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 Anti-runon
Image from Haynes Workshop Manual
Note: On 1978 models a secondary charcoal canister was connected between the anti-runon valve and the air vent pipe.
When the engine is running carb vacuum is applied via the crankcase breather pipe (2) and oil separator (1) to the crankcase and hence the rocker cover, which has a non-vented oil filler cap (6). A restrictor (3) in the rocker cover vent limits how much air can flow to avoid weakening the mixture excessively. Air is drawn through the purge line (4) via the charcoal adsorption canister (7), running-on control hose (10), anti-runon valve (9) and vent pipe (5, see Note below) from the atmosphere. As air is drawn through the charcoal canister previously adsorbed fuel fumes (see Tank Expansion and Carb Expansion) are given up to be burnt in the engine.
Note: For the vent pipe the Leyland Parts Catalogue lists BHH1026 for chassis numbers 294251 (Aug 72) to 386600 (Sep 75) and BHH1851 for 386601 on including for the 77 to 80 cars, Googling shows neither except references back to BL documents. Moss Motors show three different air-vent pipes (hoses) bringing fresh-air in to the valve during running to purge the canister of previously absorbed fumes. The 1970-74 drawing lists 367-280 for 73-74 and the 1979-80 drawing lists 367-290. But the 1975-78 drawing shows three including those two with different dates plus another - 367-280 for 1975-78, 367-285 for 1976-77 (those two date ranges overlap so maybe the first one should read 75-76) up to chassis number 415000, and 367-290 for 1977 from chassis number 415000 to 78. All are NLA so there are no pictures showing how they may differ, but the three parts drawings show a relatively short hose that looks like it goes almost straight down. However John Twist in this video indicates the hose on a 1977 going to the front of the car through the radiator diaphragm panel, and a pal with a 76 has this long hose (oil can for scale) which has never been fitted in his ownership: (Bill Etter)
Bill has fitted an oil cooler and there is no third hole in the diaphragm panel for the valve hose, so maybe this long hose is supposed to use one of the unused cooler hose holes.