70
All jetting is based on
*
Standard jetting
*
Unmodified Engine
TEMPERATURE
ALTITUDE
10,000 ft
to
7,500 ft
PS:
SJ:
JN:
NC:
MJ:
1 1/2
42
NCYR
4th
165
1 1/2
42
NCYR
4th
162
1 1/4
42
NCYR
3rd
162
1 1/4
42
NCYR
3rd
160
1 1/4
40
NCYR
3rd
160
1
40
NCYR
3rd
158
1
40
NCYR
3rd
158
7,499 ft
to
5,000 ft
PS:
SJ:
JN:
NC:
MJ:
1 1/2
42
NCYR
4th
168
1 1/2
42
NCYR
4th
165
1 1/2
42
NCYR
4th
162
1 1/4
42
NCYR
3rd
162
1 1/4
42
NCYR
3rd
160
1 1/4
40
NCYR
3rd
160
1
40
NCYR
3rd
158
4,999 ft
to
2,500 ft
PS:
SJ:
JN:
NC:
MJ:
1 3/4
42
NCYR
5th
168
1 1/2
42
NCYR
4th
168
1 1/2
42
NCYR
4th
165
1 1/2
42
NCYR
4th
162
1 1/4
42
NCYR
3rd
162
1 1/4
42
NCYR
3rd
160
1 1/4
40
NCYR
3rd
160
2,499 ft
to
1,000 ft
PS:
SJ:
JN:
NC:
MJ:
1 3/4
45
NCYR
5th
170
1 3/4
42
NCYR
5th
168
1 1/2
42
NCYR
4th
168
1 1/2
42
NCYR
4th
165
1 1/2
42
NCYR
4th
162
1 1/4
42
NCYR
3rd
162
1 1/4
42
NCYR
3rd
160
999 ft
to
Sea Level
PS:
SJ:
JN:
NC:
MJ:
Legend
PS: Pilot Screw opening from fully seated
SJ: Slow Jet
JN: Jet Needle
NC: Needle Clip position
MJ: Main Jet
General changes due to temperature and altitude
Condition
Cold temperature
Warm temperature
Dry air
High humidity
High altitude
Mixture will be
Lean
Rich
Lean
Rich
Rich
Adjust to
Richen
Lean
Richen
Lean
Lean
Component affected
Main jet (jet needle stage)
1 3/4
45
NCYR
5th
170
1 3/4
45
NCYR
5th
170
1 3/4
42
NCYR
5th
168
1 1/2
42
NCYR
4th
168
1 1/2
42
NCYR
4th
165
1 1/2
42
NCYR
4th
162
1 1/4
42
NCYR
3rd
162
FAHR.
-21
°
〜
0
°
-1
°
〜2
0
°
19
°
〜4
0
°
39
°
〜6
0
°
59
°
〜8
0
°
79
°
〜10
0
°
99
°
〜12
0
°
ST
ANDARD
JETTING
5. CARBURETOR ADJUSTMENT
TUNING FOR SPECIAL CONDITIONS
Once 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.
NOTE:
•
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 umber, when: it is very humid or
raining, or it is hotter than 113 degrees F.
NOTE:
•
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 air leaks, blocked or
restricted exhaust or fuel systems, or a dirty air cleaner.
SPARK PLUG COLORING INDICATIONS
Normal
Overtheating
Wet
―
Mixture is Lean*
Mixture is Rich
Light gray or white color
Wet and sooty
Dark brown to light tan color with
dry electrode
Remember that in addition to improper jetting:
*
A lean condition can be caused by air leaks in the inlet tract.
The same simple throttle opening identification system used in the Break-In section
of this manual can also be used to accurately identify where carburation problems
occur, should they arise (See page 3).
