Rotax MMH-912 Series, Руководство по обслуживанию

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73-00-00
page 12
May 01/2007
Effectivity 912/914 Series
Edition 1 / Rev. 0
d02623
BRP-Rotax
Maintenance Manual
2.8) BING constant depression carburetor type 64-3
See Figs. 73-6, 73-7 and 73-8.
The BING constant depression carburetor type 64-3 is a cross-draft butterfly
valve carburetor with variable venturi, double float system arranged centrally
below the venturi, and a rotary disc valve type starting carburetor. Its particular
feature is the carburetor piston (2), which is suspended on a rubber diaphragm
(3) and protrudes into the venturi. It adjusts the smallest crosssection (“air
funnel”) of the venturi depending on the vacuum prevailing there. The vacuum
is influenced by actuation of the throttle valve (1) and the carburetor piston
moves accordingly. The throttle valve diameter is 36 mm (1.42 in.).
When the fuel has reached the specified level in the float chamber, the floats
close the needle valve (4) via the float bracket, thus preventing any further
supply of fuel.
◆
NOTE: The atmospheric 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 en-
riches the mixture, too low pressure leans down the mixture,
possibly leading to engine stop.
The float chamber airspace is connected by a channel with the venting nipple
(18). The channel must be clear. With a hose connected on the venting nipple,
the correct pressure in the float chamber is established.
The amount of mixture drawn in by the engine and thus its performance are
determined by the cross-sectional area in the venturi, which is opened by the
throttle valve (1). 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 (16), which draws fuel from the float chamber through the jet
system.
The vacuum in the venturi acts on the top side of the diaphragm via the bore (12)
in the carburetor piston and attempts to lift the piston against its own weight and
the spring. The reference pressure prevailing between air filter and carburetor
(e.g. ambient air pressure) is applied to the bottom side of the diaphragm via
channel (13). The space in the cover above the piston guide is vented through
bore (14). The bore diameter determines the piston damping.