Evaporative Emissions System: Operation

As level of fuel in tank rises, vapors are discharged to atmosphere through fill control tube and a space between inner fill tube and outer neck. When fuel covers fill control tube, vapors can no longer escape and fill tube begins to fill up. When this occurs, a vapor lock is formed by the 0.040" restrictor and neither vapor nor fuel can flow through the vapor line to the canister. When thermal expansion occurs in the fuel tank, vapor is forced through the restrictor and allows vapor to pass to the canister. Vapors are stored here when engine is not running, and drawn into the carburetor intake system when the engine is started.