Carburetor Circuit Functions; Carburetor Removal - Honda 2005 CRF450R Owner's Manual

Carburetor Circuit Functions

The carburetor has several major circuits, each
providing the fuel/air mixture over a given
portion of throttle valve opening.
These major circuits overlap as shown below.
Main jet
The main jet affects fuel/air ratio from half (1/2)
to full throttle (4/4). The size should be reduced
at higher altitudes.
Honda carburetor jet sizes are numbered in
increments of 2 or 3. When changing the main jet
size, increase or decrease it gradually until the
desired jetting is obtained. Because Honda jet
size numbers do not correspond with other
carburetor manufacturer's jet size numbers, use
only Honda jets.
Jet Needle
The jet needle controls fuel/air mixture over fully
closed to 3/4 throttle. The straight section (1)
affects throttle response at smaller throttle
openings. By changing the position of the clip
(2) in its groove, you can improve acceleration at
medium low and medium speed.
1st
2nd
3rd
4th
5th
(1) straght section (2) clip
Slow Jet and Pilot Screw
The slow jet and pilot screw affects fuel/air ratio
over fully closed to 1/4 throttle.
Adjust the pilot screw to obtain the best off-idle
performance.
• If the engine blubbers (rich) exiting a corner,
turn the pilot screw clockwise to lean the
mixture.
• If the engine surges (lean) exiting a corner,
turn the pilot screw counterclockwise to
richen the mixture.
The minimum to maximum range of pilot screw
adjustment is 1 1/4 to 2 turns out from the lightly
seated position.
If you exceed 2 turns out, the next larger slow jet
is needed.
If you are under 1 1/4 turns out, the next smaller
slow jet is needed.
Carburetor Adjustments & Tuning Tips
Leak jet
The smaller the number, the smaller the jet bore
diameter. This controls the amount of fuel
returned to the float chamber under pressure,
there fore the flow amount is incleased or
decreased. With a small jet, pressure is raised
faster and the elapsed time before the fuel starts
flowing is shorten.

Carburetor Removal

(2)
1. Turn the fuel valve OFF.
2. Drain the remaining fuel in the float bowl by
loosening the drain screw (1).
(1)
Gasoline is highly flammable and
explosive. You can be burned or
seriously injured when handling fuel.
• Stop the engine and keep heat, sparks
and flame away.
• Handle fuel only outdoors.
• Wipe up spills immediately.
(1) drain screw
3. Tighten the drain screw.
WARNING
(1)
(cont'd)
119
Adjustments for Competition