carburetor vacuum system

[scott.k]

Hello, does anyone have diagrams and pictures (labeled if possible) of the vacuum system on a 911 with carbs (PMOs, webers, zeniths)...

Or if anyone could just list the vacuum ports on the carb and manifold that would help too.

Thanks for the help
Scott

[scott.k]

also, I would be interested in knowing peoples fuel system setups for these carbs as well.

thanks
scott

[1QuickS]

Webers supplied for use by Porsche added ports in the bottom flange for use as a vacuum supply, they are shown on this throttle body on the left and right flanges. Best fuel system is one that recirculates fuel through the carbs and back to tank (easy for 1969(?) and later chassis since they have two fuel lines for this purpose.) Locate fuel pump on front cross member (close to tank and low enough to get good fuel delivery to pump) and install fuel filter in the engine compartment. Easiest installation is a dead-end style with one fuel line. If a recirculating system is installed then the fuel pressure regulator is down-stream from carbs and drains excess fuel back to tank after fuel passes through carbs. Get a good quality fuel pressure regulator and set fuel pressure to 3.5 psi at the inlets to the carbs; critical also is to shim float levels while engine is running.

[snbush67]

The only vacuum port you will need is a single brake booster vacuum port. The brake booster vacuum on my set-up is on the driver side manifold in the center.

I did not need any other vacuum from the carbs or the manifolds.

For your SC, you can use the fuel block from PMO with your stock pump. It will dial the pressure down to the level needed for carbs.

[scott.k]

Originally Posted by snbush67

The only vacuum port you will need is a single brake booster vacuum port. The brake booster vacuum on my set-up is on the driver side manifold in the center.

I did not need any other vacuum from the carbs or the manifolds.

For your SC, you can use the fuel block from PMO with your stock pump. It will dial the pressure down to the level needed for carbs.

ok, thanks for the help. what about vacuum advance for the dizzy if your running it? i have an msd 6530 with programmable advance so I probably wont need it anyway but just wondering?

thanks

[snbush67]

Nope you wont need it, just plug it. You should look at finding out what your distributor is doing (how much advance are you getting out of it) if it needs a rebuild, you should opt for a re-curve as well. If not the second best option is to find an early SC (78-79) dizzy which will have a better curve more suited to the carbs.

The third option is to clean it really good and set it at full advance at 3k rpm, I think optimally you should be seeing around 26-28 degrees of advance. You might get a bit of a rough idle but that is oK as you shouldn't spend too much time down there (idle), unless you are actually stopping for stop lights and pedestrians.

[snbush67]

Oops! I think if you are using that MSD then you need to lock your distributor out. I think you are getting into some cloudy water with that unit. There was some discussion about the limits of advance etc. I will be following your progress with interest.

[scott.k]

Originally Posted by snbush67

Oops! I think if you are using that MSD then you need to lock your distributor out. I think you are getting into some cloudy water with that unit. There was some discussion about the limits of advance etc. I will be following your progress with interest.

yeah, im not sure if i want to do that, i may just use the stock timing

and thanks a lot for the help

[scott.k]

Originally Posted by snbush67

Nope you wont need it, just plug it. You should look at finding out what your distributor is doing (how much advance are you getting out of it) if it needs a rebuild, you should opt for a re-curve as well. If not the second best option is to find an early SC (78-79) dizzy which will have a better curve more suited to the carbs.

Sorry, just to clarify, when you say "you wont need it", thats referring to the vacuum advance of the carb?

thanks
scott

[scott.k]

and to clarify for the fuel system, you can use the stock pump with a regulator and return line? out of curiosity, how many psi is the stock pump?

thanks
scott

[snbush67]

You will not need the vacuum advance pod on the distributor so just put a small cap on it.

Yes you can use the stock fuel pump with a regulator and a return line. I think that the stock fuel pump puts out 78-80 psi.

