The
following is a short blog relating to that most humble of instruments, the SU
float chamber.
Without
wishing to state the obvious, a float chamber’s primary purpose is to provide a
constant head of fuel to the carburettor under all circumstances. Clearly there
will be a difference in the head of a gravity fed fuel system, between the tank
full and the tank empty. Less clear are the effects due to an ever changing
demand for fuel under engine running conditions and the gravitational forces
generated under heavy acceleration, braking and cornering. Quite simply, it is
impossible to calibrate a carburettor unless you can ensure a constant fuel
supply.
The SU did
this very well.
The design
was simple, it was easy to manufacture and was robust in use. All the above
guaranteed it had a long life, both in terms of production and under normal working
use. It was available in many different sizes and forms, but the design was
always basically the same. As such it was fitted to a large proportion of UK
car production, from when it was initially conceived in 1910, up until its
demise due to the coming of fuel injection in the 1980s.
The SU float
chamber was designed to be pressure fed, either from a mechanical fuel pump
driven off an eccentric lobe on the camshaft, or from an electrical pump wired
to the vehicle power supply. For this purpose SU manufactured a range of constant
pressure electric pumps with differing flow rates to suit all applications. The
fuel is ‘top-fed’ via a gauze filter into the lid of the float chamber. Here a
pivoted fork controls the opening and closing of a needle valve, which allows
the fuel to enter and fill the float chamber. This causes the hollow brass
float to rise, closing the valve and halting the fuel supply when the correct
level is attained. The fuel is ‘bottom-fed’ to the carburettor and as the
supply within the chamber is used, the level drops opening the needle valve
once again. The design of the later series of SU float chambers was amended to
incorporate a pivoted a plastic float within the chamber lid, but the operating
principle remained the same.
Due to
simple functionality, the SU float chamber was also used for a great number of
applications for which it was never intended. It was a very popular choice in
motorcycle racing, both solos and sidecars. On a solo it would normally be
gravity fed from the fuel tank above, but on a sidecar two float chambers would
be employed, one either side of the carburettor. This would ensure the
carburettor could never be deprived of fuel due to the corning forces generated
on a three-wheeled machine. The fuel was normally supplied to these chambers
via a fuel pump of the type described previously.
