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OK. 
Apparently, whatever is in that port never gets to the space above the diaphragm.
In your first picture, it shows that the brass nipple is not connected to the vacuum side of the diaphragm. Would be nice to know where it goes. Can you spray some carb cleaner in there and see where it comes out?
In the second picture, it appears that the vertical tube comes down to where the "choke" plunger lives, but I don't think it joins there, it might come down and forward from there. Does it open into the carb throat?
In the third picture, that is a great shot down the "choke" circuit, but it appears that the plunger is pulled out, indicating the "choke" is ON.
In the fourth picture, that appears that might answer my question about the second picture. That might be the bottom end of that vertical tube. Can you verify?
The fifth picture seems to be a duplicate of the second picture, both of them seem to show that curved tube that goes from the bottom of the carb body to the fuel inlet of the "choke" circuit. The BS carbs in our GSes have that passage built in on the carb with just a short section of brass visible. There is no picture that shows it, but I will guess that there is a tube extending into the bowl below the end of that curved tube?
Now, the only truly "unknown" is the purpose of the port on the cap that appears to go unrestricted to the intake tract downstream of the throttle plate. My guess is that it might be part of some sort of emission control system. Because it feeds unregulated air directly into the engine, there must be some sort of valve system that controls it, like maybe only on deceleration (high vacuum) and maybe over a certain RPM, but that is strictly a guess.
.
Apparently, whatever is in that port never gets to the space above the diaphragm.
In your first picture, it shows that the brass nipple is not connected to the vacuum side of the diaphragm. Would be nice to know where it goes. Can you spray some carb cleaner in there and see where it comes out?
In the second picture, it appears that the vertical tube comes down to where the "choke" plunger lives, but I don't think it joins there, it might come down and forward from there. Does it open into the carb throat?
In the third picture, that is a great shot down the "choke" circuit, but it appears that the plunger is pulled out, indicating the "choke" is ON.
In the fourth picture, that appears that might answer my question about the second picture. That might be the bottom end of that vertical tube. Can you verify?
The fifth picture seems to be a duplicate of the second picture, both of them seem to show that curved tube that goes from the bottom of the carb body to the fuel inlet of the "choke" circuit. The BS carbs in our GSes have that passage built in on the carb with just a short section of brass visible. There is no picture that shows it, but I will guess that there is a tube extending into the bowl below the end of that curved tube?
Now, the only truly "unknown" is the purpose of the port on the cap that appears to go unrestricted to the intake tract downstream of the throttle plate. My guess is that it might be part of some sort of emission control system. Because it feeds unregulated air directly into the engine, there must be some sort of valve system that controls it, like maybe only on deceleration (high vacuum) and maybe over a certain RPM, but that is strictly a guess.
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