return and supply fuel lines for Weber 45s

[goodhobbes]

I am putting in new fuel lines for my twin Weber 45's side-draught carbs. Do I still need a fuel return line if I am using an electric fuel pump at 2-4psi ?

I am also considering a 5/16th Stainless braided PTFE supply line,  Is this safe to run this fuel hose under under the car? I going round in circles with this as I search the literature, but I want to get it right for obvious reasons.  

[Son of Marty]

You would be better off to run a metal line for your fuel supply for a couple of reasons, first a metal pipe will always flow more than a hose, and second the stainless braid is designed, and it does it well, to file it's was through anything it comes in contact with and it's heavier than tube. You shouldn't need a return line with the webers but you should have a good fuel pressure regulator even if your pump is rated at 2 to 4 psi as that is almost always the delivery pressure and it will keep building pressure above that when the floats are closed. This is of course my opinion.   

[Chris_B]

+1

 

No return line needed if you are running a fuel pump with the proper, nominal pressure  (<4 PSI). Stick with the factory mechanical fuel pump if you can. The added complexity of installing an electric fuel pump is unneeded.

[Son of Marty]

If you do go with a electric pump be sure to use a inertia switch or oil pressure switch in case you accidently park your car on it's roof.

[TobyB]

Personally, I'd be the odd duck and would run a return line-

 

pump in the trunk, bypass regulator under the hood, and 3/8" return from the regulator.

 

Because I've tried almost everything else, and it has compromises....

 

And yeah, save your money and run hard line- it's far cheaper, and while

getting it in in the first place will suck, after that, it'll be all win.

 

t

 

[Hans]

Out of curiosity why would a pipe and hose of equal ID flow at different rates

[jimk]

No worries about flow.  Should be evaluating size on velocity of the gasoline in the tube at WOT.  A 3/8" ID tube or hose will have a velocity of about 35 feet per minute at WOT with no return to the tank.  This engine in NA form will draw a conservative 13 gallons per hour fuel.

[M3M3]

11 hours ago, TobyB said:

Personally, I'd be the odd duck and would run a return line-

 

pump in the trunk, bypass regulator under the hood, and 3/8" return from the regulator.

 

Because I've tried almost everything else, and it has compromises....

 

And yeah, save your money and run hard line- it's far cheaper, and while

getting it in in the first place will suck, after that, it'll be all win.

 

t

 

this...and just use the stock metal line that runs along the left side frame rail.  and you can get two of these and run them next to each other.

 

and use a pressure regulator.  webers don't like a lot of pressure, just flow.  2.5psi is MORE than enough.

[Hans]

2 hours ago, M2M3 said:

this...and just use the stock metal line that runs along the left side frame rail.  and you can get two of these and run them next to each other.

 

 

I thought it was a plastic tube through the inside.... or am I having a Covid moment?

[M3M3]

might be a square tail thing.  there is a metal fuel line that goes up the left side of the car.  look for the red line in these pictures.  i painted one of the two i have red for "return".

 

 

Edited October 25, 2021 by M2M3

[TobyB]

yeah, and maybe a late roundie as well- the fuel system went through at least 

3 iterations, with the final one being a rather large metal return line.

Which is nice.

 

I THINK that started in '72 sometime, along with the bypass valve, but

I'm basing that on old memories of taking apart parts cars. 

 

t

 

[Son of Marty]

In my memory the return started with the 2 bbl carb, but like Toby it's been a while.

[goodhobbes]

Thank you all for all your replies to this topic.  I have a length of 6.9 mm ID (0.27in) stainless fuel line that I could use, but might be too small an ID.   The stainless reinforced hose that I have is 5/16th. which I think may be a better option.  Many seem to stress the use of a pressure regulator, which I will do along with an electric fuel pump in the trunk. As for the return line, there seems to be a mix of pro's and con's on that topic.  Any comments?

 

[M3M3]

return line is a function of what pressure regulator you use........

[John76]

I thought the return line was "invented" to help with the boiling of the new (unleaded) fuel.  Constant circulation keeps it cooler and less likely to vapor lock.

The BMW fuel return line is pretty clever. It returns more fuel at idle (high vacuum) and less at WOT (low vacuum).

The variable diameter of the return orifice in the valve also controls the fuel pressure from the mechanical (or electric) pump. Larger opening at idle = lower pressure. Smaller opening at lower manifold vac = higher pressure to the carb.

A bonus for using a fuel return with an electric pump is reduced noise. E-pumps get noisy when pumping against a dead-end.

 

On 10/25/2021 at 10:41 AM, Hans said:

plastic tube through the inside.

The plastic tube running through the cabin is the supply line to the mechanical pump. It's low vacuum from the tank, so if it splits or leaks, it just sucks air, or leaks fuel at low pressure (gravity). If the plastic line is used with an E-pump near the tank, it would be pressurized at the full fuel pump pressure (before a regulator), which could spray a fine mist of gas into the cabin if it gets a pin hole. Not good!

 

On 10/24/2021 at 8:41 PM, TobyB said:

Personally, I'd be the odd duck and would run a return line-

Me too. Steel in both directions with a electric pump.

 

John (aka Odd Duck II)