OPERATION
Spark Delay Valves (SDV) are used to delay vacuum
to the distributor vacuum advance actuator during
hard acceleration, to delay the action of the
Thermactor Air Induction Reaction (AIR) system
during prolonged engine idling, and to delay the
application of vacuum to the automatic choke
pulldown diaphragm during cold engine operation.
A sintered metal valve is installed in the vacuum
advance (outer) diaphragm of the distributor control
unit on some engines. The purpose of the valve is to
delay the spark advance during rapid acceleration
to minimize the formation of NOx. The sintered
metal is porous and allows vacuum to bleed through
the valve acting like an orifice of about 0.002" in
diameter. Control is obtained by varying the number
of discs in each valve assembly so that the time
delay features can be tailored to the engine
(FIGURE 11).
SERVICE PROCEDURES
The time delay of the valve varies with engine
application. The different valves may be identified by
color and part number. Spark delay valves cannot be
repaired and must be replaced every 12,000 miles
because the pores of the sintered metal fill with dust,
which can slow the performance of the valve. NOTE:
The spark delay valve is a one-way unit that must be
installed with the Black side facing the carburetor
vacuum port.
To determine if a spark delay valve is operating
correctly, the following service procedure should
be used:
1) With the transmission in neutral, set the carburetor
to the fast-idle position, remove the spark-delay valve
and tee your vacuum pump into the hose leading to
the carburetor spark port.
2) Record the vacuum reading, which should be
between 10 to 16" Hg.
3) Pinch off the vacuum hose and observe if the
gauge maintains the vacuum level. If the gauge
shows that the vacuum drops with the hose pinched
off, the gauge or vacuum hose has an external
leak, which must be corrected.
4) Now, connect the black side of the spark-delay
valve to the vacuum hose leading to the carburetor
spark port. Connect a section of vacuum hose to
your vacuum pump and attach the other end to the
distributor end of the spark delay valve. Observe
the time in seconds for the gauge to reach 6" Hg,
with a 10 to 16" Hg vacuum source. If the vacuum
reaches the 6" Hg level in less than two seconds,
regardless of type, the SDV should be replaced.
When checking the valve, care must be taken to
prevent oil or dirt from getting into the valve as
this will impair its function.
13
SPARK DELAY VALVES (SDV)
FIGURE 10: TEST THE VACUUM SOURCE FOR
THE BPV WITH YOUR PUMP
Attach the Pump Here
Transducer
Exhaust Gas Inlet
ERG
Valve
FIGURE 11: TYPICAL SPARK DELAY VALVE
Spark
Delay
Valve
