Rotax 914 F Maintenance Manual page 39

The float chamber airspace is connected by a duct with the venting nipple y.
The duct must be clear. With a hose connected on venting nipple the correct
pressure in the float chamber is established.
The amount of mixture drawn in by the engine and thus its performance is
determined by the cross-sectional area in the venturi which is opened by the
throttle valve Q. If the throttle valve is opened while the engine is running, the
increased airflow in the venturi results in a vacuum building up at the outlet
of the needle jet r which draws fuel from the float chamber through the jet
system.
The vacuum in the venturi acts on the top of the diaphragm and the carburetor
piston via a bore } in the carburetor piston and attempts to lift the piston
against its own weight and spring. The reference pressure prevailing be-
tween airfilter and carburetor (e.g. ambient air pressure) is applied to the
underside of the diaphragm via duct q. The space in the cover above the
piston guide is vented through bore w. The bore diameter determines the
piston damping.
On its way from the float chamber to the venturi the fuel passes through the
main jet U, the jet carrier and the needle jet, and as it leaves the needle jet
it is pre-mixed with air which is brought in from the air filter via an air duct O
and the atomizer I in an annular flow around the needle jet. This air flow
assists the atomizing process and favours fuel distribution in the intake duct.
The jet needle P responsible for part load is kept in position by the retaining
clip {.
During idling and low-load operation the throttle valve is closed to such an
extent that the airflow underneath the carburetor piston W no longer forms a
sufficient vacuum. The fuel is then supplied via an auxiliary system, the idling
system, which consists of the idling jet T and the idling air jet Y .
The BING constant depression carburetor is provided with a rotary valve
starting carburetor t as an aid for starting a cold engine, using a Bowden
cable.
full
throttle
17
00128
Modification no.
- 3 -
NOTE: The air pressure in the float chamber is decisive for the
function of the carburetor and must correspond with the air
pressure at entry into the carburetor. Too high pressure
enriches the mixture, too low pressure leans down the
mixture possibly leading to engine stop.
WARNING: With throttle lever not connected the carb will remain in full
throttle position. See Pic. 12.
Therefore never start engine without connecting throt-
tle lever first.
Pic. 16
Reference
18
17
00035
Main
914 F
Pic. 17
Date Page
1999 01 01 39