Honda Crf 250 M Manual

							Adjustments for Competition117
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) 
						

							Carburetor Adjustments & Tuning Tips
118Adjustments for Competition
4. Disconnect the fuel line (2) from the fuel
valve (3).
5. Remove the fuel tank (page 30).
6. Remove the sub-frame (page 32).
7. Loosen the intake band screw (4).
(2) fuel line
(3) fuel valve
(4) intake band screw
8. Disconnect the throttle sensor connector (5).
9. Remove the carburetor.
(5) throttle sensor connector
10.Remove the hot start cable holder (6).
(6)  hot start cable holder
11. Disconnect the rear suspension (7) upper
mounting nut (8).
(7) rear suspension
(8) upper mounting nut
12.Remove the throttle drum cover bolt (9) and
throttle drum cover (10).
(9)  throttle drum cover bolt
(10)  throttle drum cover
13.Loosen the lock nut (11), adjusters (12) and
disconnect the throttle cables (13) from the
throttle drum (14).
Remove the carburetor to the rearward from
the frame
.
(11) lock nut (13) throttlr cables
(12) adjusters (14) throttle drum
(3)
(2)
(4)
(5)
(6)
(8)
(7)
(9)
(10)
(12)
(13)(11)
(14) 
						

							Carburetor Adjustments & Tuning Tips
Adjustments for Competition119
Carburetor Disassembly
1. Pull up the cable clamp (1) and remove the
carburetor top bolts (2).
Then remove the carburetor top (3).
(1) cable clamp
(2) carburetor top bolts
(3) carburetor top
2. Remove the jet needle holder (4) and jet
needle (5) from the throttle valve (6).
(4) jet needle holder
(5) jet needle
(6) throttle valve
3. Remove the link arm set screw (7).
When installing the link arm set screw, apply
a locking agent to the link arm set screw
threads.
4. Remove the throttle valve (8), throttle valve
roller (9) and floating valve (10).
(7) link arm set screw (9) throttle valve roller
(8) throttle valve (10) floating valve
When installing the floating valve (10) onto
the throttle valve (8) make sure the floating
valve’s flat side faces out and the hole (11)
faces down.
(8) throttle valve
(10) floating valve
(11) hole
(3)
(1)
(2)
(5)
(4)
(6)
(7)(8)
(9)
(10)
(8)
(11)
(8)
(10)
(cont’d) 
						

							Carburetor Adjustments & Tuning Tips
120Adjustments for Competition
5. Remove the accelerator pump cover screws
(12) and accelerator pump cover (13).
(12) accelerator pump cover screws
(13) accelerator pump cover
6. Remove the D-ring (14), O-ring (15) spring
(16) and diaphragm (17).
Clean the diaphragm.
When installing the D-ring into the accelerator
pump cover (13) make sure the D-rings
flatside faces down, away from the float bowl
(18).
(13) accelerator pump cover (16) spring
(14) D-ring (17) diaphragm
(15) O-ring (18) float bowl
7. Remove the holder screw (19), throttle stop
screw holder (20), float bowl screws (21),
tube guides (22) and float bowl (18).
(18) float bowl
(19) holder screw
(20) throttle stop screw holder
(21) float bowl screws
(22) tube guides
8. Remove the pump rod (23).
Clean the pump rod and rod passage (24).
Install the pump rod into the link lever (25).
Push the rod forcibly into the link lever until
it snaps into place.
(23) pump rod (25) link lever
(24) rod passage(26) float level
(27) float level gauge
(28) carburetor intake
10.Remove the leak jet (29) from the float bowl
(18).
Clean the leak jet. 9. Measure the float level (26) with the float
level gauge (27) during float tab just
contacting the float valve and the carburetor
intake (28) facing up. The float level should
be 0.31 in (8.0 mm).
Adjust the float level by bending the float tab
carefully.
(18) float bowl (29) leak jet
(13)(12)
(12)
(14)
(18)
(14)
(13)
(16)
(17)(15)
(21)(20)(21)
(18)
(21)
(22)
(19)
(21)
(22)
(25)
(23)(25)
(23)
(24)
(26)
(27)(28)
(29)(18) 
						

