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mbarjbar

fuel return line needed?

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Wow, I thought this was going to be a simple question. By that I mean, I was worried the needle and seat would not be up to 'dead heading' the flow. Pressure would be approx. 3.5-4.0 psi. All, in my experience, British SU equipped cars do not have return lines. My metal fuel lines are not in good shape. The engine inlet and return line are a unique piece and difficult replace with new. So I would like to do without it and the mechanical pump. On my MGB, I have wrapped the fuel line to keep it cool. No problems with the electric fuel pump.

And, please guys, take it easy on each other. This is a hobby forum, a little respect goes a long way.

Mike

Mike

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This might be a dumb question but in a return line setup is the fuel only returned to the tank if the pressure is above a certain limit? How is the fuel system setup to determine when the fuel should be returned?

My car must be deadheaded because when I first got the car the electric pump would run and then shut off if I left the key in run but didn't start the engine. It has an emergency cutoff now so the pump turns off if the engine stops turning over.

Mike

Edited by Mikes Z car

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. Six of one, half dozen of the other. If you have the right pump and properly protect the lines, I see no problems with either setup.

FWIW, a carb (on a "normal" street car) needs only a slight amount of fuel pressure to run at reasonable RPM's a few pounds of pressure is required only at high speed.

The return line has a capillary hole to maintain a tiny bit of pressure at the bowl inlet to maintain idle, the rest of the fuel flow is returned to the tank, the pressure mounts as RPM's increase to combat the dreaded VL. At some point the return flow will max out due to the capillary hole, but VL is not a problem when the car is moving forward at speed as the air flow through the engine compartment keeps the gas cool.

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I've owned 240Z's, a '66 1600, a '67 2000 and worked on a lot of other Datsuns of similar vintage and the progression of design is notable with regard to the fuel systems. The early roadsters resemble MGs in their design (no fuel return); my 1600 had twin SUs (Hitachis) and two small metal vent tubes which exited from the tops of the carb float chambers, angling down and terminating at the bottoms of the float bowls...right above the exhaust manifold (!!!). Other roadsters I've seen had longer tubes that exited underneath the chassis, away from the hot exhaust parts. With the introduction of the Z, the addition of a return line was two-fold, I believe prompted by both emissions and safety---either reason could have been the preeminent one, but I think it was equally mandated by the design requirements and the evolution of that particular set up. The 240z was a culmination of all that, just as the flat-top Hitachi carbs on '73 & '74 models could be seen as a culmination of the dual carb set-up, ending up with non-adjustable, pre-set & emissions "friendly" O.E.M. designs as required by Federal law. The basic fuel supply and routing didn't change for years afterward, on Z cars at least. Only times I've encountered non-return set-ups are in early factory original cars which essentially were drawing on an 80 year old design and modified cars with other than factory carbs which may or may not have had a provision for fuel return. My 2000 roadster had a stock-appearing Solex setup with the correct airbox, but not the authentic Datsun Competition package, I'm fairly certain. I'm not going to research it but I assume even the dealer sold roadsters with the Solex package, hotter cam & aluminum oil pans had return lines also, but I could be wrong.

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elect fuel pump has bee deleted on my 74 260z. fuel path is, tank directly to mechanical pump then on to 2 Hitachi 3 screw carbs.

the metal fuel rail (which includes the "return" to tank line) has been deleted in favor of a rubber fuel line to carbs.

Can't seem to keep new mechanical pump primed with fuel, so it does not pump fuel to the carbs.

do not see any air/fuel leaks in the line from the tank to the mechanical pump.

1. assume the fuel rail can be deleted with no return to the tank path.

2. any ideas what is causing mechanical pump to loose its prime?

thanks gary

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18 minutes ago, gary lee said:

elect fuel pump has bee deleted on my 74 260z. fuel path is, tank directly to mechanical pump then on to 2 Hitachi 3 screw carbs.

the metal fuel rail (which includes the "return" to tank line) has been deleted in favor of a rubber fuel line to carbs.

Can't seem to keep new mechanical pump primed with fuel, so it does not pump fuel to the carbs.

do not see any air/fuel leaks in the line from the tank to the mechanical pump.

1. assume the fuel rail can be deleted with no return to the tank path.

2. any ideas what is causing mechanical pump to loose its prime?

thanks gary

Are you sure the fuel pump valves are sealing? If they don't the fluid will seek it's own level which is a foot or more below the fuel pump

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