Figure 14. Main Jet.

Principles of Gasoline And Diesel Fuel Systems
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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

low-speed 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 low-speed range is as

follows:

(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

the engine.