KEEWAY 50cc Series Service Manual page 25

At idle, the fuel is sprayed in a fine mist into
the carburetor throat by the idle jet. Since
the throttle body is completely closed, air
passes through the idle passage and into the
throat as well, blending with the fuel mist.
This blending process can be adjusted
through the IDLE AIR SCREW. As the
throttle body is slowly opened, a SLOW
speed condition begins.
Under light throttle, air begins to flow
through the slightly opened throttle body,
creating small portion of the vacuum needed
to trigger the main jet. Since vacuum is
minimal, the idle jet remains active, but the
main jet operates at minimum capacity. At
the point of half throttle, the throttle body is
opened approximately halfway, which
greatly increases airflow and vacuum
pressure to the main jet. This triggers a
MIDDLE speed condition.
Under half throttle, airflow through the
throttle body increases, which causes the
vacuum pressure on the main jet to increase
just enough to take pressure from the idle
jet, ceasing its fuel spray. At this point, the
main jet is the sole source of fuel spray into
the carburetor throat, but vacuum is only
sufficient for approximately half capacity.
At this point, the air/fuel mixture becomes
optimal and combustion efficiency is
highest.