The middle one is for vacuum advance of the distributor. It connects to ported vacuum on the carburetor (which may be the capped one in the lower left of your third picture). It changes the timing when your engine isn't working hard (high vacuum) like cruising down the freeway with low throttle input and increases efficiency.

EDIT:
Actually, I'm not sure. I was thinking vacuum advance because it's on the vacuum advance, but that is the vacuum hooked up to the nipple on the end of it. I'm not sure how that works with it. Someone else may need to explain. Here's a couple of explanations I found with a quick google:

"It is possible to swap in a single port vacuum can.
The 2 ports serve different functions. The port next to the cap is a timing retard. The port on the end is a regular timing advance port usually hooked up to ported venturi vacuum and not manifold vacuum. For a Bronco that doesn't need emission control you can leave the port next to the cap not connected but you should not cap it as it will act like a timing advance restricter. Vacuum will have to over come the vacuum created by the plugged port slowing the rate of vacuum advance just off idle."

"FYI:
The rear can connection retards timing to make the exhaust hotter at idle. This makes the air injection work better for cleaning up unburned fuel in the exhaust. If the engine starts to overheat from prolonged idling, the thermal switch on the intake shuts off the retard so it runs normal timing and cools down."

 

Number one is one half of the choke stove.
There is a small tube angled out of the back of the carb’s bell. A short bit of rubber hose mates the two metal tubes.
This is the clean air-in, that goes down to the exhaust manifold, and another metal tube (surrounded by a fiber heat protective sleeve) goes up to the carburetor’s choke housing.

Number three is the “bow vent“ that goes to the charcoal canister. As opposed to the “bowl vents“ that are simply for allowing for expansion and contraction of the fuel level in the float bowl.
A rubber hose with a one-way check valve runs down to the charcoal canister on the frame.

If you don’t have a charcoal canister, cap this port.
Because it’s outside of the air filter, it will allow dirty air to enter the carburetor.

 

M

y brother’s truck shows that vent connected to vacuum advance on the distributor…???

 

Then your brother‘s truck is wrong.

It’s not hurting anything by being hooked there, except for performance on the street. Probably gas mileage as well.
As has been said already, that top bowl vent has zero vacuum. Zero.
It is not a vacuum fitting. It is a vent fitting for emissions to reduce the amount of fuel vapors escaping into the atmosphere through the air filter.

For your vacuum advance to work, you need to connect it to vacuum. Vacuum is found either at the manifold, the carb spacer, or the base of the carburetor.
These are typically the only spots for vacuum. However, in our vehicles, the vacuum advance mechanism probably went through at least one, if not more thermal vacuum switches before it reached the distributor.
I saw in one picture recently (that was maybe one of yours?) a vacuum advance distributor with a very short hose on the front of it. It was short because it only had to reach over to the radiator hose outlet area, where there was a thermal vacuum switch that turned on and off as the temperature rose and fell.

Please stop trying to connect it to the “easy” fitting on top, when it doesn’t belong there.
Especially since it’s actually not that easy because it’s the wrong size. That’s almost a dead giveaway right there.
Since the factory typically would try to use the same size fitting at both ends of a single size hose, you should be looking for a same-size fitting near the bottom of your carburetor.
For example, a 3/16 fitting on your distributor took a 3/16 diameter hose, to a 3/16 fitting somewhere else.
The emission bowl vent on top of the carburetor is not it!

 

“This is the clean air-in, that goes down to the exhaust manifold,”

….and it connects with rubber hose to where?

 

where does vacuum advance go then?

I’ve asked this question about 12 different ways and can never get a straight answer from anyone on here. Is it such a mystery? 2150 carb. Pretty common.

 

As previously described, to the top of the carburetor just underneath the air filter flange.
Do you have a picture of the backside of your carburetor from that side?
If your carburetor has one, and most of them did at one time, then it should be pretty obvious. It would be about two or 3 inches away from the end of that tube.

 

Actually, you’ve gotten at least three straight answers, one in excruciating detail, in your other threads. If I were you, I would stop creating new threads, and simply ask for clarification of a point in an existing one.
Although, I think you’ve actually done that and still didn’t understand the answers.

I think the issue is perhaps you’re not understanding the wording and terminology of all the different bits and pieces.
Because each one of your questions was in fact, answered. At least, as far as I could tell.
You kept asking if the vacuum advance should be connected to that vent on the top of the carburetor, when everyone told you it should not.
So even if you didn’t quite catch where it should go, you should’ve caught the fact that it should not go there.

 

A vacuum line should run from the fitting with the red arrows to the fitting on the end of the vacuum advance cannister.

 

Yes, that’s it. Thanks so much!! I just drove it for the first time in three months since the fire. It runs better than ever. Thanks to all who helped!!