5.5
Section 5
Fuel System and Governor
5
Main (High-Speed) Circuit:
At high speeds/loads the engine operates on the main
circuit. As a metered amount of air is drawn through
the four air jets, fuel is drawn through the main jets.
The air and fuel are mixed in the main nozzles and
then enter the main body of airflow, where further
mixing of the fuel and air occurs. This mixture is then
delivered to the combustion chamber. The carburetor
has a fixed main circuit; no adjustment is possible.
Carburetor Adjustments
Adjustment
NOTE
: Carburetor adjustments should be made
only after the engine has warmed up.
The carburetor is designed to deliver the correct fuel-
to-air mixture to the engine under all operating
conditions. The main fuel jet is calibrated at the
factory and is not adjustable*. The idle fuel adjusting
needles are also set at the factory and normally do not
need adjustment.
*
NOTE:
Engines operating at altitudes above
approximately 1500 m (5000 ft.) may require
a special high altitude main jet. Refer to High
Altitude Operation later in this section.
If, however, the engine is hard-starting or does not
operate properly, it may be necessary to adjust or
service the carburetor.
Low Idle Speed (RPM) Adjustment
1.
Low Idle Speed (RPM) Setting
: Place the throttle
control in the
idle
or
slow
position. Set the low
idle speed approximately
300 RPM
*
less
than the
intended or specified Governed Idle Speed, by
turning the low idle speed adjusting screw in or
out. Check the speed using a tachometer.
IMPORTANT: The Governed Idle Speed
Adjustment must follow any resetting of the
Low Idle Speed.
*
NOTE:
The actual low idle speed depends on
the application. Refer to the equipment
manufacturer’s recommendations. The
low idle speed for basic engines is
1200 RPM
. To ensure best results when
setting the low idle fuel needle, the low
idle speed should be
1200 RPM (± 75
RPM
).
Have a container ready to catch the fuel.
2. Wipe the tip of the solenoid with a shop towel or
blow it off with compressed air, to remove any
remaining fuel. Take the solenoid to a location
with good ventilation and no fuel vapors
present. You will also need a 12 volt power
source that can be switched on and off.
3. Be sure the power source is switched
off
. Connect
the positive power source lead to the red lead of
the solenoid. Connect the negative power source
lead to the solenoid body.
4. Turn the power source
on
and observe the pin in
the center of the solenoid. The pin should retract
with the power on and return to its original
position with the power
off
. Test several times to
verify operation.
Carburetor Details
The Keihin BK two-barrel carburetor is a side-draft
design. The circuits within the carburetor function as
described following:
Float Circuit:
The fuel level in the bowl is maintained by the float
and fuel inlet needle. The buoyant force of the float
stops fuel flow when the engine is at rest. When fuel is
being consumed, the float will drop and fuel pressure
will push the inlet needle away from the seat,
allowing more fuel to enter the bowl. When demand
ceases, the buoyant force of the float will again
overcome the fuel pressure, rising to the
predetermined setting and stop the flow.
Slow & Mid-Range Circuit:
At low speeds the engine operates only on the slow
circuit. As a metered amount of air is drawn through
the slow air bleed jets, fuel is drawn through the two
main jets and further metered through the slow jets.
Air and fuel are mixed in the body of the slow jet and
exit to the transfer port. From the transfer port the air
fuel mixture is delivered to the idle progression
chamber. From the idle progression chamber the air
fuel mixture is metered through the idle port
passages. At low idle when the vacuum signal is
weak, the air/fuel mixture is controlled by the setting
of the idle fuel adjusting screws. This mixture is then
mixed with the main body of air and delivered to the
engine. As the throttle plate opening increases,
greater amounts of air/fuel mixture are drawn in
through the fixed and metered idle progression holes.
As the throttle plate opens further the vacuum signal
becomes great enough so the main circuit begins to
work.
