How it works
The Autovac employs an auxiliary tank, mounted above the carburettor on the bulkhead. This is divided into two chambers; the inner connected to the induction manifold and the main fuel tank and the outer to the carburettor.
There is a drop valve in the base of the inner chamber and as engine suction creates a partial vacuum in the inner chamber the drop valve is held shut. Fuel then flows in via the petrol elbow in the top causing the float to rise when, at a predetermined level, the air and suction valves are operated.
The suction valve shuts and the air valve opens destroying the vacuum and allowing the fuel to fall past the drop valve into the outer chamber and thence to the carburettor by gravity. As the float falls with the outflow of fuel from the inner chamber the valve mechanism is again actuated and the process repeated.
As the outer chamber is always open to the atmosphere through the air vent fuel is always free to supply the carburettor.
The Autovac uses the "patent self-draining float". A hollow stem is used, being drilled inside at the bottom and outside above the body of the float.
Any fuel entering the float is automatically evacuated through the stem during the suction period, thereby enabling the float to function even if punctured; with the caveat, "within reason", it will not keep a split float functioning.