Return fuel line ?

[Art@NissanSport]

A friend just bought a 73 240Z. It has no return fuel line. It is blocked off (the hard line has been removed). I have never seen this before. Is this considered normal anywhere in the world? The PO said that it has always driven fine. The car came from back east. Once it got into New Mexico it started running like cr@p. The PO also said that he uses a 9 volt battery to jump the electric fuel pump to prime the line. This doesn't make sense to me either. Although I can understand not running the e-pump continuously with no place for the fuel to return. I want to help get this guys Z up and running properly but in its current state nothing makes sense. Any comments, suggestions or answers would be of great help. Thanks

[JimmyZ]

SU carbs right?

[Art@NissanSport]

Yes, three screw SU's.

[JimmyZ]

I think you know what to do and I don't blame you for not wanting to do it. :nervous: Make your return hard line out of 3/16 brake line since it will be readily available at any REAL parts store in 25ft rolls. Make sure you use a quality tight fitting flexible line where needed and clamp it well. I just measured my hard line on my 2/71. It is within a few thousandths of the 3/16 line I used for fashioning new clutch/brake lines. 3/16 line OD= .192-.194"

Before starting, check that the fuel tank return is still usable and that the fuel "rail" has it's restrictive opening flowing fuel. (Hope the rail is still usable) Hydraulic pressure is a function of restriction no restriction = no pressure. I know we're talking about low pressures with carb systems but the system needs it. The hole is the size of a needle.

I did not enjoy removing and replacing my fuel/brake hard lines on my Z. At least you'll only be doing one.

[Carl Beck]

Hi Art:

Lots of guys have been running triple side drafts for years with no return line. I believe the reason is that Cannon supplied the fittings, that did not accommodate a return line - so the Z owners just used that was there. Not the best set-up, but it usually works. They also usually run an electric pump with a fuel pressure regulator.

For the most part the return line in the OEM system is there to keep the fuel moving/circulating and thus cooler than it would be if left standing still in the lines close to the engine.

Both the mechanical and electric pumps have predetermined upper pressure limits - that is to say they put out 3 to 4 psi - at which they either stop pumping (electric) or some physical restriction stops the output of more fuel.

I'm sure you are old enough to remember that most cars in the 50's and 60's had no fuel return lines.:-)

In either case the first thing to do is measure the fuel line pressure at the carb.'s - - - if you are getting excessive pressure (6+ psi) - it's possible some PO has installed an electric pump that's output is higher than needed by the S.U.'s, or it's possible that the electric pump is not putting out enough fuel because the filters in it are clogging up..

I'm not positive - but as I recall - the OEM electric pumps on the 73's only ran on the START position of the ignition - then when the engine was running the mechanical pump actually supplied the fuel. That would explain why the PO "jumped" the electric pump to pressurize the fuel line prior to starting - it would take less cranking on the START position to keep the electric pump running. That is "IF" the car still has its OEM electric and mechanical pumps in place.... If some PO has installed an aftermarket electric pump and wired it to run all the time - then it should run any time the key is ON.. (easy enough to check)..

All the above said - the poor running condition may have nothing to do with not having a return line. Could be that the car sat, un-used back East for a prolonged period - and now you are really dealing with varnish and rust in the tank, or in the fuel filters - at the pump, the in-line filter, or at the carb.s.

Could be a weak pump as well.

I do agree that it is important to have a return line on the 240-Z's running the S.U.'s - it keeps the fuel cooler and it's part of the evaporative emission controls...

FWIW,

Carl B.

[beandip]

Art, I agree that the return line is an important item to retain , especially in the warmer climates. Along with this issue, I believe that having a safety shut off on the pump in the event of a accident or oil pump failure . I have installed a simple pressure switch by adding a tee at the oil sending unit. This switch activates the fuel pump as soon as there is 6 lbs. oil pressure. I have also added a inertia switch on the same circuit . This way if I am incompasitated in a accident the pump will not keep pumping. The engine starts on the fuel in the carbs with no problem, and has after the Z has wintered over . The switches have been in place for about 3 years now. I am running only electric , and it is a pump from a 1980 RX-7 . It supplies right at 5 psi. with plenty of flow. Gary

[JimmyZ]

Carl is right. There are many things it could be besides the non existant return. Many older vehicles I've owned/worked on had no return. (Holley, Rochester carbs) In theory the SU's don't need a return. Fuel flow problems can sound like ignition problems. This was my experience when my tank started passing rust chunks years ago. It sounded like a bad miss due to the filter clogging. The engine finally stopped at which point I'd replace the filter and drive a little more.

