Image from Haynes Workshop Manual

Note: On 1978 models a secondary charcoal cannister was connected between the anti-runon valve and the air vent pipe.

The instant the ignition is switched off the engine is still spinning. The anti-runon valve (9) operates closing off the air vent pipe (5). 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). The rocker cover restrictor (3) applies carb vacuum to the purge line (4) and charcoal adsorption cannister (9). Inlet manifold vacuum from the running-on control pipe (11) is also applied to the cannister. The cannister applies this vacuum to the vapour pipe (8) to the carb float chamber which sucks the fuel out of the jet thus preventing Dieselling as the engine runs down.

Check the anti-runon plumbing as follows: Start the engine and allow it to idle. Block the open end of the air vent pipe (5) and the engine should stop. Failure to stop indicates blockages or leaks in the running-on control pipe (11), running-on control hose (10), charcoal cannister (7), or vapour pipe (8).