PMO fuel regulator

[scott.k]

i apologize for asking a bunch of questions, but can i remove the ecu and wiring harness in the engine too? it seems that there was a wiring harness for the injection system and i wont be hooking any of the wires up with the carb and i think that harness goes to the ecu.
(and i think this would include the o2 sensor too)

thanks a lot for the help
scott

[scott.k]

Originally Posted by scott.k

i apologize for asking a bunch of questions, but can i remove the ecu and wiring harness in the engine too? it seems that there was a wiring harness for the injection system and i wont be hooking any of the wires up with the carb and i think that harness goes to the ecu.
(and i think this would include the o2 sensor too)

thanks a lot for the help
scott

anyone? im sure tons of people on the forum have done the cis to carb switch

[RoninLB]

I'm not up on the 1980 but

If distributor is a single pot it's a vacuum retard.. plug it
I feel carbs need a fairly aggressive timing curve. Initial between 5-10 BTDC and total depends on compression

I made up new engine harness and stored CIS harness. The only thing you should find out is where the tach wire is. Mine was inside a bundle outside and under the electric panel. Ignition wiring is simple. Keep it that way.
I used Waytek Inc, Home page to buy fuses, circuit breakers, cable ties, wire and other electrical supplies By Waytek Inc. double wall shrink tube in 4' lengths to mimic a stock harness. I left some slack in tube so it can make bends and looks stock. Solder all connections.

The PMO fuel regulator is designed for use with a fuel return line. It's a good unit and many pelicanheads like it.

a good fuel filter is a Racor unit with a 2 micron filter. Many auto part store filters are 10 micron and 10+

spend the bucks now and get it done right

[scott.k]

Originally Posted by RoninLB

I'm not up on the 1980 but

If distributor is a single pot it's a vacuum retard.. plug it
I feel carbs need a fairly aggressive timing curve. Initial between 5-10 BTDC and total depends on compression

I made up new engine harness and stored CIS harness. The only thing you should find out is where the tach wire is. Mine was inside a bundle outside and under the electric panel. Ignition wiring is simple. Keep it that way.
I used Waytek Inc, Home page to buy fuses, circuit breakers, cable ties, wire and other electrical supplies By Waytek Inc. double wall shrink tube in 4' lengths to mimic a stock harness. I left some slack in tube so it can make bends and looks stock. Solder all connections.

The PMO fuel regulator is designed for use with a fuel return line. It's a good unit and many pelicanheads like it.

a good fuel filter is a Racor unit with a 2 micron filter. Many auto part store filters are 10 micron and 10+

spend the bucks now and get it done right

THanks for the help. Just wondering, did you keep the ecu, or was the wiring harness you made just to replace the ignition harness?
thanks
scott

[scott.k]

in the ecu wiring harness, there is one wire that runs from the ecu to the front of the car (black with white stripes), and one dark brown wire that runs from the front of the car and join with the bunch of wires that run from the ecu to the engine compartment. I was just wondering what the black wire and the dark brown wires are? I think this would help enable me to remove the ecu.

THanks
Scott

[scott.k]

Originally Posted by scott.k

in the ecu wiring harness, there is one wire that runs from the ecu to the front of the car (black with white stripes), and one dark brown wire that runs from the front of the car and join with the bunch of wires that run from the ecu to the engine compartment. I was just wondering what the black wire and the dark brown wires are? I think this would help enable me to remove the ecu.

THanks
Scott

Maybe one of them is the tach output wire, ill take a look and try to follow them better.

[scott.k]

does anyone have a 911 sc ecu wiring diagram?

[RoninLB]

Originally Posted by scott.k

THanks for the help. Just wondering,

did you keep the ecu, or was the wiring harness you made just to replace the ignition harness?

my car didn't have an ecu

my only comment is that an engine needs fuel, ignition and air

my harness is only for ignition and other neurotic needs. The CIS auxiliaries wiring isn't needed

[scott.k]

Originally Posted by RoninLB

my car didn't have an ecu

my only comment is that an engine needs fuel, ignition and air

my harness is only for ignition and other neurotic needs. The CIS auxiliaries wiring isn't needed

great looking engine bay. and thanks for the feedback, i think im going to take out the ecu and make a wiring harness for the msd unit