Carburetor Adjustments & Tuning Tips
The carburetor on your CR 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 and slow jets are available for
your CR. See your Honda dealer.
Any engine or airbox modifications or the use of
an aftermarket exhaust system may require
jetting changes.
Carburetor Components
Starting Circuit
A very rich mixture must be delivered to the
cylinder when the engine is being started. When
the choke knob (1) is pulled up, fuel is metered
by the starter jet (2) and is mixed with air from
the air passage (3) to provide a rich mixture for
starting. The mixture discharges through the
orifice (4) into the cylinder.
(1) choke knob
(3) air passage
(2) starter jet
(4) orifice
Slow Circuit
Fuel is metered by the slow jet (5) and is mixed
with air that has been metered by the air screw
(6). The mixture enters the venturi through the
bypass (7) and slow jet outlet (8).
(5) slow jet
(7) bypass
(6) air screw
(8) slow jet outlet
Main Circuit
Fuel is metered by the main jet (9), jet needle
(10) and needle jet (11). It is then mixed with air
coming from the air jet (12) and enters the
venturi past the needle jet.
Baffle Plate
The baffle plate (13) prevents foaming of fuel or
abnormal fuel level around the main jet.
Float Chamber
The float (14) and float valve (15) operate to
maintain a constant level of fuel in the float
chamber.
(9) main jet
(13) baffle plate
(10) jet needle
(14) float
(11) needle jet
(15) float valve
(12) air jet
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.
Your CR’s carburetor jet sizes are numbered in
increments of 10. When changing the main jet
size, increase or decrease it gradually until the
desired jetting is obtained. Because your CR’s jet
size numbers do not correspond with other
carburetor manufacturer’s jet size numbers, use
genuine Honda CR250R jets.
Adjustments for Competition
103
(1)
(4)
(3)
(2)
(7)
(8)
(6)
(5)
(12)
(15)
(11)
(14)
(10)
(13)
(9)
MAIN JET
JET NEEDLE (TAPERED SECTION)
JET NEEDLE
(STRAIGHT SECTION)
SLOW JET AND AIR SCREW
FULLY
CLOSE
FULLY
OPENED
1/8
1/4
1/2
3/4
(cont’d)
CR250R/085-103(31KSK630) 6/2/06 10:01 AM Page 103
Summary of Contents for CR250R
Page 2: ......
Page 6: ...Motorcycle Safety 1 Motorcycle Safety mation 2 recautions 2 cations 3 4 ...
Page 8: ...Accessories Modifications Motorcycle Safety 3 ...
Page 9: ...replacement ...
Page 10: ...Operating Controls 5 Operating Controls Locations 6 ...
Page 11: ...kickstarter rear brake pedal throttle grip front brake lever ...
Page 12: ...Before Riding 7 Before Riding e 8 ady to Ride 9 9 ...
Page 13: ...RNING met increases the injury or death in a ys wear a helmet eye er protective apparel ...
Page 15: ......
Page 17: ......
Page 19: ......
Page 20: ...Basic Operating Instructions 15 Break in Guidelines ...
Page 21: ......
Page 23: ......
Page 27: ......
Page 36: ...Servicing Your Honda 31 7 and shroud B bolts A bolts collars 9 s e 29 Fuel Tank Removal 9 8 ...
Page 48: ...Servicing Your Honda 43 Throttle ...
Page 69: ......
Page 82: ...Servicing Your Honda 77 6 RC cover bolt olt t Additional Maintenance Procedures 1 3 2 5 ...
Page 85: ......
Page 119: ......
Page 120: ...Tips 115 torcycle 116 117 nt 118 119 Tips 115 Tips ...
Page 123: ...drained fluids is harmful to ...
Page 125: ......
Page 127: ... is stamped on the left 2 ...
Page 132: ...Technical Information 127 High Altitude Carburetor Adjustment ...
Page 135: ...ng Suspension Settings Gearing Chassis Adjustments Maintenance ...
Page 138: ...Technical Information 133 Wiring Diagram ...
Page 139: ......
Page 140: ...Consumer Information 135 136 137 138 ub USA only 139 Consumer Information ...
Page 142: ...Contacting Honda Consumer Information 137 ...
Page 144: ...Consumer Information 139 The Honda Rider s Club USA only ealer for more information on to ...
Page 148: ...Index 143 Index ...