It is a damper for the piston in the carb, you should pour oil in, till
it is covering the hollow rod you see inside. There is some debate as to
the correct grade, but standard engine oil will do the job. The purpose
of the oil is to damp the rise of the piston (the pison is lifted by the
air through the carb, on the bottom of the piston is a tappered needle
that controls the mixture at different air requirements)

hth
Drew.