Evaporative Emissions System: Description and Operation

Fig. 37 Evaporation control system:

This system, Fig. 37, prevents the emissions of gasoline vapors from the carburetor (if equipped) and fuel tank into the atmosphere.

When fuel evaporates in the fuel tank or carburetor float chamber (if equipped), the vapors pass through vent hoses or tubes to a charcoal canister where they are held until they are drawn into the intake manifold when the engine is running. Vapor flow to the engine is regulated by a vacuum port located in the base of the carburetor or throttle body. The fuel tank is vented through a vapor vent tube assembly. A tube from the vent tube assembly or the ESC dome on top of the fuel tank leads to the charcoal canister. Fuel vapor from the fuel tank flows through this tube to the canister

Fuel vapors are purged from the canister through the use of a ported vacuum connection on the carburetor which uses the throttle valve of the carburetor as a purge valve.

Some 1973-74 vehicles incorporate a canister with an integral purge valve. This canister uses four connecting hoses.

Fig.27 Dual Evaporation Control System Secondary Vapor Canister:

Some vehicles use a dual canister evaporation control system, Fig. 27. When fuel evaporates in the carburetor float chamber, the vapors pass through vent hoses or tubes to the primary canister where they are held until they are drawn into the intake manifold when the engine is running. Vapor flow to the engine is regulated by a vacuum port in the base of the carburetor.

The fuel tank is vented through a vapor vent tube assembly. A tube from the vent tube assembly leads to the secondary canister where fuel vapors from the fuel tank are held until they are drawn into the intake manifold when the engine is running. Fuel vapors are purged from the primary canister through the use of a vacuum connection on the carburetor which uses the throttle valve of the carburetor as a purge valve. Fuel vapors from the secondary canister are purged through the PCV hose to the carburetor using a vacuum signal from the distributor vacuum hose to open the canister purge switch.