Jabiru 3300, Instruction And Maintenance Manual

Page: 58 / 103

Page No: 58 Issue No: 1 Date: 031100 Issued By: PJA
IDLING SYSTEM
During idling and low-load running the throttle valve (23) is closed to such an extent that
the air flow underneath the plunger (13) no longer forms a sufficient vacuum. The fuel is
then supplied via an auxiliary system, the idling system, which consists of the idling jet
(5), the idling air jet (LLD) - no spare part - and the mixture control screw (7) which is
sealed off against the carburettor housing by the rubber ring (9) and secured by spring (8)
to prevent it from becoming slack. The fuel passes through the idling jet (5) whose bore
will determine the amount of fuel. Behind the jet bore the fuel mixes with air which is
supplied via cross ducts in the jet throat from the idling air channel, the amount of air
admitted being determined by the size of the idling air jet at the inlet of this duct. This
initial mixture then flows through the idling outlet bore (LA), the cross-sectional area of
which can be adjusted by the mixture control screw (7); it then reaches the choke tube via
bypass or transition passages (BP) where it is mixed further with pure air.
Idling should always be with the engine at operating temperature. First the mixture
control screw (7) is turned fully clockwise and then backed off by the number of turns
specified for the particular engine. Turning in clockwise direction results in a leaner
mixture and turning in anti-clockwise direction in a richer mixture.
The idling setting quoted serves as a guide only. The optimum will generally differ
slightly. First select the desired idling speed by using the idling stop screw (34). When
subsequently adjusting the mixture control screw - starting from the basic setting - a speed
drop will be noticed in both directions. The optimum setting will generally be found half-
way between the two settings at which this speed drop was noticed.
To facilitate the idling setting on engines having several carburettors where it is important
that they are evenly adjusted, it is possible to connect a pressure gauge (in the simplest
case a "U-tube pressure gauge") to a nipple below the throttle valve shaft bearing point
which is normally closed off by screw (39). To select the idling speed, the idling screw
(34) is in this case adjusted until the same vacuum is indicated for all carburettors. By
slightly opening the throttle valve via a turning handle or the accelerator it is also possible
to adjust Bowden cables or linkages evenly by making this vacuum comparison.
STARTING CARBURETTOR
BING constant depression carburettor is provided with a rotary valve starting carburettor
as an aid for staring a cold engine using a Bowden cable. A disc (47) resting against the
carburettor housing is turned via a shaft in the starting carburettor housing (48) so that the
starting carburettor chamber into which air enters from the air filter side of the carburettor
is connected to the engine side of the carburettor via a duct. The airport in the disc (47) is
shaped in such a way that depending on the disc position, more or less air is drawn in. At
the same time the disc opens the fuel system of the starting carburettor via bores matched
to the disc position. The fuel flows from the float chamber through the starting jet into
the vented starting chamber also contained in the float chamber (44) and from there
through a riser where it is pre-mixed with air via transverse bores, into the starting
carburettor. There it forms a particularly rich mixture with the air drawn in, and this