Running a return fuel line

[Yellow Truck]

Autoholic, thanks. I looked at the QFT documentation and that also made it a bit more clear. They also said it is preferable to have the same size line, but made it clear it didn't have to be the same.

I think I can lay out the system now.

[Autoholic]

Your return line can be smaller than your supply line but I would only go one size smaller. Theoretically, your return line should be able to return all of the fuel but realistically, even at idle the engine will consume a decent amount. Lets say you are idling the engine for awhile, long stop light or something. Your return is 1 size smaller. The regulator tries to send 80% of the fuel back to the tank (just an example, not sure of the percentage). The regulator is set at 7 psi, so regardless of before or after the carb, the incoming fuel is at 7 psi. The return line is a tad too small for the fuel needing to go back, this creates a restriction that remains constant and the regulator is already open. The result is that the return line would be under pressure and the incoming fuel might increase in pressure. By using the same size line, the incoming fuel can never go higher in pressure than your regulator's setting. Even if 100% of the fuel had to go back to the tank, the fuel line size wouldn't create a restriction and so worst case is the return fuel would be at 7 psi, the fuel pump would just be circulating fuel. With different sized lines, the return fuel could be at a higher pressure than the supply side. You could also see a pulsating fuel pressure on the supply side.

[Yellow Truck]

Autoholic, thank you. I'm starting to understand it. I will try with the line as planned, and I'll have a gauge on the fuel log. If I see it too high at idle, I'll have to run a new line, which will suck because it will all be back together and I won't enjoy pulling the tank again. Or I can give up on the return line and just stick in one of the other regulators.

Having said that, we will have a fuel filter in the path of the supply line and none in the return, which will help a little.

[Autoholic]

If you are running 6AN return, I would think that would be ok. You're looking at 7 psi and 40+ gph, I'm not positive but I think that would be ok. You can always ask the company you're purchasing fuel lines from if 6AN is fine with that demand. I've been thinking a long time about this and I can't see the return line ever being under pressure. In order for the return line to build pressure, it either has to pressurize your fuel tank (improbable if not impossible) or it has to run into some sort of restriction and I can't see 40-ish gph resulting in too high of a flow rate. Even under turbulent flow, if there is not a restriction to the flow there is no pressure. Too small of a fuel line can provide that restriction BTW but I think something else would fail before the return line does. By this point there would be a decent amount of back pressure in the system and your carb would most likely act up, idle bad, something due to a large spike in pressure. I think you'd end up running rich if there was a prolonged spike in fuel pressure, resulting in backfires on the overrun being common. Maybe someone else can dive into the issues you might face with a sudden spike in fuel pressure and how it will effect your carb.

[Yellow Truck]

Joe, I am planning on a 6 AN supply line and reusing the original supply line as a return. The original supply line is a 1/4 OD steel line. I don't know the inside diameter.

[Autoholic]

The ID is probably 1/8" or very close to that. I would replace this line before finishing your fuel system. You're running like a 3AN return. For a 6AN system, you want tubing with an ID of 3/8".

[TomP]

I am pretty sure the stock fuel lines are 5/16" not 1/4"

[Yellow Truck]

Tom, that could be true. Took the measurement with a not great set of callipers.

[Nightmist66]

Easy way I've found especially if you don't have a caliper handy, is use an open end  wrench and see which one fits. Works for standard and metric.