							Carburetor Adjustments & Tuning Tips
Adjustments for Competition121
Carburetor Assembly
To assemble, reverse the disassembly procedures.
To install the carburetor, reverse the removal
procedures.
•  After installing the carburetor, adjust the
throttle cable (1) free play and hot start cable
(2) free play.
•  After installing the carburetor, check the air
vent hoses (3) and overflow hose (4) for
kinking or pinching and correct routing as
necessary.
(1) throttle cables
(2) hot start cable(3) air vent hoses
(4) overflow hose
(2)
(1)
(3)
(3)
(4)
(3)
(4) 
						

							Carburetor Adjustments & Tuning Tips
122Adjustments for Competition
Standard Tuning Recommendations
For the following recommendations to be
accurate, you must use the standard settings as a
baseline.  Also, don’t change any of the settings
until you’ve determined what changes are
necessary.
Adjustment
Pilot screw opening
Slow jet
Jet needle
Needle clip position
Main jet
Float level
Identification number
Standard settings
1–3/4 turns out
#42
NCYQ
3rd groove
#172
0.31 in (8.0 mm)
FCR01B
Adjustments for Altitude & Temperature
All jetting recommendations are based on
standard jetting with an unmodified engine.  The
following conditions can affect the fuel mixture.
All jetting is based on
•    Standard jetting
•    Unmodified Engine
Legend
    PS:  Pilot Screw opening from fully seated
    SJ:  Slow Jet
    JN:  Jet Needle
    NC: Needle Clip position
    MJ: Main Jet
TEMPERATURE
ALTITUDE
10,000 ft
to
7,500 ftPS:
SJ:
JN:
NC:
MJ:1 3/4
42
NCYQ
3rd
1721 3/4
42
NCYQ
3rd
1701  1/2
42
NCYQ
2nd
1701  1/2
42
NCYQ
2nd
1681  1/2
42
NCYQ
2nd
1681 1/4
42
NCYQ
2nd
1651 1/4
42
NCYQ
2nd
165
7,499 ft
to
5,000 ftPS:
SJ:
JN:
NC:
MJ:1 3/4
42
NCYQ
3rd
1751 3/4
42
NCYQ
3rd
1721 3/4
42
NCYQ
3rd
1701  1/2
42
NCYQ
2nd
1701  1/2
42
NCYQ
2nd
1681  1/2
42
NCYQ
2nd
1681 1/4
42
NCYQ
2nd
165
4,999 ft
to
2,500 ftPS:
SJ:
JN:
NC:
MJ:2
42
NCYQ
4th
1751 3/4
42
NCYQ
3rd
1751 3/4
42
NCYQ
3rd
1721 3/4
42
NCYQ
3rd
1701  1/2
42
NCYQ
2nd
1701  1/2
42
NCYQ
2nd
1681  1/2
42
NCYQ
2nd
168
2,499 ft
to
1,000 ftPS:
SJ:
JN:
NC:
MJ:2
42
NCYQ
4th
1782
42
NCYQ
4th
1751 3/4
42
NCYQ
3rd
1751 3/4
42
NCYQ
3rd
1721 3/4
42
NCYQ
3rd
1701  1/2
42
NCYQ
2nd
1701  1/2
42
NCYQ
2nd
168
999 ft
to
Sea LevelPS:
SJ:
JN:
NC:
MJ:2
42
NCYQ
4th
1782
42
NCYQ
4th
1782
42
NCYQ
4th
1751 3/4
42
NCYQ
3rd
1751 3/4
42
NCYQ
3rd
1721 3/4
42
NCYQ
3rd
1701  1/2
42
NCYQ
2nd
170 FAHR.-21°~ 0°-1°~ 20°19°~ 40°39°~ 60°59°~ 80°79°~ 100°99°~ 120°
STANDARD
JETTING
Condition Mixture will be Component affectedAdjust to
Richen
Lean
Richen
Lean
Lean Lean
Main jet
(jet needle stage) Rich
Lean
Rich
Rich Cold temperature
Warm temperature
Dry air
High humidity
High altitude 
						

