This solenoid is turned on by
1 cooling fan
2 Headlamps
The vacuum pipe leads to a diaphragm that pulls the throttle lever slightly to increase rpm - The little pipe thats leading to the back of the carby is leading to that diaphragm. That diaphragm also has a little adjustment screw to finely adjust the rpms on load
The A/C idle up solenoid is a separate diaphragm that pulls the throttle lever to compensate for A/C load.
Some A/C mechanics just wire this solenoid to the A/C switch directly and use it for A/C idle up.. Usually done in after market A/C installation or non working A/C idle up solenoid.
The usual problem is that your radiator fan is running direct which means that this solenoid has to live in the "on" state increasing revs, mechanic ustadjee after drinking some bad chai reduces rpm by the main throttle stop to compensate for the what he thinks is excess rpms..