TUN
9-1
TUNING
ENGINE
CARBURETOR SETTING
• The role of fuel is to cool the engine, and in
the case of a 2-stroke engine, to lubricate
the engine in addition to power generation.
Accordingly, if a mixture of air and fuel is
too lean, abnormal combustion will occur,
and engine seizure may result. If the mix-
ture is too rich, spark plugs will get wet with
oil, thus making it impossible to bring the
engine into full play or if the worst comes to
the worst, the engine may stall.
• The richness of the air-fuel mixture required
for the engine will vary with atmospheric
conditions of the day and therefore, the set-
tings of the carburetor must be properly
suited to the atmospheric conditions (air
pressure, humidity and temperature).
• Finally, the rider himself must make a test
run and check his vehicle for conditions
(pick-up of engine speed, road surface con-
ditions) and for the discoloration of the
spark plug(s).
After taking these into consideration, he
must select the best possible carburetor
settings.
* It is advisable to make a note of settings,
atmospheric conditions, road surface condi-
tion, lap-time, etc. so that the memorandum
can be used as a reference useful for
future.
ATMOSPHERIC CONDITIONS AND
CARBURETOR SETTINGS
The air density (i.e., concentration of oxygen in
the air) determines the richness or leanness of
the air
/
fuel mixture. Therefore, refer to the
above table for mixture settings.
That is:
• Higher temperature expands the air with its
resultant reduced density.
• Higher humidity reduces the amount of oxy-
gen in the air by so much of the water vapor
in the same air.
• Lower atmospheric pressure (at a high alti-
tude) reduces the density of the air.
Air temp.
Humidity
Air
pressure
(altitude)
Mixture
Setting
High
High
Low (high)
Richer
Leaner
Low
Low
High (low)
Leaner
Richer
ENGINE
