We have done a lot of research and development on the working of Autovacs in response to the impact of modern fuel and now make a number of changes to the mechanism to enable it to operate correctly.
One of the common faults is that fuel is drawn along the suction line into the manifold, possibly causing erratic running. Petrol nowadays has a lower specific gravity and as it flows into the inner chamber of the Autovac it swirls as it hits the rising float and can be sucked out through the suction valve.
We now fit a longer inner tank and a float with a longer spindle to allow more space between the top of the float and the valves.
On a standard service we also change the spring, gaskets, filter and blowback valve (in the bottom of the suction elbow). We have now modified this valve with a smaller central hole (the two outer ones are blind to facilitate fitting), which reduces the severity of the "suck".
We may also need to change the air and suction valves, as the settings are critical to allow the mechanism to shut off the suction at the correct time. We measure this during testing when we observe and measure the flow of fuel into and out of the Autovac, while varying the vacuum.
With these modifications we have found that Autovacs are reliable and cycle more quickly with modern fuel.