Yamaha TZ250M1 Owner's Service Manual page 265

7
SETTING
EC700000
TUNING
EC710000
ENGINE
EC711011
Carburetor setting
• The role of fuel is not only to produce motive
power but also to cool the engine and, in the
case of a 2-stroke engine, to lubricate it.
Therefore, too lean a mixture (of air and fuel)
tends to cause an abnormal combustion (i.e.,
detonation), whereas too rich a mixture
makes it difficult for the engine to develop its
full performance, with the result that in some
cases the spark plug may be fouled, causing
the engine to stop running.
• The richness of the air-fuel mixture required
for the engine will vary with atmospheric con-
ditions of the day and therefore, the settings
of the carburetor must be properly suited to
the atmospheric conditions (air pressure,
humidity and temperature).
• As a basic setting method, only the factory set
main jet is first changed to check for the dis-
coloration of the spark plug(s) and piston(s) at
full throttle in 6th and then the setting is deter-
mined at mid-open throttle.
$ Recording and storing the data on the set-
tings, weather conditions, road surface con-
ditions of the circuit, lap times, etc. will
enable quick setting under different condi-
tions at a later time.
EC712000
Atmospheric conditions and carburetor
setting
Air temp. Humidity
High High
Low Low
The reason for the above tendency is that the
richness or leanness of a fuel mixture depends
on the density of the air (i.e. the concentration
of oxygen in it).
7-1
TUN
Air
pressure Mixture Setting
(altitude)
Low
Richer Leaner
(high)
High
Leaner Richer
(low)