Service Manual

SECTION 1 CARBURETOR
Carburetor Operation
The carburetor receives fuel from the gasoline tank and
mixes
it with air in the right proportions to provide a
highly combustible mixture to the engine.
As the piston moves up on the compression stroke, a
partial vacuum is created within the engine crankcase,
causing the greater atmospheric pressure to force air to
flowthrough the carburetor into the cylinder. Thevelocity
of the air increases as
it flows through the carburetor
venturi and the air pressure is reduced at this point to
less than atmospheric pressure. The low pressure in the
venturi of the carburetor causes atmospheric pressure
to push raw fuel from the float bowl into the air stream in
the throat of the carburetor, where
it breaks up into a fine
spray, or becomes atomized, and mixes with the air
stream.
See Figure 10.
Atmospheric
Pressure
Float Bowl
To Engine
,Venturi
Carburetor
Bore
When starting the engine an extra rich mixture is
required. The choke plate
is closed by the operator to
provide an approximate
8:l ratio of fuel to air for this rich
mixture. Closing the choke plate further reduces the air
pressure in the venturi to increase the fuel pushed into
the carburetor throat. In this condition fuel also flows
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.
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 conditions 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 premixes with the incoming
air, then
is discharged into the intake port of the engine.
See Figure
12.
Pilot Jet
Pilot System
,Pilot Air Fitting
COLD START
Figure 11
Pilot System /Pilot Jet
Fitting
NO LOAD
Figure 12
As the throttle plate is opened to compensate for engine
load, the main discharge tube becomes the main source
of fuel. Opening the throttle plate increases the flow of
air through the venturi and strengthens the low pressure
area at the main discharge tube. Fuel discharge
increases at the main discharge tube as it decreases
from the pilot system. Air is drawn from the air correction
jet through holes along the length of the main discharge
tube. This premixes air with the fuel before
it enters the
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