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SPARK CONTROL VALVE
All manifold vacuum to the distributor
passes
through the spark control valve.
Under normal
road load conditions, the spark valve
is held open
against the pressure of a pre-set
factory calibrated
spring by a combination of atmospheric
pressure
outside of the diaphragm and manifold
vacuum
from within. When accelerating,
manifold vacuum
momentarily drops below a predetermined
point
and the calibrated spring closes
the spark valve,
shutting off the largest of two
passages through
the spark valve. The passage left
open is a cali
brated by-pass which permits a small
amount of
manifold vacuum to reach the distributor
vacuum
line. Closing the valve prevents
excessive spark
advance, while the manifold vacuum
passing
through
the spark valve by-pass prevents
full spark retard.
As engine speed approaches the throttle
setting,
manifold vacuum increases sufficiently
to open
the spark valve and allow a higher
vacuum to
operate the distributor. This carburetor
also
employs a distributor passage ball
in the manifold
vacuum passage to prevent weakening
of the
vacuum
to the distributor whenever venturi
vacuum be
comes stronger than manifold vacuum.
By use of
advance or the sluggishness of wide-open
throttle
spark retard.
The spark valve functions in a similar
manner
to provide an intermediate spark
retard whenever
the load on the engine is increased
to a degree
where the normal road load spark
advance would
be too great and the wide-open throttle
spark
retard would reduce the efficiency
of the engine.
10. GOVERNOR
The model 1901-FFG truck carburetors
in
corporate the Holley Centri-Vac
vacuum operated
engine speed governor. The governor
consists of
two units, a throttle actuating
mechanism which
is attached to the throttle shaft
of the carburetor
and a controlling unit which is
an integral part of
the Holley pressure distributor
and is driven by
the distributor drive mechanism
of the engine.
The throttle actuating mechanism
consists of a
governor spring and a diaphragm
which is exposed
to carburetor vacuum. The spring
and diaphragm
are linked together by the governor
lever which is
attached to the throttle shaft
of the carburetor.
The controlling unit consists of
a rotor which is
driven by the distributor drive
mechanism and
contains a centrifugally operated
governor valve.
