to me it looks like it's connected to the crank case valve line. The acutal unit itself looks like it's a vaccum operated unit, hence the nipple. I doubt it would be a fuel tank breather as that would be EXTREMELY dangerous.
Just run a whole new metal fuel line, they cost $40 for like 6m well that price is for 3/8" metal pipe.

Cheers
damo

yes it is part of your "evaporative emission control system"
the old fuel tanks let fuel vapour to the atmosphere , then they sealed the tank and developed this system that draws crankcase and tank vapour into the intake , the top hose conection goes to the air cleaner.

I used this vapour line for my return line when I put injection on , but in hind sight should have used the old fuel line and kept the vapour system and just putin the new fuel line

do u think it will be fine to use as a return? sounds like it would, the only reason i dont wanna run a new line is coz that one is already there, and if i run a new one then this one will not be used, its just that i dunno whether the return needs to run back into the tank above fuel or below or what ??? i know nothing about fuel systems......

On your compliance plate it will have an ADR for fuel tank emissions. For engineering and when you go for blueslip, this ADR will still need to be met.
The key to running low fuel pressure at idle with a malpassi, is that your return line is big enough to flow as much as the supply line.
You should be running a bigger than stock fuel supply line, and even the stock fuel supply line used as a return would be maxed out for the pressure you're needing your carbs to be seeing at idle.
The return at the tank needs to see as little pressure also. So you need it to be higher than fuel level. You can either braze in a return line or T into the fuel breather line on the filler neck ( preferably as low as possible so fuel isnt splashing up on the cap)

so im probly gonna need two new fuel lines, damn. are u allowed to use flexible line or gotta be steel..... i can see what ur saying with the return coz i am needing to send alot of fuel back tgo the tank to regulate the flow properly. how will i know if it is regulating proerly? will it just not go below a certain level even when i adjust the fuel pressure lower ????

Most auto parts shops will sell steel fuel line for about $35 for 16metres. If you mount the rubber line ( you need efi pressure line for supply ) properly - it will pass, but it will be cheaper and better looking to use steel.( new stuff is easy to bend by hand )

On the reg, just undo the screw till its loose. Turn on your pump. Then slowly turn the screw in untill the fuel pressure guage reads as high as you want it at idle.
When the engine is idleing, you will need to check the pressure again and mabe need to turn the screw in more since you now have vacuum and the pressure will have dropped a little.
Only way to see if it regulates, is to watch the fuel pressure guage and see that pressure rises as boost pressure increases.

what sort of pump are you usig to need a big return for a carbie application the standard line will flow more fuel than you can burn I cant see it making enough power to need !!! 8 or 10 mm lines I wish it does for your sake , but.
the return wont need to return more fuel than is supplied but less.

When you need fuel pressure to be greater than boost pressure, you run an efi pump.
An efi pump pumping fuel to a carbie is no good as fuel pressure is to high.
Using a malpassi fuel reg allows fuel pressure upon demand and reduced pressure when high pressure is not wanted.
If you want to run 4psi fuel pressure to a carbie you need to be able to bleed off the excess fuel and fuel pressure. 4psi is squat to what an efi fuel pump will supply - so you need to be able to return just under what is able to be supplied.( definately not more )
Fuel line size?
I have never tried this setup with the carbies kegs is using. I only ever used modified stock A series hitachi's. 2 Stock fuel lines, one for return and one for supply, feels like;
Accelerating and then feeling like a brick has been put under the pedal.
Not real good for the rings and not exciting either.
New lines fixed this.

This part on the firewall with three hose connections is called the "Flow Guide Valve". As mentioned, it is part of the "Evaporative Emission Control System".

This is a good thing, it does not hurt performance in any way, but does prevent fuel tank fumes from venting directly to the atmosphere (when hooked up). Result = happy planet.

In fact, on a 1200 (71-74) it is the way the fuel tank breathes. Blocking the hoses would be a bad idea. 1200s use a fuel tank cap that has a safety-pressure release valve and pressure will build until it "pops" when these hoses are blocked.

1. The side inlet is from the fuel tank. This lets the tank breathe

2. The lower hose (connected in the pic) goes to the crankcase, to the large pipe at back of engine (visible in pic). The fuel fumes are allowed to exand into the crankcase, but not to escape into the atmosphere. There is an anti-backfire valve in the Guide to prevent bad stuff from happening

3. The top hose (not connected in the pic) goes to the air cleaner. If the pressure drops in the tank, the one-way valve inside the Guide allows the tank to breathe in

North American model 1975-up added a carbon charcoal canister to 'store' the fumes.