RBS Low Speed Circuit

Fuel for idle and the early part of throttle operation is metered through the low-speed circuit. The low-speed jet is pressed into place within a passage in the casting to prevent possible damage. It cannot be removed but it can be cleaned by blowing compressed air through the step-up rod jet or through either of the idle passage air bleeds located in the carburetor bore. We also sell a set of nylon brushes that are extremely helpful during the cleaning process.

Gasoline from the fuel bowl is metered as it enters the lower end of the jet and flows up through the tube where air metered through the “by-pass” mixes with the fuel.

Both fuel and air then pass through the “economizer” and on to the “idle bleed” where a second metered amount of air is introduced. This air-fuel mixture is discharged into the intake manifold through the “idle port” and the idle adjustment screw port.

An idle adjusting screw is used for trimming the idle mixture to individual engine requirements for satisfactory idle.

The idle adjustment screw controls the amount of air-fuel mixture admitted to the intake manifold. Turning the screw outward increases the amount of mixture; turning the screw toward the seat decreases the amount of mixture.

During curb idle, only a small amount of the idle port is exposed to intake manifold vacuum.
As the throttle is opened, more of the idle port is exposed to manifold vacuum to allow a calibrated increase in the amount of air-fuel mixture admitted to the engine.

The idle jet, economizer and both air bleeds are calibrated and pressed into place. All must be free from dirt.

On some applications the idle air bleed consists of a pressed-in tube which extends upward inside the air horn to a point above the closed choke valve. The choke valve is notched to accommodate the tube.

Emission laws require use of an idle adjusting screw with limited adjustability. This allows for proper idle adjustment while assuring the emission limits will not be exceeded.

To meet these requirements, later models use a limiter cap to prevent over rich idle adjustment.

Updated on 06/22/2022

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