Carburetor Adjustments & Tuning Tips; Carburetor Components; Cold/Hot Start Circuit - Honda 2005 CRF450R Owner's Manual

The carburetor on your CRF should perform
suitably with the standard recommended settings
under average load, climatic, and barometric
conditions. However, to fine tune the engine's
power output, the carburetor may require
adjustments for specific competition needs.
Optional main jets, slow jets and leak jets are
available for your CRF. See your Honda dealer.
Any engine or air cleaner modifications or the
use of an aftermarket exhaust system may require
jetting changes.

Carburetor Components

Cold Start Circuit
A very rich mixture must be delivered to the
cylinder when a cold engine is being started.
When the choke knob (1) is pulled out, fuel is
metered by the starter jet (2) and is mixed with
air from the air passage (3) (located above the
throttle valve (4)) to provide a rich mixture for
starting. The mixture discharges through the
orifice (5) into the cylinder.
(3)
(4)
(2)
(1) choke knob (4) throttle valve
(2) starter jet (5) orifice
(3) air passage
Carburetor Adjustments & Tuning Tips
Hot Start Circuit
A lean mixture must be delivered to the cylinder
when a hot engine is being started. When the hot
start lever is pulled back, the hot start valve (1)
opens, allowing air to be supplied to the main
bore (2) through the hot start air passage (3). This
extra air enters the air-fuel mixture from the slow
circuit resulting in a lean condition.
(1)
(5)
(1) hot start valve
(2) main bore
(3) hot start air passage
(1)
(3)
(2)
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Adjustments for Competition