The Stovebolt Site Forums The Shop Area Engine Shop 6 volt electric fuel pump issues

I have a 54 gmc 302 that I have rebuilt, currently it is installed on my running stand.
This install uses an 6 bolt electric fuel pump, the one on the truck is 40 years old, not surprised when it did not work.
So I installed one bought at the local parts store, along with a fuel pressure regulator,
It worked fine for about 30 minutes then quit pumping.
So I tried another new pump, same results.
Pretty basic installation , 2 wires one is grounded.
I found it strange that both pumps never quit running, I would think that when the pump reaches pressure it should have stopped pumping? When the pressure drops it start up again?
Anybody had experience with pumps?

Your post is somewhat confusing. You say both pumps quit pumping and then further along in your story you say both pumps never quit running. Both things can't be true. If a pump is running, its pumping.

My experience with standard electric pumps from the parts store is that as long as they are switched on, they continue pumping until powered off. A pressure regulator down the line will have no effect on the fuel pump. The pump will keep pumping if its turned on.

I would eliminate the pressure regulator and buy a pump that operates within your carburetor's specified pressure range. If the service manual says the fuel pressure should be 5-7 psi, buy an electric fuel pump that meets that requirement without going higher or lower.

Electric pumps are designed as pushers, not pullers. They should be installed within about 1 foot from the fuel tank outlet to work properly.

Like Otto said, unless the impeller fell off, a pump will deliver fuel if it's running. There aren't any that I know of that have built in pressure switches to shut them off if they develop pressure.

There is a difference between pumping and running.
Typically a fuel pump will run until it reaches the maximum pressure then quit running, until the pressure drops, then it will start running again
(And resume pumping)

No, that is not typical of off the shelf, universal fuel pumps. No after market electric fuel pump I've ever encountered operates like that. They're designed to pump/run continuously as long as power is applied.

What does the paperwork that came with your pump say about its operation? What brand and style pump are you talking about.

Well I have encountered a different style of pump.
However this doesn’t explain why two different pumps would cease to work after 30 minutes of operation.

Can you post what pump you have that quit pumping? A centrifugal pump will stall out when it reaches it's maximum delivery pressure, aka shutoff head.

The first pump was via Amazon, no paperwork it lasted 30 minutes.
The second pump was Spectra model sp8108
The paper is basic showing the connections,
I reached out to them via email, haven’t heard back yet.
I’m trying pump # 3 also a Spectra product, I have removed the pressure regulator.

Dave

I looked up the specs on the Spectra 8108. Lots of useful information on their site including installation instructions and a chart with specifications for all fuel pumps for any application. www.spectrapremium.com

It is a typical universal fuel pump that runs continuously when power is applied. That means it pumps continuously when power is applied.

It should be connected to the run circuit of the ignition so that it automatically shuts off when the ignition key is switched off.

6V

3.9- 6.4psi

5A draw

One of our AI overlords says that the fuel pressure specification for a 1954 GMC 302 is 3.0 - 5.0 psi. If this is accurate, then the Spectra 8108 might over-pressurize the needle valve in the carb and cause flooding

To emphasize what Otto said, that pump is probably too much for your truck. Browsing through the Spectra catalog, THIS PUMP would be more suitable for your application. The only problem is that it's a 12V pump. I didn't find a suitable 6V pump in their lineup.

Additionally, for safety, any electric pump should have a shutoff relay to kill power to the pump when the engine is not running, with an override from the start circuit. That keeps the pump from dumping gasoline in the instance where the engine stalls for whatever reason, including a feed line brake that starves the engine of fuel - which might then be dumping on a hot exhaust manifold.
Spectra 8108
Average Flow Rating: 41 gph (THIS MAKES NO SENSE BASED ON THE FOLLOWING INFO)
Maximum Flow Rating: 28 gph (THIS CORRESPONDS WITH NOTHING IN THE WAY ON THE OUTLET - AS IN DISCONNECTED FUEL LINE)
Maximum Pressure: 6.4 PSI (THIS WOULD CORRESPOND WITH A NO-FLOW SITUATION - ASSUMING THE NEEDLE VALVE IN THE CARB CAN HOLD THAT MUCH PRESSURE)
Minimum Flow Rating: 18 gph (THIS RATING WOULD BE THE MINIMUM YOU EVER NEED - SHUTTING OFF THE FLOW FOR AN EXTENDED PERIOD WOULD TEND TO DAMAGE THE PUMP)
Minimum Pressure: 3.9 PSI (THIS IS HOW MUCH PRESSURE IS AVAILABLE AT THE OUTLET OF THE PUMP WITH NOTHING CONNECTED.)

