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EFI fuel return lines

Colin R

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I used the carb feed pipe as the return pipe when I converted to Throttle bodies. Worked fine.

Not sure what the pressure on the other side of the pressure reg (set to 50psi) is but it has been fine.


Some recommend a filter in the low pressure circuit too. I have a Halfords plastic one before the tank so that you can see any crud build up.


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Edited by - stevefoster on 3 May 2003 08:33:23

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The return pipe is connected via an unrestricted connection to the fuel tank which is at atmospheric pressure. The pressure in the return line can only be that due to pressure drop along it plus the head of fuel above it's discharge point (as it used to be a suction line it will end below the fuel in the tank).


The pressure drop along the line must be very small as when it used to be supply line for the carbs it was only fed with a max of say 7 psi and needed to deliver at least 5 psi at the carbs.


The only difference is when you run a surge pot filled by a LP pump and fit a restrictor in the overflow back to the main tank. The surge pot could then be running at up to the LP pump pressure. But LP pump pressure is exactly what the old carb feed line was designed for.


So without an LP pump the return line will see a few psi at most. With an LP pump it'll see a maximum of LP pump pressure. Either way it'll be fine.

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It stops the LP pump that fills the surge pot from running flat out all the time. Depending on the pump this may or may not be a problem to the pump but it'll certainly be ticking away merrily to itself flat out all the time which is irritating.


The LP pumps are generally designed to fill a carb float chamber then tick over against a closed needle valve, with the restrictor they effectively do the same, pump to the surge pot with air going through the restrictor. Then once it's full the fuel flow through the restrictor is much much less and the pump then ticks over against the restrictor.

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