Evaporative Emissions System: Description and Operation

Fig. 14b Evaporative emission control system.:

This system, Fig. 14b, 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.
A liquid/vapor separator permits liquid fuel to return to tank through a return line. The vapor flows through a vent line to the charcoal canister, where it is absorbed by activated charcoal. When engine is off, the air vent passages in the main and auxiliary float chambers are shut off by the air vent cut-off diaphragm, causing vapors in the float chambers to be vented to the charcoal canister. When engine is running, manifold vacuum holds the cut-off diaphragm open.
The vacuum holding solenoid valve stabilizes manifold vacuum at the diaphragm. When engine coolant temperature is above the set temperature of thermovalve, the valve closes and the purge control diaphragm valve in the canister is opened by intake manifold vacuum. Fuel vapor is then purged from the 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 the carburetor bore. When ignition switch is off, the power valve control solenoid is off, the power valve control solenoid valve is deactivated, trapping vacuum in hose between power valve and check valve. The power valve is now closed, shutting off the fuel passage.
A two-way valve in the fuel filler cap acts as a safety device is this system malfunctions.