SECTION
1
CARBURETOR
Carburetor Description
The carburetor used on the
GTS
150 is a dual circuit (pilot
and main), float bowl design with fixed main jet, fixed
nonremovable pilot jet and an adjustable pilot circuit
mixture screw. The carburetor is fitted with a standard
number
30
(.0295") main jet.
A
high attitude main jet
number 28 (.0280") is also available. The carburetor is
fitted with a stainless steel inlet needle and a replaceable
inlet seat. The throttle and choke shaft are equipped with
dust seals.
Carburetor
Theory
and Operation
The carburetor receives fuel from the tank and mixes
it
with air in the right proportions to provide a highly combus-
tible mixture to the engine.
As
the piston moves down on the intake stroke, a partial
vacuum is created within the cylinder, causing the greater
atmospheric pressure to force air to flow through the
carburetor into the cylinder. The velocity of the air
in-
creases as
it
flows through the carburetor venturi and the
air pressure is reduced at this point to less than atmos-
pheric pressure. The differences of pressure in the venturi
of the carburetor causes atmospheric pressure to push
raw fuel from the float bowl into the air stream where it
breaks up into a fine spray, or becomes atomized, and
mixes with the air stream. See Figure 10.
VENTURI
CARBURETOR
BORE
FLOAT BOWL
TO
ENGINE
When starting the engine, an extra rich mixture is required.
The choke plate is closed by the operator to provide an
approximate 8:l (approximately six times richer than
normal) ratio of fuel to air for this rich mixture. Closing the
choke plate further reduces the air pressure area in the
venturi to increase the fuel drawn into the carburetor bore.
In this condition, fuel is drawn from the float bowl through
the pilot system ports as well as the main discharge tube
to achieve the proper starting mixture. See Figure 11.
PILOT
SYSTEM
COLD START
Figure
11
Fuel atomization becomes more efficient due to heat, once
the engine has reached normal operating temperature.
As
a result, the engine does not require the rich mixture it did
for starting and the choke plate must be moved to the open
position. The engine speed is now regulated by the throttle
plate. In no load condition, a small portion of the fuel may
be drawn from the main discharge tube, however the
primary fuel supply is drawn from the pilot circuit. Air
passing through the pilot jet from the pilot air fitting draws
fuel
out
of the pilot jet orifice from the float bowl. This fuel
pre-mixes with the incoming air and is discharged into the
carburetor bore where the fuel becomes atomized. See
Figure 12.
--
''LOT
I
I
PILOT
AIR
FITTING
PILOT
JET
GTS
150
19
Figure
12
Carburetor
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