Mechanical AND electric fuel pump combo

[JumboFett]

Hi fam,

As we are all wont to do from time to time, I’m just pondering technical plausibilities while I wait for the summer heat to die down.

 

What do you think the viability is of using an electric fuel pump as a primer pump on the starter switch circuit then a mechanical fuel pump on the keyed ignition circuit as the main pump feeding the engine? 
Would an electric pump allow fuel to pass thru while is wasn’t on, like a default bypass mode?

I’m really only interested because there doesn’t seem to be an answer for which electric pump is the best, most reliable pump. It appears that the mechanical OEM Japanese pump is the best, most long lasting solution but it still has a hard time dealing with pulling the fuel up from the tank if it’s not started everyday. 
 

Maybe it’s a convoluted idea, or maybe it’s a valid epiphany! Idk… thoughts? 😎

[frankendat]

I think the idea has merit . This might not solve you problem, but it appears (on paper) that an electric pump inline after a mechanical pump and with a pressure regulator will raise the fuel pressure, above what is necessary for fuel injection. It appears (on paper) that raising the pressure with stock injectors will create more flow and will allow continued use of stock injectors even after boring the block and incorporating 1mm over pistons. That's what I am hoping, but my optimism has cost money, time and heartbreak on occasion. Trying to get more power out of less fuel is a tired and true method of building a bomb. I also like redundant systems.

[JumboFett]

34 minutes ago, frankendat said:

I think the idea has merit . This might not solve you problem, but it appears (on paper) that an electric pump inline after a mechanical pump and with a pressure regulator will raise the fuel pressure, above what is necessary for fuel injection. It appears (on paper) that raising the pressure with stock injectors will create more flow and will allow continued use of stock injectors even after boring the block and incorporating 1mm over pistons. That's what I am hoping, but my optimism has cost money, time and heartbreak on occasion. Trying to get more power out of less fuel is a tired and true method of building a bomb. I also like redundant systems.

The engine is Weber carbureted, and they do make electric fuel pumps that only push about 3-5psi, so a regulator might be unnecessary. My goal is to prime the system by default while actuating the starter, but immediately switching the electric pump off once the engine is running, relying solely on the mechanical pump. Thereby eliminating the difficulty the OEM pump has when pulling fuel all the way from the tank during start up. 
 

I feel like I can’t be the first person to ever think of this as a solution. I just hope I’m not creating a solution in search of a problem. 

[datzenmike]

  For a carbureted engine? The stock Hitachi has a float chamber half filled with gas left over from the last time it was run. You could go out and remove the stock mechanical pump and tomorrow start it up and drive down the block, well part way down the block, anyway. You do not need a working pump to start a carbureted engine.

 

When shut off, the fuel shouldn't drain back into that tank anyway. Only way it might is if a hose clamp on the pump or the fuel filter is loose or not sealing, allowing air into the line as the fuel siphons back into the tank. Possibly there is a crack in the hose also.

[JumboFett]

2 minutes ago, datzenmike said:

  For a carbureted engine? The stock Hitachi has a float chamber half filled with gas left over from the last time it was run. You could go out and remove the stock mechanical pump and tomorrow start it up and drive down the block, well part way down the block, anyway. You do not need a working pump to start a carbureted engine.

 

When shut off, the fuel shouldn't drain back into that tank anyway. Only way it might is if a hose clamp on the pump or the fuel filter is loose or not sealing, allowing air into the line as the fuel siphons back into the tank. Possibly there is a crack in the hose also.

It’s a Weber carb, but all fuel bowls act the same, so I take your point. I think my concern stems from not starting it daily. Fuel drains back into the tank after not being started for a few days, and I’m thinking of ways to mitigate that issue. I’m fully aware that this is some real mental masturbation type stuff though. 😅

[datzenmike]

2 hours ago, datzenmike said:

 

 

When shut off, the fuel shouldn't drain back into that tank anyway. Only way it might is if a hose clamp on the pump or the fuel filter is loose or not sealing, allowing air into the line as the fuel siphons back into the tank. Possibly there is a crack in the hose also.

 

Worth repeating. There has to be an air leak on the suction side of the fuel pump. Won't show as a leak... when pump working it might suck some air in, when pump is off, the fuel runs down hill towards the tank pulling in air to displace it. Look for loose hose or even a replacement hose that's the wrong diameter or cracks in the hose. 

[thisismatt]

If you continue down the initial thought path (whether it has merit or not), you could add a bypass line around the electric pump with a check valve.

[frankendat]

I remembered systems that do what you originally requested (although DatzenMike gives good advice and I would work to fully disprove his theory before spending any time or money on the original request)

 

If I understand your original request, then wouldn't something like https://blog.cantonracingproducts.com/blog/accusump-basics

work. It is marketed for oil, so I would check all the gasket material, before trying to install it (gas eats many gaskets) The accumulator would collect whatever pressure was there during operation and you could manually release it when starting. As long as you remember to close the valve before shutdown, there will be a bit of pressure available for startup.

