Marvel Schebler Idle System

How it works

This illustration shows the Marvel Schebler’s idle circuit.

The idle system controls the flow of fuel at idle speed and at slow speeds until the throttle is opened wide enough to allow the power fuel feed system to function.

When the throttle valve (6) is in the idle position the edge of the valve is between the primary idle orifice (7) and the secondar idle orifice (8). With the valve in this position the air pressure (manifold vacuum) at the primary idle orifice is lower than the air pressure in the fuel bowl chamber (9) and fuel is forced from the fuel bowl (1) into the idle fuel passage (10).

AS the fuel travels through the idle fuel passage it passes through the metering orifice of the idle jet (11) to the point where it is combined with air entering through the idle adjusting needle seat (12). The mixing of air with gasoline helps to atomize the fuel and this process is repeated at the secondary idle orifice as the fuel travels through the idle fuel passage. As this rich mixture of fuel and air emerges from the primary idle orifice it is reduced to correct proportions by the air which passes around the throttle valve since this valve must be slightly open to permit the engine to idle. The resultant mixture is correct for operating engine at idle speed, provided the idle adjustment needle (13) is properly adjusted.

As the throttle valve is slowly opened from the slow idle position it gradually subjects the secondary idle orifice to intake manifold vacuum, and the secondary idle orifice no longer bleeds air to the idle fuel passage but feeds an additional quantity of fuel into the engine. This is proper since the throttle valve is now open wider and will admit a greater amount of air to blend with this additional fuel to maintain the correct proportions of fuel and air for the engine.

As the throttle valve is opened still woder, the idle fuel delivery begins to fade out, however, the throttle vavle at this point is far enough open for the power fuel feed system to begin functioning.

Rough Idle

  • A vacuum leak at the intake or carburetor mounting would cause low vacuum at the idle circuit giving you a rough idle.
  • An incorrectly adjusted, or damaged, idle mixture screw will cause a rough idle. Find replacements here.
  • A clogged, or partially clogged idle fuel passage #10 will cause a lean condition at idle. Use nylon brushes to clear debris from the passages.
  • Either the secondary idle orifice, or primary idle orifice being clogged will cause a rich condition
Carburetor Parts

Updated on 05/24/2022

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