What we're actually crapping on about here are hitachi sidedraughts which are a copy of the pommie SU's. In addition to a butterfly valve, they have a piston in the venturi that moves up and down to vary the size of the venturi. The piston is contolled by the amount of vacum (I think), so it closes up the throat at low throttle openings, giving better economy and drivability at low revs. Have a look in the tech section- there are also a few dedicated SU sites out there.