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Electric fuel pump


Cannuck
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Hi I recently installed an electric fuel pump close to the gas tank . I have tried it a few times with mixed results . 

The last time the car would not start  I had to resort to poring gas into the carb ,as soon as I did away it went .

What I have been doing is a couple of pumps on the gas full choke and the electric pump for about 5 seconds 

don't know if this is the right way or do I just let the pump run until it starts ? I tried it once and could smell gas 

which concerned me . I checked for leaks around the new pump and line  could smell gas around the mechanical

pump no drips just the smell . Does the Mechanical pump block the flow of gas until the starter is turning the engine ?

If i was to let the pump run does it stop when the carb bowl is full ?

Any suggestions would be appreciated.

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13 minutes ago, Cannuck said:

What I have been doing is a couple of pumps on the gas full choke and the electric pump for about 5 seconds

 

It seems you are pumping the gas first then turning on the pump?  I don't know if you can hear the pump or not but there should be a noticeable change in it's sound once the bowl fills, the flat shuts the incoming fuel off and the pump dead heads.  If you can hear that, this is the point where I would shut off the pump then do the gas pedal routine.  The procedure outlined in my owner's manual, 51 Cambridge, is one pump and hold the pedal about a 1/3 of the way open, crank the engine over.

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I have an electric pump on my 39 Desoto. If the car has sat for several days after running there is a good chnace that the fuel from the mechanical pump up to the carb and also in the float area has evaporated away. So running the electric pump only approx 5 seconds was not long enough to get the fuel from the tank throught the mechanical pump and then up the fuel line and even start to fill the bowl on the carb.  You will have to run it longer. If you have an auxillary fuel filter jsut prior to the carb you can then at least make a visual sighing and when that gets full with gas then try to start the car. The gas needs to be in the fuel bowel of the carb to ensure there is fuel to keep the car running.

 

Just keep try longer time frame with the electrical pump, you will then determine the length of time to run the electric pump.

 

Also on my car I have the foot pedal manual start floor plunger so I do not turn on the ignition. I crank over the car several time just to get oil onto the cylinder walls prior to having gas in the carb.

Rich hartung

desoto1939@aol.com

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If your electric pump is pushing fuel through the mechanical pump it may not be able to push the fuel through. I had mine plumbed that way at one time and found that sometimes i couldn’t prime the carb until I cranked the engine a little bit. I think it depended on the position of the diaphragm on if it would allow fuel to pass through or not.  I have since re-plumbed my electric pump to run in parallel to the mechanical pump. That solved that problem.

 

As to your question of if it will stop pumping when the float bowl gets full, that depends on the pressure rating of your pump. As I recall these carbs like 3-6 PSI of fuel pressure. Any more than that and it could over power the needle valve and float.   

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My electric pump will push fuel through the mechanical pump and when using the electric pump I can see fuel filling my glass gas filter unit just prior to the carb. I have used the electric pump for 25+ years as a backup and primer pump.

 

Rich Hartung

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Just as a data point, the Facet electric pump on my airplane will push fuel through the mechanical pump which is the same type of pump as used on our flatheads. There are check valves in the mechanical pump but apparently the electric pump supplies enough pressure to force fuel past the check valves. There is a fuel pressure port between the mechanical pump and carb and I've never seen an instance where the electric pump didn't boost pressure prior to engine start.

 

But this is a moot point on my P15, I removed the mechanical pump and run an electric pump fulltime.......just like nearly every vehicle on the road today.   :)

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Thanks to everyone for their input solving this problem . I will definitely run it first and longer and listen for the change .

This forum has helped me again and again and I highly recommend it to everyone on other forums that I frequent.

 

Thank you again

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Are you folks just running a push button swich to use the electric pump to prime?  Or are you running a fuel pump constantly on a switch?

 

I had a friend spend almost a year in the hospital, much of that time in the burn unit, because he was in a relatively minor accident (he would've suffered a broken leg and probably a concussion if not for the fire) in a more modern vehicle where someone had rigged the electric fuel pump to run whenever the key was on. 

 

Always wire any electric fuel pump so it can only run when the engine is actually spinning.   If the engine stops turning, the electric fuel pump should shut off within a few seconds. Your life could depend on this.

