A question about an external fuel filter

[beginner49]

.
My old car has a plastic fuel filter located between the fuel pump and the carburator fuel inlet.

Whatever the engine RPMs are, this filter is only about 40% filled, it is never replete.

This happens whatever the filter position is, vertical, horizontal, tilted...

Currently the filter is positioned vertically, input from pump is downward, output to carb is upward.

I do not observe air bubbles in the liquid when it is flowing.

All people I know, who use this kind of filters, do observe the same; the filters are never fully filled, but half filled.

The filtering element of these transparent plastic filter is paper. I guess most of you know the type of filter I am talking about.

Is there any physical explanation to this partially charged filter when the fuel is flowing throu. Even when no liquid is observe reaching the filter out tip.

Thank you very much in advance.

 

[Valerie Park]

The phenomenon you are observing is known as cavitation. Cavitation occurs when the pressure of the liquid drops below its vapor pressure. This causes bubbles to form in the liquid which can impede the flow. When the pressure increases, the bubbles collapse and the flow resumes. In your fuel filter, the pressure is likely dropping below the vapor pressure of the fuel when it passes through the filter, causing cavitation and a partial filling of the filter.