In order to use that pump, you should install a pressure relief valve with the outlet directed back to the tank. This is different than a pressure regulator valve which controls pressure on the outlet of the valve, driving the pump outlet pressure up. A relief valve controls the pressure at the outlet of the pump by bypassing flow to a low pressure point (the tank). That would bring the delivery pressure within the spec that Otto posted for your carburetor.

What about a fuel pressure regulator, the one I have purchased is adjustable from 1 psi to 5 psi.
I was informed that the use of pressure regulator
( will not) harm the pump, however I had the regulator installed on the two failed pumps.

What brand and part number pressure regulator?

Is it a Mr gasket brand by any chance?

This is getting complicated!
When last running (40 years ago) the engines,
Correct, it has twin GMC 302s it was set up with two 6 volt AC fuel pumps. Upon testing they both ran but didn’t pump fuel.

Yes the pressure regulator is a Mr Gasket brand

Yes, these carbs , Holley AA-1 are temperamental to high pressure, apparently they like no more than 2 1/2 lbs

Can you use the original mechanical pumps?

My truck also requires only 2 - 4psi. I had Then and Now Automotive rebuild my original pump and it measures 2.5psi at the carburetor inlet.

If the regulator is the one I'm thinking of, it has a chrome dial about the size of a half dollar that clicks into 6 positions. Each position ostensibly represents the psi at the outlet. In reality I've found that the numbers are only relative. Put a pressure gauge on the fuel line at the carburetor to test what the actual psi is.

I’m starting to think a mechanical pump may be way to go, one of the concerns is space. With the two engines side by side, there is not a lot of room.
Must be a reason the builder used electric fuel pumps, and no pressure regulator or bypass line was used. Considering the age of the truck, asking the builder is no longer an option.

Don't you need a pressure regulator AND a bypass line with an electric pump?
One to provide proper pressure to the carb and the other to prevent back pressure on the pump?
Or are the pumps built with internals to accomplish this?

That may well be the case, apparently back in the day you could buy a low pressure electric pump,
Not so today.

I’m wondering if the pump has a range of 3.9 - 6 psi
How far can you increase the back pressure, or reduce the output pressure before damaging the pump? I guess this is a good question for the pump manufacturer.

I needed a new electric pump to replace a 30? year old pump. The new pump was the same make and model as the one it replaced, but it put out more pressure. The carb did not like the extra pressure, so I added a 12-803 regulator and gauge. This regulator has 2 outlets - perhaps you could have one pump feeding two carbs? I did not have to add a return hose to the tank.

Before connecting it to the carb, I ran the pump with the hose discharging into a gas can. This primed and flushed the system.

A company called Vintage Auto Garage has a 6 volt fuel pump that is rated at 2 1/2 - 4 1/2 psi
This would be a good place to start , if the carb is still not happy ( flooding) a pressure regulator to drop the psi down to 3 psi may work.
I am going to test the output of the regulator, to verify the psi.

That sounds like a good plan.

Plug your fuel pressure gauge with a T connection into the line after the fuel pump and regulator to see what it is when running.

I have a by pass fuel filter that works great on a mechanical fuel pump, to remove vapour lock issues.
It has two outlet ports, one of course goes to the carburetor, the other line goes the fuel tank.
Just for fun I will install and check just how much pressure is lost due to the rescirculation ,

Just my thought on the subject, but if they are continuous duty pumps, using the bypass filter and return line to the tank shouldn't really affect your fuel pressure.

It does depend on what the original flow and pressure the bypass filter is designed to be used for, as the return to tank line should have a metering restriction to only allow over pressure/flow to go back to the tank at the designed rates.

Theory testing needed to determine the filter output to carb pressure, which I gather is your plan.

I look forward to seeing your results.

I believe you are correct, the difference being between pressure and flow rate.
To date I have purchased a pressure gauge and connected the Spectra sp8108 pump
(3.9 - 6.4 psi) along with a Mr. gasket pressure regulator….. waiting for the low pressure pump to arrive. Turns out the Mr Gasket pressure regulator is not accurate ( as I was advised )
Close as I can get is the actual pressure varies about 1 1/2 psi from the setting on the regulator.
Will see how the low pressure pump does when it arrives.

Next tool I need is low pressure hand (or thumb) pump and a gauge to accurately measure the carburetor fuel needle holding pressure.