G424 Service Manual LPG Fuel System76
Carburetor
LP Fuel Carburetor Schematic
(1) Idle air screw. (2) Air valve. (3) Diaphragm.
(4) Metering spring. (5) Passage. (6) LP gas metering valve.
(7) Power mixture adjustment. (8) Carburetor. (9) Piston.
(10) Throttle valve.
Lift trucks with LP (Liquid Petroleum) fuel system use
an air valve type of carburetor (8). The operation of the
carburetor controls the fuel mixture flow into the
engine with the movement of throttle valve (10) and
mixes the air flow with the LP fuel flow.
The air flow is measured by the operation of air valve
(2) in the carburetor bowl. The stronger the air flow
through the carburetor, the higher the air valve moves
up. LP gas metering valve (6) is connected directly to
air valve (2). As the air valve moves up, LP gas
metering valve (6) also moves up. LP gas metering
valve (6) is shaped to let in the correct amount of LP
gas at any height related to air valve (2) upper
movement. The air valve activates a pressure drop
which gives a very high metering force (measured
suction) to the fuel that flows in the carburetor at low
engine speeds and it permits easy starting. At full
engine speeds, the air valve is at the top of its upper
movement. The valve then becomes an inverted (up
side down) venturi and will let a large amount of air
pass through.
When starting the engine, the intake stroke of piston
(9) causes lower than atmosphere pressure (vacuum)
in the carburetor. The vacuum is felt through passage
(5) in air valve (2) and in the upper side of diaphragm
(3). As a result, the atmospheric pressure flows in the
carburetor and pushes up on the diaphragm.
Diaphragm (3) is lifted against the down pressure of
metering spring (4). The vacuum that is felt on
diaphragm (3) is variable according to engine speed
and the position of throttle valve (10) opening. Air
valve (2) measures the air flow that goes into the
engine, respective to the demands of the engine and
throttle valve (10) position.
The air pressure drop of 1.3 to 2.7 kPa (0.2 to 0.4 psi)
is controlled by metering spring (4) which permits the
force necessary to pull in the fuel into the air flow. LP
gas metering valve (6) is connected to the air valve
and lets in the correct amount of fuel from the LP
converter to mix with the air at any opening of the air
valve.
The carburetor is equipped with two limited mixture
adjustments. Idle air screw (1) directs atmosphere
(air) around the opening of air valve (2). As the idle air
screw is adjusted open, the air valve loses a little more
and causes the gas metering valve to close. The result
will be less fuel in the mixture at idle. The second
adjustment is power mixture adjustment (7). It controls
the fuel mixture when gas metering valve (6) is fully in
the up position. This adjustment is done only when the
engine is at a load condition. A CO meter or exhaust
analyzer is used for the power mixture adjustment.
This adjustment has no effect at idle or in the light load
range.
IDES070S
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