What happens when a back-suction type mixture control is in idle cutoff?

When a back-suction type mixture control is in idle cutoff, the float chamber is vented to a negative pressure area. This is a critical function of the fuel metering system where the intent is to disrupt fuel flow to the engine during idle conditions. By venting the float chamber to a negative pressure, the system effectively reduces the amount of fuel that can enter the engine, which is essential for maintaining an appropriate mixture for combustion when the engine is not under load.

This mechanism helps prevent excessive fuel delivery during idle, which could lead to fouling of the spark plugs or other performance issues. The negative pressure acts to draw fuel away from the engine, ensuring that only the necessary minimum fuel is present for operation. Thus, the back-suction type design effectively manages the fuel-air mixture at low engine speeds, contributing to operational efficiency and engine health.