Fuel Evaporation System: Operation
When engine is not running, fuel vapor from tank vents through check and relief valve, or depending on model, into charcoal canister. On carbureted models, the vent switching valve opens to allow carburetor vapor to vent to charcoal canister. Purge control valve is closed when engine is not running.
When engine is running, vent switching valve closes by solenoid action (if equipped). Any vacuum in fuel tank exceeding .2-.6 in. Hg will open relief side of check and relief valve to atmosphere. Purge control valve opens on vacuum signal from distributor advance mechanism and allows canister vapor to be drawn into intake manifold.