Carburetor Tuning In Practice - Suzuki RM85 Owner's Service Manual

CARBURETOR TUNING IN PRACTICE

In the previous section, basic principles of carburetor tuning have been discussed. Described in this section
are the bases for carburetor tuning required when coping with variations in air (oxygen) concentration.
VARIATION IN AIR (OXYGEN) CONCENTRATION AND CARBURETOR CONDITIONS
As the air, affected by the temperature, altitude and humidity, gets expanded or compressed, air (oxygen)
concentration varies accordingly. Each of these three factors affects the air in different ways, and the follow-
ing graphs show their effects respectively in terms of the oxygen concentration.
100
90
80
-10 0 10
10 30 50
ATOMOSPHERIC TEMPERATURE
In the above graphs, oxygen concentration is graduated on the vertical axis while the temperature, altitude
and humidity are on the horizontal axis respectively. Oxygen concentration is set 100% under the conditions
of 20 °C (68 °F), 0 m (0 ft) and 50% humidity. The standard carburetor setting is chosen to obtain the best
engine performance under these conditions.
The graph at the left shows that the oxygen concentration changes about 10% in the 0 to 40 °C temperature
difference, the one in the center shows about 20% change in the 0 to 2 000 m altitude difference and the
one at the right shows about 5% change in the 0 to 100% humidity difference. As for humidity, its normal
range is from 20 to 95%. Therefore the possible effect of humidity on the oxygen concentration is so little
that it can be disregarded. Consequently, we can say that the oxygen concentration varies by as much as
20% depending on the temperature and altitude under normal riding conditions. On the other hand, different
from the air, the fuel (gasoline) hardly changes in volume even when such environmental conditions change.
Therefore, increase in oxygen concentration will make the air/fuel mixture richer and decrease will make it
lean.
As the carburetor mixes gasoline and air, which are metered by each jet in varying proportions to suit throttle
opening, the air/fuel mixture is affected if the air concentration itself varies as described above. Then proper
engine power output can not be attained and, should the mixture become too lean, a piston seizure may
result. To compensate for such change in the air concentration, it is required to carry out carburetor tuning
beforehand. This requirement applies to all models of motorcycles and ATVs if they are used in areas where
temperature and altitude range widely. The next section describes the procedure of the above tuning in
detail.
20 30 40 [˚C] 500 1000 1500 2000 2500[m]
70 90 110 [˚F] 1600 3200 5000 6500 8200 [ft]
ALTITUDE
MACHINE TUNING
20 40 60 80 100 [%]
HUMIDITY
4-5
[%]
100
90
80