							Carburetor Adjustments & Tuning Tips
Adjustments for Competition123
Special Tuning Conditions
There are some unique atmospheric conditions or
situations that may require additional
adjustments.
It should not be necessary to go more than one
jet size richer or leaner to fine tune your
motorcycle.  If larger jetting changes are
necessary, check for air leaks, blocked or
restricted exhaust or fuel systems, or a dirty air
cleaner.
Condition
long straights
uphill sections
sand
mud
high humidity
raining
hotter than 113 °F (59 °C)
Main Jet Adjustment
next larger
next smaller
Onece you’ve adjusted the carburetor for
temperature and altitude, it shouldn’t need major
readjustment unless race conditions change
drastically.  However, there are some unique
atmospheric conditions or race day situations that
may require additional adjustments.  They are as
follows.
To prevent engine damage, always adjust the
main jet before adjusting the jet needle.  Always
choose the next richer jet if there is any doubt so
there is a margin for safety.
Main Jet • Go richer on the main jet, by one
number, when: the track has a very
long straightaway or uphill section,
a high percentage of sand, or the
track is muddy.
• Go leaner on the main jet, by one
number, when: it is very humid or
raining, or it is hotter than 113
degrees F.
After using the chart, and making any
adjustments for special conditions, it shouldn’t be
necessary to go more than one jet size richer or
leaner to fine tune your CRF.  If larger jetting
changes are necessary, check for worn crankshaft
seals, air leaks, blocked or restricted exhaust or
fuel systems, or a dirty air cleaner.Pre-Adjustment Checks
Before adjusting carburetor settings, check the
following:
• air cleaner
• air leaks
• float level
• clogged carburetor jets
• spark plug fouling (improper heat range or
other cause)
• freshness of fuel
• owner modifications (such as exhaust system,
holes in the air box, etc.)
• ignition timing
• compression
If the above check out, then, adjust the carburetor
for your specific racing conditions.  Engine
response and appearance of the firing end of a
spark plug are highly indicative of the engine
condition.
Refer to the list on page 147 for the optional
carburetor parts.
To prevent engine damage, always adjust the
main jet before adjusting the jet needle.  If there
is any doubt — always choose the next richer jet
so there is a margin for safety. 
						

							Carburetor Adjustments & Tuning Tips
124Adjustments for Competition
Carburetor Minor Adjustments
The standard carburetor  settings are ideal for the
following conditions: sea level altitude, and 20°C
(68 °F) air temperature.  If your conditions are
different, you may need to adjust the carburetor
settings, using the tuning information on page
122 and 123. Confirm your settings are correct
before proceeding.
Minor 
Adjustments
1. Adjust the carburetor setting using the tuning
information on page 122 and 123.
2. When the engine is warm enough to run
without the choke, push the choke knob down
to its off position.
3. Adjust the pilot screw (1) 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 counter clockwise to
richen the mixture.
The minimum to maximum range of pilot screw
adjustments is 1 to 3 turns out from the lightly
seated position. If you exceed 3 turns out, the
next smallest slow jet is needed.  If you are under
1 turn out, the next larger slow jet is needed.
(1) pilot screw
Adjustment Procedure
1. Turn the pilot screw in until it is lightly seated
and record the number of turns. Turn the pilot
screw out the same number of turns.
2. Warm up the engine.
3. Adjust the engine idle speed (page 48).
4. Make 2 – 3 laps of a course with the standard
setting or corrected jetting (page 122). and
spark plug.  Note engine acceleration and
other engine conditions in relation to throttle
opening.  Verify carburetion by removing the
spark plug and reading its firing end (page
54). It may take more than two or three laps
to get a good spark plug reading with a new
spark plug.
WARNING
Gasoline is highly flammable and
explosive.  You can be burned or
seriously injured when handing fuel.
•Stop the engine and keep heat, sparks
and flame away.
•Handle fuel only outdoors.
•Wipe up spills immediately.
8. Turn the fuel valve OFF, and disconnect the
fuel line from the fuel valve.
9. Loosen the connecting and insulator band
screws.  Rotate the carburetor.  
(2) float bowl plug
(1)
(1)
(2)
5. Change carburetor settings or select suitable
carburetor jets, taking into consideration the
engine conditions and factors for temperature
and altitude (page 122).
6. Adjust the pilot screw as required.
7. If you’ve determined that the main and slow
jets must be changed, you must rotate the
carburetor and remove the float bowl plug.
10.  Remove the float bowl plug (2). 
						

