This photo is of the engine fitted to a 1949 James Trials bike but it was fitted to a great many other makes in Britain and many other countries. 

It is shown here with a Villiers two lever carburettor.  One 'lever' (the twist grip) controls the main throttle slide.  The second lever raises or lowers the throttle needle to richen or weaken the mixture as a starting aid.  The air cleaner does not have a choke shutter.