Suzuki RM125P

4-6 MACHINĘ TUNING

CARBURETOR TUNING IN PRACTICE

In the previous section. basie 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 temperaturę, altitude and humidity, gets expanded or compressed, air (oxygen) concentration varies accordingly. Each of these three factors atfects the air in different ways, and the follow-ing graphs show their effects respectively in terms of the oxygen concentration.

tions 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 °C (32 ^::F) to 40 °C (104 °F) temperaturę difference, the one in the center shows about 20 % change in the 0 to 2 000 m (6 562 ft) altitude difference and the one at the right shows about 5 % change in the 0 to 100 % humidity difference. As for humidity, its normal rangę 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 temperaturę and altitude under normal riding conditions. On the other hand, different from the air, the fuel (gasoline) hardly changes in volume even when such environ-mental conditions change. Therefore, inerease 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 temperaturę and altitude rangę widely. The next section describes the procedurę of the above tuning in detail.