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DISTRIBUTOR VACUUM PASSAGES
B. DISTRIBUTOR VACUUM PASSAGES
The spark advance mechanism of the
Holley
pressure type distributor is controlled
by a vacu
um operated diaphragm operating against
the
tension of calibrated springs. The
degree of
spark advance is primarily regulated
by the
strength of the vacuum acting on
the diaphragm.
The vacuum is transmitted from two
interconnected
passages within the carburetor through
tubing to
the distributor diaphragm chamber.
One of the
passages is open at the venturi
throat of the car
buretor and the other opens into
the throttle bore.
The two passages are calibrated
to supply vacuum
of the proper strength for the required
spark ad
vance during operation under all
engine speed and
load conditions. Any time the vacuum
from the
venturi becomes strong
e
r than the vacuum from
the throttle bore, the distributor
passage ball in
the vertical passage from the throttle
bore will be
drawn up to its seat, closing off
the lower passage.
due to air bleeding from the lower
passage. Care
should always be exercised when
performing any
service operations on a carburetor
to prevent
damage or distortion of the distributor
vacuum
passages.
9. SPARK CONTROL VALVE
The spark control valve provides
an efficient
degree of spark advance during periods
of accel
eration by momentarily providing
a partially re
tarded spark. This partially retarded
spark mini
mizes detonation (or pinging) and
increases engine
responsiveness while accelerating.
In effect, the
spark valve provides an intermediate
spark advance
curve between a fully retarded spark
and the nor
mal spark advance which would be
excessive when
the engine is suddenly accelerated.
The spark
valve accomplishes this by controlling
the amount
of manifold vacuum admitted to the
distributor
vacuum line.
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