Hover over a number for the key, click on the menu items below to show the vapour/vacuum routings.
Fuel Flow Tank Expansion/Contraction Carb Expansion Crankcase Breathing
Image from the British Leyland Workshop Manual
Ignoring the external expansion tank which was not fitted to the MGB.
Any expansion and contraction of fuel or air in the tank while the engine is switched off moves through the separator tank (7), vapour lines (3, 5), the charcoal adsorption cylinder (8) and the vent pipe (11) to/from open air. Petrol fumes from the vapours are adsorbed by the charcoal and not discharged to the atmosphere. If the engine is running any fumes in the canister are drawn into the engine and burnt, and the charcoal granules are purged of any vapours previously adsorbed, see Crankcase Breathing.