It is your decision whether to try to reconnect everything (if your car still has the charcoal cannister - which my be tucked way up under the fender on the radiator side of the car), or whether to remove the remaining components of an incomplete system that is interfering with smooth operation of the engine.

MPlayle, I think I might know now what used to be connected to the broken port on my thermostatic switch, in one of the pics there is a white-ish tube below the electronic vac device I traced back to the gas tank in the boot. I found it disconnected there too (I assume it should be put in the breather tube), and based on another forum, it seems like that should have been connected to the charcoal canister in a way to reburn vapor fumes. I wonder if it was directly connected to the switch (as-is it won't reach it though). Once I get an intact part I am thinking to connect it to complete that system.  Good or bad idea?

Since the charcoal canister has already been "removed" from your car, I would suggest disabling/removing the reset of that system and cap off that one connection at the carb(s).  You can leave the thermo switch in place as a "plug".

Speaking very quietly, it's probably not a 1985 car...

I did some research on this after getting more pieces of the puzzle and found this forum that explains some of this as well.

https://www.theminiforum.co.uk/forums/topic/275373-charcoal-canister-connecting/

You drag or drop pictures directly by using the green button to the left and above the space where you are typing in your message.

Ok, thanks for the diagram, but it looks like the mystery deepens because that exact canister is nowhere to be found in my car. I experimented a little and bypassed the switch and the electronic gizmo on firewall and ran the vacuum line from the manifold to the actuator disk on the carb, and it seemed to like that, but it did idle about 200-300 rpms higher than I would like, but it was like an open vac but without the sucking sound I get with the switch when it clicks to open.

 

I've concluded then I have a hacked setup based on the comments about the dual carb and all. I'm fairly new to the ins-and-outs of classic minis.

 

Any recommendations and advice for my situation would be welcomed for consideration. I don't know if I want to start adding emissions parts to this system that would complicate the simplicity, or the fact it's currently not required in my state to pass any emissions tests (it's licensed as a collector vehicle, and exempt).

The switch in question is #27 in the diagram at the link below.  Your missing line runs to a valve on the top of a charcoal canister as part of the emissions/tank vapor recovery system.

https://www.minispares.com/catalogues/classic/Classic~Mechanical~Parts~Manual/Engine/Engine~Breathers~.aspx?2~12~117

The diagram has a boxed off section in the lower right that shows the thermo switch and the line from it to the charcoal cannister.

I was able to see your images OK.

It appears your distributor vacuum advance is connected directly to the carb port separately, which seems OK to me (throwing my wild guess out!).

Dual carbs definitely seem non-standard! The other collection of vacuum lines draw from the intake manifold port and eventually connect to the round gizmo beside the carb, which appears to be a positive crankcase ventilation valve. No idea what the actuator on the firewall does - maybe tracing the wire colours in a wiring diagram might tell you what it does. Maybe it is an electrically controlled vacuum valve managed by the ignition switch??

I am wondering if in the original configuration the system used the open port to control the distributor vacuum advance and when the twin carbs were added, the carb port was used for the dizzy.

Thank you for the replies. I do suspect some previous hacking and/or transplant and suspect that the open switch port was a hack to get it to run better since the open vacuum makes it idle fine.  I am wondering if it is even needed, but plugging it causes very rough idling. I just don't like the idea of something switching from time to time and also sucking in straight air. I attached some pictures and there is a Tee in the vacuum lines to the switch from the manifold that ultimately go to the carb actuator.

 

As for the vacuum from the carb to the distributor, that is there and a totally different dedicated line.

 

Mike: I'm not sure about all Mayfairs, but mine of a similar age was built with a 998cc engine and later received a 1275 Metro transplant somewhat bodged in. It does not have that valve/switch or even (i suspect) the correct thermostat cover!. Mine is also a carbed car. It came with a worn out HS4 on a 998 siamesed cast iron manifold.

My other guess is that flopek's car has received a transplant.

Would an 85 Mayfair have a 1275cc carbed engine?

I was wondering what this switch was, as I have an 87 Mini City-E (not far from your 85 Mayfair) so something is "off".

From the link you quoted:

"...Fitted to 1275cc carb type engines from 1992 to 1994. Has 2 vacuum pipes conected to it. Screws into thermostat housing under top radiator hose. Use with special washer MAW10001...."

This is a wild guess:

Assuming by your post that one leg is connected to the carb or intake manifold, my guess is that the other leg would be connected to the vacuum advance of a distributor. My theory is that when the engine is cold, the valve is closed, preventing vacuum advance from occurring to allow easier starting. Once it opens, vacuum advance is permitted but since the system is closed, there is no air leak.

If this is the case, It should run fine once connected.
 
If your engine does not have a distributor with vacuum advance, you could plug the line going to the carb. Vacuum advance generally works to provide earlier spark at idle or part throttle to allow leaner carb settings during those events, to improve fuel economy in road cars. As soon as you open the throttle, vacuum reduces and the vac advance goes way to be replaced with mechanical advance inside the distributor.

Depending on your engine and distributor, the specs may require you to disconnect the vacuum advance when tuning the engine.  IF so, you would have to disconnect one of the lines and plug it for the work.

I bought a 1985 Mini Mayfair a few years ago, so I am not the original owner ??, and I found a vacuum leak of sorts.  There is a thermostatic vacuum switch installed on my Mini and one of the legs is broken. When the switch closes, it idles very rough and wants to stall, but when open, it idles fine.  I need to buy a new switch since one of the legs is broken, but I have no idea where the other line connects to (it was never installed when I bought it.)  Anyone have any idea where the second leg of the vacuum connects to?  I cannot seem to find any information about this.

Thank you,
Ian