Evaporative Emissions System: Description and Operation

Fig. 19 Evaporative emission control system.:

This system, Fig. 19, prevents the escape of gasoline vapors from fuel tank into atmosphere. A two-way valve regulates pressure or vacuum caused by changes in temperature and/or fuel level inside tank. Fuel vapor is allowed to escape as pressure increases and vacuum is relieved when temperature decreases or fuel is pumped out to carburetor.
When the engine coolant temperature is above the set temperature of thermovalve B, the valve closes and the purge control diaphragm valve in the canister is opened by intake manifold vacuum. fuel vapor is then purged from canister, through the purge control diaphragm valve, by venturi vacuum.
When engine is off, the main and slow primary fuel metering system fuel passages are cut-off by solenoid valves so that fuel in the float chamber cannot enter carburetor bore.
The air vent cut-off solenoid valve regulates air flow to the carburetor float bowls. When engine is off, the outer air vent passage opens to allow fuel vapors to be vented from the float bowls to the charcoal canister. When engine is running, the inner air vent passage is opened by the air vent cut-off solenoid valve to allow fuel vapors to be vented from the float bowls to the air cleaner. When engine is started with coolant temperature above 158 deg. F, the opening of the inner air vent passage is momentarily delayed to improve engine startability.
The inner vent solenoid valve normally opens the sub inner air vent passage when engine is running, but will only allow fuel vapors to be vented to the air cleaner.
A two-way valve in the fuel filler cap acts as a safety device if this system malfunctions.