See Figure 1
The Improved Combustion (IMCO) Emission Control system was first fitted in 1968. Unlike Thermactor, which consists of bolt-on components, the IMCO system involves both modifications to the engine itself and bolt-on equipment. As the past several years have seen increasingly rigid government-imposed emission control standards, the IMCO system has been modified in order to meet the requirements.
For the 1968 and 1969 model years, the IMCO system was used only in non-high-performance engines with automatic transmissions. Cars, with standard transmissions and all high-performance cars regardless of transmission type still used Thermactor systems. In 1970 and 1971, the IMCO system was used in all models, and certain high-performance engine options combined the Thermactor system with IMCO engine modifications. For 1972, the Thermactor system is not used.
In addition to the closed crankcase which was used beginning in 1968, the IMCO system in its original form consisted of a dual-diaphragm distributor and distributor vacuum (temperature sensing) valve, idle mixture limiter caps (on the carburetor), and a heated-air-intake system. These components reduce exhaust emissions by promoting the complete and efficient combustion of fuel within the engine rather than by burning the exhaust gases in the manifold as in the Thermactor system.
The dual-diaphragm distributor consists of two diaphragms which operate independently of each other. The outer (or primary) diaphragm uses carburetor vacuum to advance the ignition timing. The inner (or secondary) diaphragm uses intake manifold vacuum to provide additional retardation of ignition timing during periods of closed throttle deceleration and idle.
The distributor vacuum control valve, or temperature-sensing valve, is in the distributor vacuum advance supply line, and is installed in the coolant outlet elbow. During prolonged periods of idle, or any other situation which causes higher-than-normal engine operating temperatures, the valve, which under normal conditions simply connects the vacuum advance diaphragm to its vacuum source within the carburetor, closes the normal source vacuum port and engages the alternate source vacuum port. This alternate source is from the intake manifold which, under idle conditions, maintains a high vacuum. This increase in vacuum supply to the distributor diaphragm advances the timing, increasing the idle speed and speeding up the fan. When the engine has sufficiently cooled, the vacuum supply is returned to its normal source-the carburetor.
The idle mixture limiter caps found on the carburetors of IMCO-equipped engines restrict the degree of richness to which the carburetor may be set. The leanness of air-fuel mixture, combined with the advance/retard characteristics of the distributor, results in more complete combustion of fuel within the engine.
The heated air-intake system improves the efficiency of cold-weather and cold-start operation of the engine. A hot-air duct, which runs from the exhaust manifold to the air cleaner, supplies air to the carburetor which has been warmed by the exhaust manifold. The result is reduced throttle-plate icing, and better performance from the lean mixtures which are necessary to successful emission control. A temperature-sensing unit is included in this system, to decrease the supply of manifold-warmed air as the engine reaches its normal operating temperatures.