Page "Carburetor" Paragraph 58

To ensure a ready mixture, the carburetor has a " float chamber " ( or " bowl ") that contains a quantity of fuel at near-atmospheric pressure, ready for use.

This reservoir is constantly replenished with fuel supplied by a fuel pump.

The correct fuel level in the bowl is maintained by means of a float controlling an inlet valve, in a manner very similar to that employed in a cistern ( e. g. a toilet tank ).

As fuel is used up, the float drops, opening the inlet valve and admitting fuel.

As the fuel level rises, the float rises and closes the inlet valve.

The level of fuel maintained in the float bowl can usually be adjusted, whether by a setscrew or by something crude such as bending the arm to which the float is connected.

This is usually a critical adjustment, and the proper adjustment is indicated by lines inscribed into a window on the float bowl, or a measurement of how far the float hangs below the top of the carburetor when disassembled, or similar.

Floats can be made of different materials, such as sheet brass soldered into a hollow shape, or of plastic ; hollow floats can spring small leaks and plastic floats can eventually become porous and lose their flotation ; in either case the float will fail to float, fuel level will be too high, and the engine will not run unless the float is replaced.

The valve itself becomes worn on its sides by its motion in its " seat " and will eventually try to close at an angle, and thus fails to shut off the fuel completely ; again, this will cause excessive fuel flow and poor engine operation.

Conversely, as the fuel evaporates from the float bowl, it leaves sediment, residue, and varnishes behind, which clog the passages and can interfere with the float operation.

This is particularly a problem in automobiles operated for only part of the year and left to stand with full float chambers for months at a time ; commercial fuel stabilizer additives are available that reduce this problem.