Technical Vacuum fitting on SBC fuel pump?

Hi guys. I ordered a fuel pump from Inline Tube that seems to have a vacuum fitting. It's going in my '40 with a SBC, so I just clicked on the pump that came up when I ordered a pump to carb line to go with the 4GC. They sent a Carter M3120. Nominally this is going to be a '62-'64 looking engine and is supposed to be a 62-64 type pump. Is this really a vacuum fitting? Or some kind of emissions thing?
JHMFC. will post a pic soon. Sorry about the pics guys, mobile HAMB app just wouldn't let me upload pics.

 

Have a picture?
Could be a vent line to go to the carburetor incase of diaphragm failure like would be on a boat engine.

 

It's a return line. Excess pressure/fuel(regulated) is released back to the tank. Helps prevent vapor lock.

As much as everyone hates emissions era vehicles, they did get a lot of nifty devices that can be useful on older carb'd vehicles. If you don't want to run a return line you are going to have to either return the pump for an older pump or secure that return line to prevent fuel leakage.

 

From what I've been able to find , you may have received an M 3880 carter , all the pictures and descriptions of M 3120 as having one outlet port & one inlet , the 3880 has the third port .

 

Marine Application...

 

does it look like this?

 

Summit shows the application to be for a 70-73 Ford 600

M3880 that is.

 

It has M3120 stamped on it as well as on the box. see pics.

 

Yes sir, that,s it.

 

Being that's a performance/aftermarket type pump and the connection is above the diaphragm it's most likely intended for boost referencing with a blow through turbo or supercharger application. Would be connected to the hat or carb box and never see vacuum but would up fuel pressure incrementally with boost. Common on old McCulloch and Paxton setups like on 50s T Birds.

 

I apologize , in looking back &forth I managed to confuse myself . carter m6626 is the 3 line design that I had on an older truck . I didn't see the 3rd fitting on top originally . sorry for the confusion ..

 

Seems to be a tiny hole and unusual fitting for a return line........

 

No fuel should be above the diaphragm, @landseaandair is probably correct.

 

Vacuum port for vacuum operated windshield wipers, converting them to dual action ? Seem to recall that on a car I had long time ago.

 

That style had 2 fittings to ***ist the engine vacuum.

 

bumping this because none of these is a firm answer. not a high performance pump to boost referencing is doubtful

 

It's just a vent. Could also be used as a return line to top of carb on a marine application in case of diaphragm failure. Probably just use the same housing for a bunch of different applications these days.

 

I'd have to go out in my boat and look but a search brought up a Nova page discussing that pump and one person said that that setup was a marine setup to vent fuel back to the carb/intake in the possibility of a leaking diaphragm. It could be part of some application's emission setup too.

 

Did anybody come up with a definite answer

 

That fitting is there in place of a weep hole. Most mechanical fuel pumps have a weep hole on the bottom side of the pump. When the diaphragm leaks,fuel escapes through the weep hole instead of saturating the engine oil with gasoline.

In a boat there needs to be a line from the weep hole to the carb to prevent gas from going into the bilge.
Carter probably fitted the that pump with that fitting so it could be sold for Marine use also.
So basically it is just a ventline and no need to worry about it.

 

That is for a supercharged engine. That port goes to the carb box that is pressurized so the top part of the diaphragm reads the supercharged carb pressure not the atmospheric pressure.

 

This is the real answer

I haven't crawled up in My boat to check but if it isn't raining tomorrow I will. One way or another it is to route any gas that might leak away from the engine compartment. There may be what amounts to a drain hose on some late models out there.

 

Not sure I understand how boost on that port will increase fuel pressure.

 

I am restoring a 63 Avanti R2. It has a port on the topside of the fuel pump. Car is supercharged. Line goes from supercharger to pump.

The fuel pump pressure would be trumped by the lbs/sq in of air pressure provided by the supercharger if Studebaker did not add that 1/4" line at the air volute of the blower....That line increases pressure on the fuel pump diaphragm as boost goes up, thereby increasing fuel pump pressure, overcoming supercharger boost pressure, which in turn allows the needed extra fuel to enter the carb............

 

Hook up a vacuum gauge see if it pulls vacuum

 

Here is an interesting diagram

the vacuum port is also on marine pumps.
Wonder if both theories are correct.

 

Here is your answer. Now who's laughing?

 

GM never made a supercharged SBC, why would carter make a boost reference fuel pump for it?
As all boat guys already have said it's a vent line.

 

In order for it to be effective in a turbocharged/supercharged application, wouldn't the carburetor have to be inside a pressurized "box"? I've seen blow-through turbo applications where this might be applicable, not sure.

In any supercharged application, isn't the carburetor always at atmospheric pressure, sitting on the intake side of the supercharger? Or are there also "blow through" supercharger setups?