Definition
A type of mixture control used in float-type carburetors that leans the fuel-air mixture by reducing the pressure above the fuel in the float chamber. A small line vents float chamber pressure toward the carburetor venturi, lowering the pressure differential that pushes fuel through the main metering jet, so less fuel is drawn into the airstream.
Plain English
A way of leaning the engine by lowering the air pressure sitting on top of the fuel in the carburetor's fuel chamber. With less pressure pushing the fuel out, less fuel mixes with the incoming air, making the mixture leaner.
Context Anchor
Seen in aircraft carburetor descriptions, mixture-control system maintenance, and troubleshooting of engines that run too rich or too lean.
Derivation
Back-suction' describes the method: instead of restricting the fuel flow directly, the system applies suction to the back side of the fuel — the air space above it in the float chamber — to weaken how hard the fuel is pushed into the venturi.
Why Pilots Care
Allows controlled leaning at altitude to avoid fuel waste, plug fouling, and power loss from an overly rich mixture.
Intuition Check
Do not read “back-suction” as fuel being sucked backward. Here it means air pressure is reduced in the fuel chamber so less fuel flows forward into the engine.
Example Sentence 1
The float-type carburetor on the trainer uses a back-suction mixture control to lean the engine at altitude.
Example Sentence 2
The mechanic verified that the back-suction mixture control valve opened smoothly when manifold suction was applied.