This is part of the carburator right?

 

Incorrect. The shutter valve is mounted in the rear primary runner of the intake. Like that quote says, it's controlled by vacuum, although like the others, I can't remember where from off the top of my head (it's from one of the solenoids on the rack, but I don't recall which - the rack also differs from year to year). Essentially, the valve closes when you let off the gas as an extra means to stop air/fuel from going to the rear rotor, preventing backfiring (afterburning). These can get stuck sometimes in any position and it's a good idea to check it if you're running a stock FB set up and having issues. SA's have a different system built into the Air Control Valve (ACV) that accomplishes the task instead.

Also, this is a rather old thread, so, just a heads up.

 

A little correction on the operation of shutter valve. It is controlled by ecu/tps/solenoid rack. The shutter valve is normally closed. The tps tells the ecu when the throttle angle is decreased either full deceleration to return to idle or while letting off to shift.
The ecu monitors engine speed and receives a closed throttle signal from tps,ecu energizes solenoid on rack to supply vacuum to shutter valve which opens to allow clean filtered air from air cleaner to flow into rear rotor to make mixture there so lean it cannot backfire on closed throttle decel. When rpm is just above idle speed,ecu turns solenoid off,cutting vacuum to shutter valve which restores balance of air/fuel to rear rotor so engine does not stall when engine speed comes back to idle.

 

Gotta love it when I'm wrong , gives me a chance to learn even more. I had always figured the operation I detailed was how it worked, but since I've got an SA (albeit with an FB motor, modded intake, carb, yada yada...) my theory was really just that; a theory. Nice to know how it actually works.