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45 minutes ago, Racer-X- said:

Are you folks just running a push button swich to use the electric pump to prime?  Or are you running a fuel pump constantly on a switch?

 

I had a friend spend almost a year in the hospital, much of that time in the burn unit, because he was in a relatively minor accident (he would've suffered a broken leg and probably a concussion if not for the fire) in a more modern vehicle where someone had rigged the electric fuel pump to run whenever the key was on. 

 

Always wire any electric fuel pump so it can only run when the engine is actually spinning.   If the engine stops turning, the electric fuel pump should shut off within a few seconds. Your life could depend on this.

 

 

Good point, you can usually use an oil pressure switch to control the fuel pump relay, this way if there is no oil pressure there is no fuel.  Also, some have retrofitted an inertia switch to kill the pump with a big enough jolt.

 

 

 

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12 hours ago, Racer-X- said:

Are you folks just running a push button swich to use the electric pump to prime?  Or are you running a fuel pump constantly on a switch?

 

I had a friend spend almost a year in the hospital, much of that time in the burn unit, because he was in a relatively minor accident (he would've suffered a broken leg and probably a concussion if not for the fire) in a more modern vehicle where someone had rigged the electric fuel pump to run whenever the key was on. 

 

Always wire any electric fuel pump so it can only run when the engine is actually spinning.   If the engine stops turning, the electric fuel pump should shut off within a few seconds. Your life could depend on this.

 

I have a switch under the dash for my pump. One direction is momentary and the other direction it stays on. I have it wired to either direction will run the pump. I usually only use the full ON position when I need the pump on for extended periods, such as when I’ve suffered fuel starvation due to high under hood temps. I then switch it off again when things cool down. I’ve thought about adding an oil pressure switch on the constant ON circuit, but haven’t got a round to it yet. 

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Well, in the interest of presenting solutions, not just problems, here's a wiring diagram that I generally use when wiring a electric fuel pump in a carbureted car:

 

 

 

fuel-pump-relay-for-carbs.png.10c031fdf83f048ddfe675e41d023768.png

I mostly work with negative ground vehicles, so it's drawn that way.   Minor adjustments for positive ground are possible.  If you have positive ground 6V system, you're probably used to doing this.

 

Things to note on this:

 

  • The power to the relay coil should be only from the "run" position of the key.  The coil should not be powered while cranking. That's the pink wire to the 85 terminal of the relay that doesn't show well on this background.
  • The 87a "normally closed" connection goes to the starter solenoid.  When you're cranking the engine on the starter, this powers the fuel pump for priming. That's why you don't want the coil powered during cranking. Actually, I'm not sure it matters because without oil pressure, it should get power from here as well.
  • The wire colors shown match standard GM harness colors.   Use what makes sense for an older Mopar wiring harness.
  • Obviously, this setup requires a 5 pin relay.
Edited by Racer-X-
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1 hour ago, Merle Coggins said:

I like that diagram, but I'd have to modify it as I don't have a starter solenoid on my truck with a foot activated starter. 

Sniper, thanks for the Bosch number for a 6v relay. I've always struggled finding such a mythical beast. 

 

Easy enough.   Just connect the 87a terminal to the switched starter power then.  Either at the floor switch or at the starter itself.

 

The key is, pump runs only with starter power when there's no oil pressure (and/or no ignition power).  With engine runnning, ignition power on, the fuel pump runs from fuel pump fuse from the battery when there's oil pressure present, but shuts off when no oil pressure and starter not energized.

Edited by Racer-X-
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1 hour ago, Racer-X- said:

 

Easy enough.   Just connect the 87a terminal to the switched starter power then.  Either at the floor switch or at the starter itself.

 

The key is, pump runs only with starter power when there's no oil pressure (and/or no ignition power).  With engine runnning, ignition power on, the fuel pump runs from fuel pump fuse from the battery when there's oil pressure present, but shuts off when no oil pressure and starter not energized.

 

That's not an option. There is no switched starter power. The foot activated starter mechanically connects a set of contacts that connect the battery cable connector to the field coils/armature. The foot activated lever also engages the bendix gear to the flywheel. 

 

If I used this circuit I would just continue to use my momentary toggle switch to the 87a contact for priming only. 

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