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The Marvel Schebler Carburetor Idle Circuit

If you look down into the barrel of a Marvel Schebler carburetor, you see three small holes in the side of the barrel about the size of pencil points. The top hole is the idle nozzle and the other two holes are bleed holes for the idle circuit. When the throttle plate is completely closed, the two bleed holes are under the throttle, on the high pressure side (atmospheric). These become air bleed holes which further emulsify or vaporize the fuel and air coming up the passageway on its way to the idle jet.

Down inside the carburetor there is an idle jet with holes drilled in the side for air reference. The idle jet mixes in some air into the idle fuel right from down at the bottom of the carburetor. This is a long way to draw the idle fuel. The air-bleed holes provide a little more air pressure to help push the fuel up. As you open the throttle you get a little more air into the engine didn't have those air/fuel-bleed holes it would have a flat spot off idle. The engine would falter. Some aircraft do that. The Lycoming engine problems.

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided is not intended to supercede or supplement the F.A.A. approved maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.