Honda CRF250R 2005, Owner'S Manual

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Adjustments for Competition
117
Carburetor Adjustments & Tuning Tips
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.
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 1/2 to 2 1/2 turns out from the
lightly seated position.
If you exceed 2 1/2 turns out, the next larger
slow jet is needed.
If you are under 1/2 turns out, the next smaller
slow jet is needed.
Carburetor Removal
1. Turn the fuel valve OFF.
2. Drain the remaining fuel in the float bowl by
removing the float bowl plug (1).
WARNING
Gasoline is highly flammable and
explosive. You can be burmed or
seriously injured when handing fuel.
• Stop the engine and keep heat, sparks
and flame away.
• Handle fuel only outdoors.
• Wipe up spills immediately.
(1) float bowl plug
3. Install the float bowl plug.
(2)
(1)
(1) straight section (2) clip
(1)
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.
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.
(cont’d)