The plastic "box" near front of LP regulator is a "baffle chamber" (aka resonance defeat chamber). Its purpose is to provide a clean, dust free pathway for the regulator secondary diaphragm to be able to move freely when it needs to operate. The hose runs to the air cleaner so any incoming air will not be dusty. The baffling effect is so the diaphragm is not likely to react to induction vacuum pulses originating at the air pipe the hose connects to.

The valve on the side of the mixer/throttle body is not an air valve. It is a fuel valve. Toyota/Aisan call it the fuel control motor.

I have found 2 main causes of hunting/surging at idle, AND a flat spot or hesitation when you press the accelerator on the 8F trucks.

The tamper resistant cap covering the idle fuel adjusting screw on top of the regulator needs to be removed so the idle fuel screw can be adjusted slightly. Often you only need to turn the screw CCW about 1/2 to 3/4 turn and the surging will go away.

The second thing that can have a profound effect on idle and take off is a gummy mixer/throttle body.
We remove the mixer/TB and clean out the main throat plenum, throttle plate, and the fuel control motor tip and passages.
I recommend that you orient the mixer so that the throttle plate operating shaft coming out of the throttle motor is always "up hill" while you are wetting and scrubbing the carbon deposits out with a toothbrush and spray carb cleaner. Keeping the throttle shaft up hill will keep carb cleaner from leaking past the throttle shaft bushing and into the TS motor case.