PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 1/TASK 2
FIGURE 14. MAIN JET.
b. Idle and LowSpeed System.
(1) Purpose. The idle and lowspeed system provides the proper air
fuel mixture when the engine is at idle and during other periods of small
throttle opening. During these periods, there is not enough air flowing
through the throat to make the discharge nozzle work.
(2) Operation (figure 15 on the following page). The idle and the
lowspeed portions of the system are really separate circuits in operation.
The idle circuit sustains the engine at idle. As the throttle begins to
open, the effectiveness of the idle circuit falls off gradually as the low
speed circuit takes over. The transition between the two circuits is a
smooth one. Operation from engine idle through lowspeed range is as
(a) The throttle valve is almost closed at engine idle. This
creates a high vacuum in the area of the carburetor under the throttle
valve. This high vacuum causes atmospheric pressure to push gasoline
through the idle port from the float bowl. The gasoline mixes with the air
that is drawn in around the throttle valve. The mixture then is drawn into