G424 Service Manual Ignition System53
Vacuum advance mechanism
There is a diaphragm within the vacuum controller,
with a spring at the negative-pressure (vacuum) side
which pressure against the diaphragm. A link is
attached to the diaphragm and is connected to the
pickup plate. The pickup assy is installed on the
pickup plate, and can rotate together with the pickup
plate.
The negative pressure (vacuum) side of the vacuum
controller is connected to the vacuum port of the
carburetor by a tube.
When the throttle valve of the carburetor is closed, the
engine is in the idling condition, and there is no
vacuum inside the tube at this time. When the throttle
valve is partially open, the air within the tube is drawn
out by the air/fuel mixture flowing near the vacuum
port, thus resulting in a negative-pressure (vacuum)
condition. Because the tube is connected to the
negative-pressure (vacuum) port of the vacuum
controller, the diaphragm pressure against the spring,
and it moves in the direction of the arrow. Then the
link rotates the pickup plate and, because the position
of the pickup changes, the timing also changes. The
direction is the direction opposite to the direction of
signal rotor rotation, so that the projection of signal
rotor will pass the pickup quickly. In this way, then, the
ignition timing advances.
The figure at the left shows the advance characteristic curve
of the vacuum-advance mechanism. Point A is the start of
the advance, and point B is the end of the advance.
Because the magnitude of the degree of advance determined
by balance of the strength of the vacuum and the pressure of
the spring, if the spring is weak the degree of advance will be
great at a small vacuum.
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