4> Main Regulating System With Pressure Control - Rotax 912-UL Repair Manual

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In addition, the float needle (10) is connected to the float hinge by the retaining
spring (11) topreventit from movingbetween floatand valve seatand thusreducing
the fuel supply. Spring and retaining guide make a considerable contribution towards
keeping the fuel level in the float chamber constant.
When fitting a new float, the fuel level must be adjusted. When doing this, care must
be taken to ensure that the fuel needle spring is not compressed by the float weight.
It is therefore advisable to put the carburetor in a horizontal position until the float
just contacts the float needle. In this position the point on the float hinge is set in such
a way that the float top edges are parallel to the top edge of the float chamber.
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The float chamber (15) is secured to the carburetor housing by a spring yoke (18).
A gasket (17) is provided between float chamber and carburetor housing. The space
above the fuel level is connected to atmosphere by two ducts. When these ducts are
blocked, an air cushion forms above the fuel. The fuel will not lift the float
sufficiently to close the needle valve and the carburetor overflows.
The float chamber (15) incorporates an overflow pipe to allow fuel to drain off if the
specified level in the float chamber is exceeded substantially due to a leaking needle
valve.
7.15.4) Main regulating system with pressure regulator
The amount of mixture drawn in by the engine and thus its performance is determined
by the cross-sectional area in the choke tube which is opened by the throttle valve
(a1). The throttle valve is secured to the valve shaft (39) by two screws (42). The end
projecting from the carburetor housing carries the throttle levers (a6) and (a7)
which are secured by the nut (37) and washer (36) to which the Bowden cable
operating the throttle shaft is attached.
The sealing ring (40) provides the seal between valve shaft and housing. The
retaining arm (43) attached to the carburetor housing by means of screws (44) and
washers engages in the notch in the valve shaft and thus prevents it from moving in
axial direction. The return spring (48) whose action opposes the Bowden cable is
attached between a bent-over tab at the lower end of the retaining arm and the throttle
lever (47).
If the throttle valve (41) is opened while the engine is running, the increased air flow
in the choke tube results in a vacuum building up at the outlet of the needle jet (3)
which draws fuel from the float chamber through the jet system. At low speeds and
in particular in the case of four-stroke engines, this vacuum is not sufficient for an
adequate fuel supply; it must therefore be increased artificially by using a pressure
regulator.