Honda CRF450X 2017 Owner's Manual & Competition Handbook page 140

Accelerator Pump Circuit
The accelerator pump circuit operates when
the throttle is opened. As the throttle valve (1)
opens, the pump rod (2) depress the diaphragm
(3). At this time, the inlet check valve (4) is shut
resulting in a sharp increase in pressure in the
pump chamber (5). The outlet check valve (6)
then opens, supplying fuel to the main bore via
the accelerator nozzle (7).
(1)
(1) throttle valve (5) pump chamber
(2) pump rod (6) outlet check valve
(3) diaphragm (7) accelerator nozzle
(4) inlet check valve
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Slow Circuit
Fuel is metered by the slow jet (1) and mixed
with air from the air passage (2). The mixture
enters the venturi through the bypass (3) and
pilot outlet (4) that has been metered by the pilot
screw (5).
(4)
(7)
(5)
(6)
(1)
(3)
(2)
(1) slow jet
(2) air passage
(3) bypass
(5)
(4)
Carburetor Adjustment & Tuning Tips
Main Circuit
Fuel is metered by the main jet (1), jet needle
(2) and needle jet (3). It is then mixed with air
coming from the air jet (4) and enters the venturi
past the needle jet.
Baffle Plate
The baffle plate (5) prevents foaming of fuel or
abnormal fuel level around the main jet.
Float Bowl
The float (6) and float valve (7) operate to
maintain a constant level of fuel in the float bowl.
(3)
(2)
(4) pilot outlet
(5) pilot screw
(1) main jet
(2) jet needle
(3) needle jet
(4) air jet
(2)
(4)
(7)
(6)
(3)
(5)
(1)
(5) baffle plate
(6) fl oat
(7) fl oat valve
Adjustments for Competition
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