Starting 1975, non-California models have a spark delay valve (SDV) in the distributor vacuum line. The valve has a small orifice in it, which slows down the vacuum flow to the vacuum advance unit on the distributor. By delaying the vacuum to the distributor, a reduction in HC and CO emissions is possible.
When the coolant temperature is below 95°F, a coolant temperature operated vacuum control valve is opened, allowing the distributor to receive undelayed, ported vacuum through a separate vacuum line. Above 95°F, this line is blocked and all ported vacuum must go through the spark delay valve.
- Allow the engine to cool, so that the coolant temperature is below 95°F.
- Disconnect the vacuum line which runs from the coolant temperature operated vacuum valve to the vacuum advance unit at the advance unit end. Connect a vacuum gauge to this line.
- Start the engine. Increase the engine speed; the gauge should indicate a vacuum.
- Allow the engine to warm-up to normal operating temperature. Increase the engine speed; this time the vacuum gauge should read zero.
- Replace the coolant temperature operated vacuum valve, if it fails either of these tests. Disconnect the vacuum gauge and reconnect the vacuum lines.
- Remove the spark delay valve from the vacuum lines, noting which side faces the distributor.
- Connect a hand operated vacuum pump which has a built-in vacuum gauge to the carburetor side of the spark delay valve.
- Connect a vacuum gauge to the distributor side of the valve.
- Operate the hand pump to create a vacuum. The vacuum gauge on the distributor side should show a hesitation before registering.
- The gauge reading on the pump side should drop slightly, taking several seconds for it to balance with the reading on the other gauge.
- If Steps 9 and 10 are negative, replace the spark delay valve.
- Remove the vacuum gauge from the distributor side of the valve. Cover the distributor side of the valve with your finger and operate the pump to create a vacuum of 15 in.Hg.
- The reading on the pump gauge should remain steady. If the gauge reading drops, replace the valve.
- Remove your finger; the reading of the gauge should drop slowly. If the reading goes to zero rapidly, replace the valve.