[jagman]

I did the two pumps on my old dodge because the cam was worn and the mechanical pump did not provide enough fuel for a start and run but at higher revs was fine, I put a tee in the gas line near the tank pickup and ran an electric pump to a tee near the carb with a fuel shutoff valve in the start circuit.  When the mechanical pump finally gave it up all I had to do was switch wiring to the run circuit and delete the line to the old pump.

[Crashtd420]

The facet low pressure pumps are supposed to be the correct psi and equivalent to a factory pump from the later 720s....

They also make a oil pressure switch that's good to use with it....

Sends power while cranking and then the oil pressure keeps it active.... loose pressure pump shuts off.....

You could even add a switch or push button to the power relay that powers the pump, use that as a primer....

 

Have a look here....

https://www.pegasusautoracing.com/document.asp?DocID=TECH00150

 

Has exactly your question with an answer.... 

 

If you want just a good electric without the mechanical go this route... these are the ones similar to factory....

 

The info says fuel can flow through this one also if off... no anti drain back valve.... so more money but better pump... 

 

[Crashtd420]

Fuel pump safety switch just in case...

Carter Fuel Systems Carter Engine Oil Pressure Switch (A68301) https://a.co/d/itpjGYR

[datzenmike]

Again starting does not need the fuel system primed. There should be gas in the carburetor just waiting and the fuel line to the tank full. 

 

Oil pressure sender is only at ground with oil pressure below about 8PSI so pump would only be on with engine stopped. You could use a relay with normally closed contacts that the oil pressure switch (when grounded) would turn a relay from normally closed to open and stop the pump.

 

This is even better: as the switch is opposite with closed contacts when there IS pressure.

 

Needs BSPT threads or an adapter is used on Nissan engines.

 

The four and six cylinder L series engines all ran just fine with a mechanical pump.

 

 

[Crashtd420]

I was just giving an answer to his original question..... 

[frankendat]

8 hours ago, datzenmike said:

Again starting does not need the fuel system primed. There should be gas in the carburetor just waiting and the fuel line to the tank full. 

 

Oil pressure sender is only at ground with oil pressure below about 8PSI so pump would only be on with engine stopped. You could use a relay with normally closed contacts that the oil pressure switch (when grounded) would turn a relay from normally closed to open and stop the pump.

 

This is even better: as the switch is opposite with closed contacts when there IS pressure.

 

Needs BSPT threads or an adapter is used on Nissan engines.

 

The four and six cylinder L series engines all ran just fine with a mechanical pump.

 

 

But DatzenMike this is fun

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[frankendat]

9 hours ago, Crashtd420 said:

The facet low pressure pumps are supposed to be the correct psi and equivalent to a factory pump from the later 720s....

They also make a oil pressure switch that's good to use with it....

Sends power while cranking and then the oil pressure keeps it active.... loose pressure pump shuts off.....

You could even add a switch or push button to the power relay that powers the pump, use that as a primer....

 

Have a look here....

https://www.pegasusautoracing.com/document.asp?DocID=TECH00150

 

Has exactly your question with an answer.... 

 

If you want just a good electric without the mechanical go this route... these are the ones similar to factory....

 

The info says fuel can flow through this one also if off... no anti drain back valve.... so more money but better pump... 

 

I didn't know such things existed. Neat!

[JumboFett]

A lot of really good input on this thought exercise. Thanks everyone!!

Edited September 11, 2023 by JumboFett

[EDM620]

What is the target pressure or upper threshold for a carb'ed engine? 

[victhewhimpykid]

My 76 would do that too, the problem wasnt a fuel delivery issue. As most have pointed out there is fuel left over in the carb. Since you have a weber you can easily remove the six bolts and the small c clip to verify there is already fuel in the bowl even when it has sat for a few days. For me at least it was a spark issue. My spark was pretty weak because of the stupid coil and ballast resistor. Not sure why datsun put it on trucks from 76 and up that came with electronic distributor. After I upgraded to a 12 volt coil that didnt require a ballast resistor and I bypassed the resistor it would fire right up specially if you have your electronic choke set up. Months later I put an electronic fuel pump. 

[datzenmike]

California trucks got a form of EI ignition a year or two before all got them in '78. I don't know if they can handle the current draw from using a lower resistance coil and no ballast. You don't have points, correct?

 

 

L20B fuel pump....... 3-3.9 psi. At 1,000 engine rpm and 1 liter per min.

Electric pump.......... 4.6psi. 1.4 liters per min.

 

720 electric pump...... 2.7-3.4 psi and 1.4 liters per min.

 

These are FSM numbers so the Hitachi carburetors should obviously handle them.

 

[Stoffregen Motorsports]

Ok, I didn't read all the replies, but yes, a low pressure electric pump can indeed help the mechanical pump. This is an old solution when vapor lock was an issue.