Craftsman 20930 Operator's Manual

Contents

OPERATIONSpark Delay Valves (SDV) are used to delay vacuumto the distributor vacuum advance actuator duringhard acceleration, to delay the action of theThermactor Air Induction Reaction (AIR) systemduring prolonged engine idling, and to delay theapplication of vacuum to the automatic chokepulldown diaphragm during cold engine operation.A sintered metal valve is installed in the vacuumadvance (outer) diaphragm of the distributor controlunit on some engines. The purpose of the valve is todelay the spark advance during rapid accelerationto minimize the formation of NOx. The sinteredmetal is porous and allows vacuum to bleed throughthe valve acting like an orifice of about 0.002" indiameter. Control is obtained by varying the numberof discs in each valve assembly so that the timedelay features can be tailored to the engine(FIGURE 11).SERVICE PROCEDURESThe time delay of the valve varies with engineapplication. The different valves may be identified bycolor and part number. Spark delay valves cannot berepaired and must be replaced every 12,000 milesbecause 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 beinstalled with the Black side facing the carburetorvacuum port.To determine if a spark delay valve is operatingcorrectly, the following service procedure shouldbe used:1) With the transmission in neutral, set the carburetorto the fast-idle position, remove the spark-delay valveand tee your vacuum pump into the hose leading tothe carburetor spark port.2) Record the vacuum reading, which should bebetween 10 to 16" Hg.3) Pinch off the vacuum hose and observe if thegauge maintains the vacuum level. If the gaugeshows that the vacuum drops with the hose pinchedoff, the gauge or vacuum hose has an externalleak, which must be corrected.4) Now, connect the black side of the spark-delayvalve to the vacuum hose leading to the carburetorspark port. Connect a section of vacuum hose toyour vacuum pump and attach the other end to thedistributor end of the spark delay valve. Observethe time in seconds for the gauge to reach 6" Hg,with a 10 to 16" Hg vacuum source. If the vacuumreaches 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 toprevent oil or dirt from getting into the valve asthis will impair its function.13SPARK DELAY VALVES (SDV)FIGURE 10: TEST THE VACUUM SOURCE FORTHE BPV WITH YOUR PUMPAttach the Pump HereTransducerExhaust Gas InletERGValveFIGURE 11: TYPICAL SPARK DELAY VALVESparkDelayValve