Kohler COMMAND CH18, Service Manual

Page: 44 / 232

5.11
Section 5
Fuel System and Governor
5
Components such as the throttle and choke shaft
assemblies, throttle plate, choke plate, low idle fuel
needle, and others, are available separately.
Always refer to the Parts Manual for the engine being
serviced, to ensure the correct repair kits and
replacement parts are ordered. Service/repair kits
available for the carburetor and affiliated
components are:
Carburetor Repair Kit
Float Kit
High Altitude Kit (1525-3048 m/5,000-10,000 ft.)
High Altitude Kit (over 3048 m/10,000 ft.)
Solenoid Assembly Kit
Accelerator Pump Seal and Bushing Kit
Accelerator Pump Diaphragm Kit
Choke Repair Kit
#
NOTE: If accelerator pump rod movement is
restricted or corrosion exists in the pump
rod housing hole; use a 0.153 in. (3.9 mm) or
No. 23 drill and carefully clean (resize) the
hole to restore proper operation. Blow out
the hole with air and lightly lubricate with
oil on reassembly.
Carburetor
Keihin BK Two-Barrel Carburetor (CH750)
The carburetor used on CH750 engines is a Keihin
two-barrel side draft design with fixed main jets. See
Figure 5-12. A self-relieving choke similar to that used
on single venturi carburetors is also contained in the
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.
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
needle is also set at the factory and normally does not
need adjustment.
Depending on model and application, engines may
also be equipped with a “Governed Idle System.” If
equipped with a “Governed Idle System”, refer to
“
Models with Governed Idle System ” when
performing any carburetor adjustment, as an
additional step to the listed adjustment procedure(s)
is required.