Any Harm in Running 2 Fuel Pumps?

[Hardway]

Almost a year ago I installed a Carter P60504 electric fuel pump and Holley adj. fuel pressure regulator.  I also removed the mechanical pump that had been restored by the previous owner and installed a billet alum. block off plate.  I made this change since I only drive the car every 3-5 weeks and the engine would need a lot of rotations to get fuel to the pump and fills the carb bowls enough to start.  The electric pump gets the line and carb bowls filled much faster thus making it easier to start the car with less revolutions of the motor.  I have been working to make my car as correct as possible while maintaining as much reliability that newer components offer.  With this in mind I am considering putting the mechanical pump back on and removing the adj. fuel press. regulator as the pump only puts out 3 - 3.5 psi.  My question is, if the electric pump runs all the time while the car is running, is there any harm in this setup?  Should I just use the electric pump as a helper pump to get fuel up to the mechanical pump and then turn it off?  The issue I see with this is the mech. pump would be pulling fuel through the non-running elec. pump. Are there any other issues you foresee with running the two pumps?

[Chickenman]

If your mechanical pump needed a lot of rotations to get fuel flowing there must be something wrong with it. The pump sits at the top of the system and should stay filled with fuel after even months of inactivity. Siphon effect . A few rotations and those float bowls should be filled. A valve can cause the system to take a longer time to build fuel flow. If it's an OEM pump that you can take apart, it may be worthwhile taking it apart. Inspecting it, cleaning and replacing or servicing parts. Sometimes it's just " gunk " on the valve seats. You can still find parts for these if you look hard enough.

You can't run the Carter in the system as a helper pump and then turn it off. That will cause a restriction in the line and the mechanical pump's output will drop. You could run it as a secondary pump. It only puts out 4 psi. But then what's the point?

 

[Walter Moore]

Isn't that what Nissan did on the late model 240s and 260s? Run an electric pump in the back to boost the pressure to the mechanical pump in the front? If the Carter puts out more than 7psi it might damage the mechanical pump unless he put his regulator ahead of the mechanical pump. (The mechanical pump will not lower the pressure.) This was one of the vapor lock fixes that they put on the carbed cars. There is even wiring for an electric pump in the factory harness. (Which I wouldn't use on a bet.)

 

[Chickenman]

Yes, Nissan did that with the horrible 1973 and later Square Top carbs,  to  correct Vapor lock issues as you mentioned. However the OP wanted to return his car to original Series 1 condition, which did not have an auxiliary electric pump.

That being said,  OP lives in Texas, so there may be some issues depending on ambient temperature ( over 90F ) . A weak mechanical Fuel Pump really aggravates Vapor lock issues.

[Mark Maras]

  Chickenman or anyone else, were the 70 & 71s plagued with vapor lock and or percolation as much as the later SUs? Having driven an early 71 for years and never had a problem with it, It seems to me, every time they "updated" the SUs the problem became worse. Any truth in this theory?

[Chickenman]

The later Hitachi style " emissions " carbs with the square throttle inlets  were just terrible, devices. Even in mild climates they were prone to all sorts of issues.

The early style " SU's " were dead nuts simple and reliable. We had no issues racing in 90F weather at all, as long as you took the normal SU or carburator precautions. Mind you, oxygenated fuels these days cause a lot of problems. Reid Vapor values are a lot different from what they were in the 70' through early 90's. And that can wreak havoc with any carb in extreme Hot weather.

[Mark Maras]

 Thanks, That explains why I had no problems with mine.

 

[Hardway]

Thank you for the info Chickenman.  Just to circle back, I can run both the Carter electrical pump and the mechanical pump on the side of the block if I want to.  I understand with the electric pump turned off it would cause a massive restriction.  

[Chickenman]

Yes, that should be OK. Make sure that the electric pump is installed as a " Pusher " pump, as close to the fuel tank as possible. Electric pumps don't work well as " Puller " pumps. Don't forget to run a fuel filter between the tank and the electric pump inlet. ( I believe the Carter comes with one supplied )

Edited March 15, 2016 by Chickenman

[Stanley]

It would be interesting to see what the fuel pressure is, measured between the mechanical fuel pump and the fuel rail, with both mechanical pump and stock (or otherwise) electric pump both hooked up and working.

My '73 had both but the stock electric pump by the tank wasn't working. I removed it and tested it hooked up to the battery and it sounded fine. Guess it was unconnected somewhere at the harness. 73's came with flattops, don't know what recommended fuel pressure was for them but Nissan must have figured it was OK to run both pumps.

I'm running SU's with stock mech. pump only, it puts out about 4.0 psi with the return line plugged. I've seen fuel pressure drop to about 3 psi on very hot days on the freeway when speeds vary suddenly from 75 mph to "parking lot". Might have indicated a borderline vapor lock condition, but the F.P. gauge was the only indication. 

[Johnny wick]

I have an Airtex 8012S electric fuel pump rated 5 to 9 psi and still have vapor lock issues. Whenever my car sits on traffic for long time plugs get black and rear carb gets really hot.  My father who is an industrial engineer and has quite some knowledge on mechanical engineering concurs with the opinión that its fuel percolation. Too much heat from the manifold to the float bowls. The stock heat shield on the 240z must be modified. Maybe using stainless Steel. Just too much heat in Panama: 33 degrees Celsius + Engine heat. Imagine???

 

 

Edited March 15, 2016 by jalexquijano

[siteunseen]

I honestly don't see how vapor lock would make your plugs black.  Seems to me right the opposite. too much fuel. 5 to 9 psi is too much for SUs. You can buy an adjustable fpr at any parts store. The knob style is what they'll have, 0-10 psi. My Dad bought a Puralator pump and it came with the fpr, $20, for his lawnmower.