Ford Full-Size Cars 1968-1988 Repair Guide

Spark Delay Valve


See Figure 1

The spark delay valve is a plastic, spring-loaded, color coded valve which is installed in the vacuum line to the distributor advance diaphragm on many 1971 and later models. Under heavy throttle applications, the valve will close, blocking normal carburetor vacuum to the distributor. After the designated period of closed time, the valve opens, restoring the carburetor vacuum to the distributor.

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Fig. Fig. 1: Spark delay valve


If the distributor vacuum line contains a cut-off solenoid, it must be open during this test.

  1. Detach the vacuum line from the distributor at the spark delay valve end. Connect a vacuum gauge to the line, in its place.
  3. Block the wheels so the vehicle can not move.
  5. Connect a tachometer to the engine. Start the engine and rapidly increase its speed to 2000 rpm with the transmission in NEUTRAL or PARK.
  7. As soon as the engine speed is increased, the vacuum gauge reading should drop to zero.
  9. Hold the engine speed at a steady 2000 rpm. It should take longer than two seconds for the gauge to register 6 in. Hg. (20 kPa). If it takes less than two seconds, the valve is defective and must be replaced.
  11. If it takes longer than the number of seconds specified for the gauge to reach 6 in. Hg. (20 kPa), disconnect the vacuum gauge from the spark delay valve. Disconnect the hose which runs from the spark delay valve to the carburetor at the valve end. Connect the vacuum gauge to this hose.
  13. Start the engine and increase its speed to 2000 rpm. The gauge should indicate 10-16 in. Hg. (33-53 kPa). If it does not, there is a blockage in the carburetor vacuum port or else the hose itself is plugged or broken. If the gauge reading is within specification, the valve is defective.
  15. Reconnect all vacuum lines and remove the tachometer, once testing is completed.


  1. Locate the spark delay valve in the distributor vacuum line and disconnect it from the line.
  3. Install a new spark delay valve in the line, making sure that the black end of the valve is connected to the line from the carburetor and the color coded end is connected to the line from the spark delay valve to the distributor.