The emulsion tube is a clever and important part of the flatslide carburettors as it holds a charge of fuel for a quicker throttle response. It screws into the carburettor body under the pilot/idle jet.  This should be the same or bigger than your idle jet.
Suitable for any Lambretta or Vespa fitted with a VHSH or VHSB flatslide carburettor.
 
This is a CD1 emulsion tube, which still holds a charge of fuel but is a 'straight through' tube (it doesn't have a size or any numbers)