This section contains only tune-up adjustment procedures for carburetors. Descriptions, adjustments, and overhaul procedures for carburetors can be found in the Fuel System section.
When the engine in your Subaru is running, the air-fuel mixture from the carburetor is being drawn into the engine by a partial vacuum which is created by the movement of the pistons downward on the intake stroke. The amount of air-fuel mixture that enters into the engine is controlled by the throttle plates in the bottom of the carburetor. The throttle plates are connected by the throttle linkage to the accelerator pedal in the passenger compartment of the Subaru. When you depress the pedal, you open the throttle plates in the carburetor to admit more air-fuel mixture to the engine.
When the engine is idling, it is necessary to have the throttle plates open slightly. To prevent having to hold your foot on the pedal when the engine is idling, an idle speed adjusting screw is added to the carburetor linkage.
The idle adjusting screw contacts a lever (throttle lever) on the outside of the carburetor. When the screw is turned, it either opens or closes the throttle plates of the carburetor, raising or lowering the idle speed of the engine. This screw is called the curb idle adjusting screw.
A special mixture circuit is incorporated into the carburetor to enable the engine to run smoothly at idle. This circuit is controlled by the mixture screw, which determines the amount of fuel admitted at idle.
IDLE SPEED & MIXTURE
- Run the engine and allow it to reach normal operating temperature.
- Stop the engine and connect a tachometer in accordance with the manufacturer's instructions.
- Then, do one of the following:
- On ff-1 and 1300G models which have air injection (air pump), disconnect the air hoses from the air distribution manifolds. Plug the hoses and the manifold openings.
- On other models, disconnect and plug the hose that runs to the distributor vacuum retard unit, if equipped.
- Models that have a secondary air cleaner, or purge valve and hose: Remove and plug the hose that connects to the engine.
- Remove the air cleaner by disconnecting the emission control system hoses from it, unfastening the wing nut(s), and removing the screws which secure it to its mounting brackets.
- Check proper idle speed in Tune-Up Specifications chart and adjust to that setting by turning the throttle adjusting screw.
- Continue adjusting the idle, this time by means of the throttle adjusting screw and the idle mixture adjusting screw, until a reading of 50 rpm above the proper idle setting is attained.
- Then, turn the idle mixture adjusting screw clockwise until the idle speed drops to the figure given in the Tune-Up Specifications chart.
- Disconnect the tachometer. Reconnect any hoses that were removed. Install the air cleaner.