More recently, when assembling my car after a complete disassembly I got some crud in my pump and lines. The parts had been sitting on the shelf for years. (Had to slap the car together quick as I was moving) I replaced the mechanical pump but took the old one apart and found that the one-way valves were jammed by debris. My fuel return opening in the rail was also clogged. My SU's didn't want to take on fuel with the return plugged for some weird reason. As metioned before they should be able to fill w/o a return line.

The rough running is probably due to something else or related to something caused by the lack of a return line.

Either way it seems best to put the return back in. I'm assuming you checked ignition issues before assuming fuel. The possible over pressure to the SU's may have made things run rich and fouled some plugs. Don't recall seeing a bypass measure on the mechanical pump. Vapor lock also comes to mind with the temps being what they are and no circulation.

2c

Jim

[Art@NissanSport]

The car supposedly ran fine until about 150 miles from Albuquerque. They got gas at a station and it ran bad from that point on. We drained the tank and put new fuel filter and gas in. It still runs rough. There is no way this car could have been driven 1000+ miles in this condition. I think the floats need adjusting since it is coughing out of the carbs and could have something to do with the return lines (who knows). The gas tank is brand new, the carbs are newly rebuilt. If the car was running fine for the first 1000 miles, I don't think it is a timing issue. The car starts and runs but it is rough at low idle. It smooths out as the rpm's increase.

I have a very good set of SU's that I know are perfectly tuned and plan to swap them with the ones currently on the car. That should go a long way in helping me find an answer to our little delemma.

Now maybe because the car came from the Chicago area and the air is thicker, and because it is now a mile high and the air is much thinner, seems that there is way to much fuel being feed to the carbs. But leaning them out has not made it any better.

I hope to have this figured out later this week. My wife thinks I'm having an affair since I'm spending so much time trying to help a friend.

[JimmyZ]

Did you pull a couple plugs and see if they were fouled. (One off each bank #2 ) I'm thinking that maybe a little excess fuel is getting by the float/needle due to pressure. A rich mixture would be the result. Return line would fix this of course.

It could also be something as simple as the points not being tightened enough and vibrating to a smaller gap. Possibly a stuck piston in the carb?

Let us know... I'm sure you will

Cya,

Jim

[Art@NissanSport]

Update time. I worked on the 240Z all day today. To try and eliminate possible problems I decided to put on a perfectly tuned set of SU's. I also ran a new return fuel line (temporary for now). The car does not backfire out of the carbs anymore but does backfire out the exhaust. In trying to reset the timing I noticed that the timing mark is jumping all over the place (timing chain stretch?) The motor I was told was recently rebuilt (could mean a year or two, I have no idea). With that being the case, wouldn't you think that the PO would have put a new timing chain on. The next thing on my list is to pull the valve cover, adjust the valves and check the timing chain. I would have to figure that if I can get enough adjustment out of it, that it could fix all the problems.

I just took my project 240Z for a test drive (I had all the same issues as my friends 240Z-minus the return line problem) and it purrs like a kitten. Proper float adjustment was one of my problems but is not an issue on my friends. Now to start on the suspension, man is it shot. But I don't care, I'm thrilled that the engine runs so smooth. I can rest my hand on the valve cover and it barely moves.

I won't be able to work on my friends Z for at least a week (Nissan Sport goes into production and it takes all my time) so I'll let you know how it turns out in early September.

PS: for anyone that wants to know, I think the return line is needed. Once I hooked it up again you could see/hear a noticable difference. Before I hooked up the line to the tank I ran a hose to a gas can and watched fuel pump out. So with fuel pumping like that, it wants to go somewhere and since it had no where to go except back to the carbs my guess is I would never have been able to tune them properly (remember, the vent line had been removed as well). Just my 2 cents.

[Carl Beck]

....... In trying to reset the timing I noticed that the timing mark is jumping all over the place (timing chain stretch?)

Broken bond on the damper pulley - or loose crank pulley bolt. Teeth on the oil pump/distributor drive broken/missing.

FWIW,

Carl B.

[hd240z]

A loose timing chain should not cause the ignition timing to vary since the distributor is driven directly from the crankshaft. I would take a close look at the distributor for wear on the bearings, etc. As Carl said the damper pulley could also be failing.

Good Luck