A spark delay system is used on 1977–78 1,796cc engines, and 1979 and later 1,970cc engines. Its purpose is to reduce the formation of CO and NO$SUB2 emissions by delaying the vacuum advance to the distributor during normal acceleration. The system consists of a spark delay valve installed in the vacuum hose between the carburetor and the distributor vacuum advance diaphragm, on all 1977–78 models, as well as 1979 and later 49 States models. The spark delay valve is installed in the vacuum line between the water thermo valve and the distributor diaphragm on 1979 and later models sold in California. The water thermo valve is installed in the intake manifold. The spark delay valve has an internal restrictor to slow the air flow in one direction and a check valve which allows air to flow freely in the opposite direction.