Confusion about original fuel setup

[obxtrainman]

We're talking about a 1973 Datsun 240Z. Some background on the car. An old man/ dealer had bought the car at a show 12 years ago. He drove it home some eight hour drive, then stored it for the last eleven years. His people had been working on it some when I happened upon it. He sold it to me after I talked to him for some time about it. I was told they had been having problems keeping it running. Then they figured out it was trash in the tank. They had finally swapped the tank for a so-called refurbished tank from Washington state. This is where I step in. They have an aftermarket electric pump (rated at 5-9 PSI) mounted in the engine bay.

In the Haynes manual it says the 240Z had a mechanical fuel pump. It also says the 260Z had a supplementary electric fuel pump that at engine speeds above 400rev/min (cranking) worked with mechanical pump. I read this as saying it worked together during normal running. Not as a booster to get the fuel to engine compartment on startup, then not powered after initial ignition.

The BD catalog refers to the 240, and 260 as having two fuel pumps, one mechanical in the engine compartment, and one electronic pump located in the rear near the fuel tank. However they don't list an electronic fuel pump for said cars. Only for 1975 - 1996.

My 240Z has two unused wires near the fuel tank, and an in-line fuse holder beside of the fuse panel. It is marked fuel with a stick on paper label.

Is everyone with me so far???

I allowed my neighbor to talk me into blocking off the intake side of the newer looking mechanical fuel pump one evening. The pump had nothing connected to it. At that point in time, the pump had good suction on the intake side. Me being an idiot, went along with his suggestion. Even though the car had been working fine, and the pump didn't seem to be causing a problem at that time. I only had the intake blocked off for minimal running and driving. A few evenings at the most. I then decided it might harm the pump, and since I didn't see how it could damage it if unblocked ...

I just disconnected the electric pump, and hooked the mechanical pump back up. This is the only time the mechanical pump has been connected since I have owned this car. It ran the fuel out of the carbs, and emptied the clear filter, then ran out of gas.

I've been having problems getting the new Ztherapy carbs adjusted, and running properly. My gut feeling for quite some time has been leaning toward fuel setup.

I quess my question is. What is the original setup? Do I need a second pump in the back, near the tank? I've ordered a mechanical pump from NAPA. I am also running out of hair to pull out. Any suggestions???

OBX

[kenz240z]

My '73 has an electric fuel pump mounted back by the tank. It is on the passenger side behind the mustache bar. It is in the yellow box in the attached photo. Check with MSA, the last time I looked in their catalog they had this part available.

What type of carbies does your Z have, the flat top SU's or the round top SU's. If you have the round top SU's you may not need the electric fuel pump. I think the electric fuel pump was one of the remedies for reducing vapor lock on the Z's with the flat top SU's.

The electric pump is "controlled" by the ignition circuit, so to speak. It will run when the car is running. The input side is connected to the fuel tank, the output side is connected to the fuel line that runs to the engine bay. The fuel line in the engine bay then runs into a fuel filter on the passenger side inner fender and from there to the input of the mechanical fuel pump. The output of the mechanical pump runs to the fuel rail that feeds the carbies. Then there is a fuel return line that runs back to the tank.

You may want to check and see if your Z still has the electric pump plumbed into the fuel line. If so, there is a fuel filter inside the bottom of the pump. Be sure to replace it. You may also want to replace the filter on the inner fire wall.

A car that has been sitting for several years will most likely have issues with sediment in the fuel. You mention the tank was swapped, but were the fuel filters replaced? Also, inspect the various sections of flexible fuel hose for cracks/splits that can allow air to be sucked into the fuel line. Your best bet would be to replace all the old flexible fuel hoses from the tank to the carbies.

[beandip]

I have a '73 z and am running a electric pump only and have been for 8 years. I am using a RX-7 pump and it runs great with it. U-pull-it $10.00. Now your '73 is wired for the pump , however it will run as long as the key is in the run position , and of course also in the start position. As a safety measure I added a pressure switch at the oil sender that powers the fuel pump at 5 psi. This way if I crash or have a oil pump failure the gas will stop flowing. Cheap and easy to do. When starting the engine, it starts on the fuel in the carbs then the gas flows. I have let the Z set over winter and she started fine with out my having to bridge the wires at the pressure switch , which you would need to do if you ran our of gas. I have a gear drive starter but don't think it spins the engine enough to pressurize the oil to 5 to 7 lbs and switch the pump on. 5 to 9 lbs is too much pressure for the SUs to handle. Your problems with the Therapy carbs is likely caused by flooding the float shut offs. 5 psi is on the high side of max.

[BRE-240Z]

Are you sure about the condition of the tank? What do your fuel filters look like?

You may want to assume that the entire fuel system is suspect, and start at the tank and work your way forward. I had some issues with fuel delivery when I first purchased my car, so I drained the tank, installed new lines all the way to the carbs, installed see-through filters (Fram G2's) front and rear, and also replaced the mechanical fuel pump with a brand new unit from Nissan.

I haven't had any issues with fuel delivery since. My car has an electric pump installed back at the rear, under the tank, but the mechanical pump is quieter and more reliable (in my opinion).

[obxtrainman]

The replacement Ztherapy carbs are early 4 screw round top.

No, I am not sure of the tank. I plan to drop it once it goes to paint, have it boiled and sealed at radiator shop.

Do I need one, the other, or both. As in fuel pumps.

Thanx for clearing up the issue about the fuel pumps Beandip. My original setup was flat top 73 SU's. So apparently I had the electric pump also.

