Evaporative Emissions System: Description and Operation

Description
The evaporative control system consists of a charcoal canister, a vapor purge control system, a carburetor vapor control system, and a fuel tank vapor control system.
The evaporative control system minimizes the amount of fuel vapor escaping into the atmosphere.

Charcoal Canister
The charcoal canister is a container for the temporary storage of fuel vapor until it can be purged from the canister into the engine and burned.

Vapor Purge Control System
The vapor purge control system controls when the canister is to be purged. When the coolant temperature is above the value for stage 1 of thermovalve A, manifold vacuum is directed to the purge control valve, which allows venturi vacuum to draw fresh air through the bottom of the canister into the engine, purging the fuel vapor from the canister.

Carburetor Vapor Control System
The carburetor vapor control system consists of the air vent cut-off diaphragm, the vacuum holding solenoid valve, and inner and outer vents.
Fuel vapor from the float chamber is directed into the charcoal canister through the outer vent passage when the engine is not running and the thermovalve in the air vent cut-off diaphragm is above its set value. If the engine is running, fuel vapor is directed into the air cleaner.
The inner vents are always open to the air cleaner.

Fuel Tank Vapor Control System
The fuel tank vapor control system consists of a fuel cut-off valve, liquid/vapor separator, a two-way valve, and fuel filler cap. All fuel vapor inside the fuel tank is directed to the charcoal canister through the fuel cut-off valve and the liquid vapor separator to prevent build up of vapor in the tank which might otherwise escape when the fuel filler cap is removed.
The two-way valve regulates both the pressure and vacuum in the tank.
In addition there is a relief valve in the fuel filler cap in case there is an excessive build-up of pressure or vacuum.