							Carburetor Adjustments & Tuning Tips
Adjustments for Competition125 11. Change the main jet (3) and slow jet (4) as
required.
(3) main jet
(4) slow jet
12. Reinstall the float bowl plug.
13.If you’ve determined that the jet needle or clip
position of jet needle must be changed, you
must remove the carburetor top.14.Remove the fuel tank (page 30).
15. Rotate the carburetor to the left and remove the
carburetor top bolts (5) and carburetor top (6).
(5) carburetor top bolts
(6) carburetor top
16.Remove the jet needle holder (7), insert the
carburetor maintenance tool (8) to the jet
needle (9) from the throttle valve (10), and
remove it.
(7) jet needle holder
(8) carburetor maintenance tool
(9) jet needle 
(10) throttle valve
17.Change the clip position of jet needle or jet
needle as required.18.Reinstall the jet needle, jet needle holder
carburetor top and carburetor top bolts.
19.Install the fuel tank (page 31).
20.Rotate the carburetor and align the lug (11) on
the carburetor with the slot (12) on the intake
tube.
(11) lug (12) slot
21.Tighten the connecting and intake tube band
screws.
22.Connect the fuel line.
23.Start the engine.
If the engine idle speed is too high or too low
or engine is not idling, adjust the engine idle
speed (page 48).
24.Repeat steps 4 – 23 until the engine gives
maximum power with the correct spark plug
reading. It is always better to jet a little rich
than a little lean. It is advisable to record the
settings, course conditions, lap times, and
climatic and barometric conditions for future
reference.
(3)(4)
(3)(4)
(5)
(6)
(7)(8)
(10)
(9)
(11)
(12) 
						

							Carburetor Adjustments & Tuning Tips
126Adjustments for Competition
Spark Plug Reading
Refer to Spark Plugon page 54.
Check any jetting change by reading the spark
plug.  The following procedure is recommended.
You may not get an accurate reading if you
simply turn off the engine and pull the plug for
inspection.
Use a new spark plug.  Inspect the plug before
installing it.Using spark plugs with an improper heat range
or incorrect reach can cause engine damage.
Before removing the spark plug, clean the spark
plug area thoroughly to prevent dirt from
entering the cylinder.
Ride for 10 - 15 minutes before taking a plug
reading.  A new plug will not color immediately.
To obtain an accurate reading of a new spark
plug:
1. Accelerate at full-throttle on a straight.
2. Push the engine stop button and pull the
clutch lever in to release the clutch.
3. Coast to a stop.
4. Remove the spark plug.
5. Use a magnifying glass to inspect the spark
plug.  The porcelain insulator (1) around the
center electrode (2) should appear clean and
colorless with a gray ring around the center
electrode where it exits the porcelain.
Metallic specks indicate lean jetting that is
removing metal from the piston.  Black sooty
streaks on the porcelain indicate rich jetting.
NOTICE
Condition
Normal
Overheating(Lean)
Wet
(Rich)
Spark Plug Appearance
Dark brown to light tan
color with dry electrode
Light gray or white color
Wet or sooty
Mixture
correct
lean
rich
Improperly tightened spark plugs can damage the
engine.  If a plug is too loose, a piston may be
damaged.  If a plug is too tight, the threads may
be damaged.
NOTICE
(1) insulator (3) side electrode
(2) center electrode
Spark Plug Coloring Guidelines
Remember that in addition to improper jetting:
• A lean condition can be caused by air leaks in
the inlet tract or exhaust system, the passage
of too much air because of the use of the
wrong air cleaner, use of a less-restrictive
aftermarket exhaust system, or a hole or holes
(deliberate or unitentional) in the air box.
• A rich condition can be caused by a plugged
or dirty air cleaner, use of a more-restrictive
aftermarket exhaust system, a clogged spark
arrester, or excessive oil on the air cleaner.
Excessive smoking will occur.
(2)
(3)
(1)