Weber Carbs Fuel Lines Help

[supradan]

Greetings my fellow Pelicanights!

Last year I was lucky enough to purchase a 1977 911S with a 2.7 and weber carbs. I am not new to Porsches, had a 930 in the past that I regrettably sold. Anyway, my experience is more with turbo cars and not carburetors, so I’m hoping someone can answer a few questions for me.

I’m getting ready to drop the engine and trans so I can replace the clutch. While I have the engine out, I plan to adjust the valves, install a MSD ignition system, replace carb gaskets, redo the fuel lines within the engine bay with new FPR and fix a small oil leak.

Having said that, I would like to change the fuel pressure regulator and the fuel lines, however I’m a little confused by the way the fuel system is plumbed, specifically the left bank of carbs. On the left side there is a fuel line with a T connector that looks to have another line connected that seem to both go into the firewall. This I’m assuming is the fuel return line, but can’t confirm as the T connector with another line is baffling me.

I’ve attached a picture of the hose in question for those carb gurus to help identify. In addition to, I went ahead and drew a diagram of the current plumbing and what I think the future setup would look like.

Can you help me identify what the line is used for?
I’m planning on adding a PMO or Holley type FPR, with the current setup, where would I connect the fuel line in question on the FPR?

Any help or direction is greatly appreciated. Thank you!
Daniel

[75 911s]

I think those are the old fuel emissions system. The two lines are lines that come off the back of the charcoal canister. I deleted my charcoal canister when I installed the carbs.

edit: Oops vacuum line for brake booster. My car has manual brakes so I missed that. :-D

[Tom_in_NH]

Those lines provide manifold vacuum to the power brake booster.

[supradan]

Just to make sure I’m not hurting anything with the carbs; can I cap this line? Thank you!

[Malachi]

Thank you for sharing Nice explantation.

DGCustomerFirst

[tk911]

Originally Posted by supradan

Just to make sure I’m not hurting anything with the carbs; can I cap this line? Thank you!

The line you highlighted has nothing to do with fuel, it provides vacuum, usually for a brake booster. Capping it won’t be a problem for the carbs, but if it is connected to your power brakes they won’t like it too much.

[Gordo2]

Here's a thread showing how I ran mine with braided AN6 lines:

AN-6 Fuel Line Plan for CIS to PMO Carb Conversion

You need vacuum / the vacuum line you highlighted for your brakes to work effectively (significantly reduces brake pedal effort to actuate brakes).

Not sure why your vacuum line has a T with another line coming off of it - it's typically just a single line with an inline check valve.

Good luck,

[supradan]

Thank you folks for the responses!

Now this begs the question, what do I do with the hose, sounds like I should leave it in place. However if I decide to go with a PMO or Holley FPR, do I only connect the fuel line coming from the fuel pumped and the two banks of carbs to FPR and no return line (much like what I have currently)?

Gordo - Love the AN fittings and hoses you upgraded to, you are giving me some inspiration to do the same.

[Locker537]

Originally Posted by supradan

Thank you folks for the responses!

Now this begs the question, what do I do with the hose, sounds like I should leave it in place. However if I decide to go with a PMO or Holley FPR, do I only connect the fuel line coming from the fuel pumped and the two banks of carbs to FPR and no return line (much like what I have currently)?

Gordo - Love the AN fittings and hoses you upgraded to, you are giving me some inspiration to do the same.

The PMO Pressure Control Unit uses a return line.

One input line.
One return line.

Two output lines; one for each carb.

Regarding the vacuum line, one end is surely running to your brake booster. Trace the other line (the braided one coming off the T) and see where it's going. Does the car have a vacuum gauge?

[BoxsterGT]

A 77 911 has a Fuel Feed and a Fuel Return line, so the PMO FPR will work just fine.

For your Carbs, for the Feed you need a Hose from the Tunnel to the Filter and then to the FPR and then individual filters to the carbs.

You also need a Return from the FPR to the Return at the Tunnel.

This photo is a of 77 911 showing the Feed that connects to the top of Accumulator and the Return pointing into the engine bay. The Big Hose is the Power Brake Booster Hose that must connect to a vacuum source.



Hope this helps.

Len

[supradan]

Originally Posted by Locker537

The PMO Pressure Control Unit uses a return line.

One input line.
One return line.

Two output lines; one for each carb.

Regarding the vacuum line, one end is surely running to your brake booster. Trace the other line (the braided one coming off the T) and see where it's going. Does the car have a vacuum gauge?

Good question, no vacuum gauge to speak of. I plan to trace where the hose goes this weekend. Stay tuned.

[supradan]

Originally Posted by BoxsterGT

A 77 911 has a Fuel Feed and a Fuel Return line, so the PMO FPR will work just fine.

For your Carbs, for the Feed you need a Hose from the Tunnel to the Filter and then to the FPR and then individual filters to the carbs.

You also need a Return from the FPR to the Return at the Tunnel.

This photo is a of 77 911 showing the Feed that connects to the top of Accumulator and the Return pointing into the engine bay. The Big Hose is the Power Brake Booster Hose that must connect to a vacuum source.



Hope this helps.

Len

Thanks Len for the guidance.