Ford Mustang/Cougar 1964-1973 Repair Guide

Deceleration Valve



Beginning in the 1969 model year, some engines were equipped with a distributor vacuum advance control valve (deceleration valve) which is used with dual-diaphragm distributors to further aid in controlling ignition timing. The deceleration valve is in the vacuum line which runs from the outer (or advance) diaphragm to the carburetor-the normal source of vacuum supply for the distributor vacuum advance. During periods of deceleration, the intake manifold vacuum pressure rises causing the deceleration valve to close the carburetor vacuum source and open the intake manifold vacuum source to the distributor advance diaphragm. The increase in vacuum pressure provides maximum ignition timing advance, resulting in more complete fuel combustion within the engine, and decreasing exhaust system backfiring.