Now I am running early roundtops. So I can maybe get by with just the new mechanical pump???

[BRE-240Z]

Now I am running early roundtops. So I can maybe get by with just the new mechanical pump???

Yes. That was the factory setup on the 240Z's. The wiring was put back there for an electric pump, but there was no pump from the factory.

My Z had an electric pump added at some point by a previous owner, but I don't use it (unless the car hasn't been run in a while, and I'll use the electric as a 'kicker' to get fuel up to the carbs).

During normal operation, I only use the mechanical pump, and I've had no issues at all.

[beandip]

The replacement Ztherapy carbs are early 4 screw round top.

No, I am not sure of the tank. I plan to drop it once it goes to paint, have it boiled and sealed at radiator shop.

Do I need one, the other, or both. As in fuel pumps.

Thanx for clearing up the issue about the fuel pumps Beandip. My original setup was flat top 73 SU's. So apparently I had the electric pump also.

Now I am running early roundtops. So I can maybe get by with just the new mechanical pump???

The '73 Z came factory with the electric pump and mechanical. If you live in warm climate like Cal or Az the electric will help to eliminate the problem referred to as vapor lock. The mechanical pump heats from conducted transfer from the head, this can contribute to the problem. The engine will run fine with either or both pumps. Some aftermarket electric pumps are vary noisy . This is why I like the RX-7 pump because it delivers plenty of volume and no more than 5 psi. Just be sure it is a NON injected RX. My engine now is a '82ZX still with SUs, and it supplies my application vary well. The P-79 head does not accept a mechanical pump in any event. My friend a few years back was running triples on a stroker , we tried a RX pump on his Z and it supplied it just as well. One thing though use a in line filter between the tank and pump, it will save the pump if there is crud in the tank. My friend recently had his mechanic install a new Nissan mechanical pump, $125.00 for the pump. Gary If your filters are not collecting crud , don't bother pulling the tank.

[bjhines]

I have to chime in here on some misinformation on the operation of the ELECTRIC FUEL PUMP WIRING. The 1973 240Z uses ONLY THE MECHANICAL PUMP during normal operation. The electric fuel pump is for priming the system ONLY!!!!! It will not operate once the engine is running. None of this is detailed in the electrical diagrams. There are hundreds of things not shown in any of the wiring diagrams.

There are a bank of relays on the passenger kick panel for the 1973 240Zs. There are 2 relays associated with the electric fuel pump.

Relay 1) cuts the fuel pump OFF once the alternator produces 12v(engine running).

Relay 2) cuts the fuel pump OFF when the starter is engaged(start pos on the key).

The operation of the electric fuel pump is only to PRIME or REPRIME(vapor lock) the fuel system. It will operate when you turn the key to the RUN position. The relays cause it to shut off once the car is running or the starter is engaged.

The electric fuel pump has a filter on the bottom that is often neglected. Check it or change it.

There are many other components on the passenger kick panel that operate flashers, wiper delay, wiper auto run for the washer pump, EGR relay and temp switch, accessory relay, etc. The electrical diagrams only brush over the most basic systems in your car. There are hundreds of wires that are simply not shown in ANY of the available diagrams. This is especially true for the bastard 1973 model 240Zs.

[bjhines]

The original mechanical pump crosses over to many other vehicles. It was originally made by Kyosan-Denki(sp). They are still available through a variety of sources including E-Bay for less than $50. I don't doubt that full mark up on these pumps can be over $100 but that is lining someone's pockets with your money.

[BRE-240Z]

I paid $70 for mine, new from Nissan.

[beandip]

bjhines, What you are saying may be true . I can only attest to my 8/73 240 Z. My Z is and has been running with a electric fuel pump only for more than the 8 years I have owned it. The pump is connected to all the factory wiring and would run as long as the ignition switch was either in the start detente or the run detente. This is why I added a pressure switch in the circuit to denergize the fuel pump if there is less than 5 lbs oil pressure. I have had this Z completely disassembled and reassembled by my self and I haven't found any relay control for the pump. I am not saying that you are wrong , just that at least on my late '73 this is the case. I have only owned the car for 8 years . It is a possibility that the other items have been removed by a PO. However , One other thing , I replaced the entire wire harness when I was doing the project. Gary

[bjhines]

It is true that many previous owners have changed the wiring configuration... But I have owned 2 1973 240Zs that had unadulterated wiring with the relay system I have described. I also have a 1972 that has all of the wiring in the dash harness minus the add-on relay harness for the fuel pump. This car needed to have the fuel pump connections completed in order to provide power to the fuel pump at all.

Another point about a kill system.. That might work fine on a farm tractor(or a drag race car).. But regardless of oil pressure... a sudden loss of engine torque while cornering at the limits is a sure way to go OFF-Road.

I would much rather loose an engine than loose the entire chassis because my fail-safe kicked in at the WRONG TIME.

I see a lot of bright ideas like this as an instructor and tech inspector. I will fail the cars with these systems in place. The worst is the IGNITION kill systems guys come up with. There is a good chance that oil pressure may drop in a high-G turn. The sudden loss of ignition(engine power) under hard cornering will kill you fast. this is an ABSOLUTE NO- NO on track!!!

GOTO Radio Shack and pick up a piezo buzzer and wire it up to buzz when your oil pressure drops. Then YOU can decide when to get out of the throttle instead of your fail safe sending you OFF-course.

I have 2 different buzzers that buzz when the oil accumulator valve operates and another when system pressure drops significantly. They have different tones so I know what is going on at all times.