Page "Carburetor" Paragraph 22

The main disadvantage of basing a carburetor's operation on Bernoulli's principle is that, being a fluid dynamic device, the pressure reduction in a venturi tends to be proportional to the square of the intake air speed.

The fuel jets are much smaller and limited mainly by viscosity, so that the fuel flow tends to be proportional to the pressure difference.

So jets sized for full power tend to starve the engine at lower speed and part throttle.

Most commonly this has been corrected by using multiple jets.

In SU and other movable jet carburetors, it was corrected by varying the jet size.

For cold starting, a different principle was used in multi-jet carburetors.

A flow resisting valve called a choke, similar to the throttle valve, was placed upstream of the main jet to reduce the intake pressure and suck additional fuel out of the jets.