Fuel Evaporation System: Operation
The fuel evaporation system is controlled by a bimetal vacuum switching valve (BVSV) and a vacuum control valve (VCV). At low temperature, all vacuum to the VCV is cut off and no fuel vapors are drawn into intake manifold. Once engine coolant warms up, the BVSV opens to allow vacuum to reach the VCV and draw fuel vapors from charcoal canister to intake manifold.
When the engine is stopped or at idle, a spring loaded ball in the check valve causes check valve to close, routing fuel vapors from fuel tank to the charcoal canister where they are adsorbed.
When engine speed increases, the spring loaded ball opens and allows intake manifold vacuum to draw accumulated vapors from the charcoal canister and the fuel tank into the carburetor.
The check valve also functions along with the fuel tank safety cap, to balance fuel tank pressure and prevent fuel tank collapse. The outer vent control valve is a solenoid which opens a passage from the carburetor float bowl to the carbon canister when the ignition is "OFF". It closes when the engine is running.