Carburetor Adjustment; Carburetor - Honda CRF450R Owner's Manual

5. CARBURETOR ADJUSTMENT

CARBURETOR

The carburetor used on your CRF will seldom experi-
ence trouble with the standard settings under average
load, and average climatic and barometric conditions.
However, to fine tune the engine's power output, the
carburetor may require adjustments for specific racing
conditions. To change the carburetor settings, observe
the following instructions.
Construction
• Cold Start Circuit
A very rich mixture must be delivered to the cylinder
when cold engine is being started . When the choke
knob is pulled out, fuel is metered by the starter jet and
is mixed with air from the air passage (located above
the throttle valve) to provide a rich mixture for starting.
The mixture discharges through the orifice into the cyl-
inder.
(1) CHOKE KNOB (2) STARTER JET
(3) AIR PASSAGE (4) THROTTLE VALVE
(5) ORIFICE
• Hot Start Circuit
A lean mixture must be delivered to the cylinder when
hot engine is being started. When the hot start lever is
pulled back, the hot start valve opens, allowing air is
supplied to the main bore through the hot start air pas-
sage. This extra air enters the air-fuel mixture from the
slow circuit resulting in a lean condition.
(1) HOT START VALVE
(2) MAIN BORE
(3) HOT START AIR PASSAGE
• Accelerator Pump Circuit
The accelerator pump circuit operates when the throttle
is opened. As the throttle valve opens, the pump rod
depress the diaphragm. At this time, the inlet check
valve is shut resulting in a sharp increase in pressure
in the pump chamber. The outlet check valve then
opens, supplying fuel to the main bore via the accel-
erator nozzle.
(1) THROTTLE VALVE
(2) PUMP ROD
(3) DIAPHRAGM
(4) INLET CHECK VALVE
(5) PUMP CHAMBER
(6) OUTLET CHECK VALVE
(7) ACCELERATOR NOZZLE
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