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 TM 9-8000
tapped into the carburetor throat just above the throttle
7-11. Fuel Evaporation Control System (Fig. 7-12).
valve.
a. Purpose. Gasoline evaporation is a major
(3) The bottom of the charcoal canister is open.
source of hydrocarbon emissions in automotive
The opening is covered by a fiberglass filter.
equipment.
Gasoline, because it is very volatile,
evaporates In the fuel tank and the carburetor float bowl.
(4) Fuel vapors that are stored in the canister
The vaporized fuel must be vented. On uncontrolled
are purged as the throttle is open after the engine is
vehicles, fuel vapor is vented to the atmosphere through
started. The vacuum at the purge line causes fresh air to
the gas cap and the idle vent (para 4-14). The discharge
enter through the bottom of the canister. The filter keeps
of these vapors to the atmosphere no longer is
foreign matter from entering the canister. As the air
acceptable on automotive vehicles. As a result, all
passes through the charcoal it removes the stored fuel
currently produced vehicles are equipped with a fuel
vapors. The vapors then are drawn into the carburetor
evaporation control system.
through the purge line, where they are reburned.
b.
Construction and Operation.
The fuel
(5) The fuel cap is a pressure-vacuum type.
evaporation control system Is integrated to control fuel
Air can be drawn into the tank to occupy the space left by
tank and carburetor evaporation together.
the gasoline as it is burned. However, it will not allow any
fumes to pass out of the tank through it.
(1) A charcoal-filled canister Is used to store
gasoline vapors. The fuel tank and the idle vent are
(6) A valve is installed in series in the vent line
connected to the canister. Any evaporation that occurs
to the fuel tank. It will prevent gasoline from passing
during periods of vehicle shutdown is collected in the
from the tank to the canister in the event of overfill or
canister and stored in the charcoal.
vehicle rollover.
(2) A purge line connects the canister to the
carburetor through a vacuum port. The port is
7-13
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