Some 1968-72 engines with dual diaphram distributors are equipped with a distributor vacuum advance control valve (deceleration valve) which is used to further aid in controlling ignition timing. The deceleration valve is installed in the vacuum line which runs from the outer (advance) diaphragm to the carburetor; the normal vacuum supply for the distributor. During deceleration, the intake manifold vacuum rises causing the deceleration valve to close off any carburetor vacuum source, and connecting the intake manifold vacuum to the distributor advance diaphragm. The resulting increase in vacuum pressure provides maximum ignition timing advance, thus providing more complete fuel combustion, and decreasing exhaust system backfiring.
- Connect a tachometer to the engine according to the manufacture's instructions. Bring the engine to the normal operating temperature.
- Check the idle speed and set it to specifications with the headlights on high beam, as necessary.
- Turn OFF the headlights and note the idle rpm.
- Remove the plastic cover from the valve. Slowly turn the adjusting screw counterclockwise without pressing in. After 5 turns, no more than 6 turns, the idle speed should suddenly increase to about 1000 rpm. If the speed does not increase after six turns, push inward on the valve spring retainer and release. Speed should now increase.
- Slowly turn the adjusting screw clockwise until the idle speed drops to the speed noted in Step 3. Make one more turn clockwise.
- Increase the engine speed to 2000 rpm, hold for 5 seconds, and release the throttle. The engine speed should return idle speed within 4 seconds. If idle is not resumed in 4 seconds, back off the dashpot adjustment and repeat the check. If the idle is not resumed in 3 seconds with the dashpot back off, turn the deceleration valve adjustment screw an addition quarter turn clockwise and repeat the check. Repeat the quarter turn adjustment and idle return checks until the engine returns to idle within the required time.
- If it takes more than one complete turn from Step 5 to meet the idle return time specifications, replace the valve.