a part number for a vented gas cap, please post it.
Thx, Dan

 

[HB Chris]

All coupes came with non vented caps.

 

[bavbob]

As Chris says...................but I think the original thought, at least for US cars, was to run it thru the charcoal filter up front and then to the carb to burn off. This would be bypassed if the cap was vented. At this point, vented cap would have no downside since that EGR-like 70's anti-pollution thing is probably non-functional. In fact, it may help with any gas smell in the trunk.

 

[Ohmess]

I should add that my car is a Euro with a non-venting gas cap. My only vent is a 6mm line from the tank neck that travels to a location under the rear bumper. This may be an issue when using my pump. I now wait a few seconds after priming before I start the car. No issues.

Bob - why would the venting be an issue with your pump? This tube was the German design for our cars from the beginning when all models were carbureted. I think the vent tube would prevent any vacuum formation inside the tank and above the fuel.

Just trying to understand what it is about your setup that might be problematic.

 

[bavbob]

My thought was that the electric pump flow rate exceeds the mechanical one. The venting was designed for the mech pump. Perhaps the electric pump creates a vacuum that the venting system cannot keep up with. Anyway, I have no issues with the pump in parallel.

 

[Mike Goble]

My thought was that the electric pump flow rate exceeds the mechanical one. The venting was designed for the mech pump. Perhaps the electric pump creates a vacuum that the venting system cannot keep up with. Anyway, I have no issues with the pump in parallel.

The vent works at the rate that fuel is used, not at the rate that the pump can provide.

 

[bavbob]

We can agree to disagree.

 

[Mike Goble]

Agree to disagree on what? It's simple physics - the vent only flows air when there is a difference in pressure between the atmosphere and the inside of the tank. If very little fuel leaves the tank, very little air goes in. When more fuel leaves the tank, more air goes in. Hopefully all this fuel is being consumed by the engine.
When the tank heats up while the car is sitting, the vent flows in the opposite direction.

 

[lip277]

I agree with Mike here...
Not sure where the 'agree to disagree' comment comes from.
Simple mechanical properties at work here. For our situation, the only thing that will contribute to a vacuum in the tank is the amount of fuel consumed by the engine - not the amount of fuel flowing through the pump (to which 99% returns to the tank via the return line).

This diagram is from 2002FAQ and while it is for the M10 motor, the idea is the same....
You'll notice here there is even both a mechanical and electrical pump.

For venting the tank - I deleted all the US mandated stuff (carbon canister, vent line to the front of the car and other stuff).
I went back to the Eurp set-up: a simple line from the filler inlet hose out the back of the trunk down near the bumper.

I did that as I was getting a gas smell in the car and figured I had a leak in the vent line as it ran from the trunk to the engine (running inside the cabin - under the carpet as I recall). It was plastic and ... I figure had developed either a small leak or such. It does not take much to have a smell... After I removed it all - was not an issue for the next 25 years I had the car.

 

[bavbob]

Empty plastic bottle of soda. Put a 2mm hole in the side then apply a huge vacuum at the bottle's opening and tell me the plastic bottle will not implode even a little.

Only fuel injected cars have return lines and if the car was fuel injected, it would not need a primer pump.

Agree that vacuum in the tank depends on fuel being used by the engine but that is not the situation I am talking about. I am talking about sucking gas at a high rate with a mechanical pump to fill the volume of the lines before the car starts.

Both you guys are talking about a steady state situation where equilibrium has been met while I am not.

 

[lip277]

My car has a return line. Factory 1000000%. Carb car. Never fuel injected.


From there - the volume of the carb float bowl(s) and fuel line (if it is involved) is not an issue if the tank is properly vented.
By the time the fuel pump even thinks something is going on, the vent has done its job and all is good.

 

[HB Chris]

The return line began with 74 MY.

 

[jmackro]

My thought was that the electric pump flow rate exceeds the mechanical one.

No, both types of pumps vary their flow based on demand. Neither a mechanical, nor an electric pump produces a fixed volume; they produce a fixed pressure. So whether the car is speeding up a hill or idling, the carburetors receive the proper amount of fuel. If that wasn't the case, and the pumps were designed to always deliver enough fuel for the "speeding up a hill" condition, the carbs would flood under prolonged idling.

 

[lip277]

The return line began with 74 MY.

And earlier that that for the 2002 (as mine was a 73).

 

[HB Chris]

And earlier that that for the 2002 (as mine was a 73).

Good to know, that is a new one for me.

 

[Dan Wood]

Update on the Pierburg pump. As @JFENG predicted, yesterday after starting the car and driving about 1/8 mile, it had fuel starvation. This is the first time since adding the electric pump. Powering the electric pump got it started again and no issues after that. I believe that if I can get past that initial distance after start up, it won't starve again. Not sure I want to go with @bavbob "Roman Aqueduct system" or seeing if @Dick Steinkamp larger diameter pump will fit. I am considering options including staying with what I have.