In what condition will a float and inlet needle open during operation?

The float and inlet needle in a carbureted engine system are designed to maintain the proper fuel level in the float chamber. When the fuel level drops, typically due to the engine consuming fuel during operation, the float will descend. This downward movement of the float opens the inlet needle, allowing fuel to flow from the fuel supply into the float chamber.

This mechanism is essential for ensuring that the engine continues to receive an adequate amount of fuel for optimal operation. If the fuel level reduces, the float's position changes and triggers the opening of the inlet needle, which in turn facilitates the replenishment of fuel in the float chamber. This function helps regulate fuel levels consistently, which is critical for maintaining engine performance.

The other conditions do not lead to the opening of the inlet needle in a typical operational scenario. For instance, an increase in fuel pressure generally causes the needle to remain closed to prevent over-fueling. When the engine is not running, there is no fuel consumption or change in fuel level that activates this mechanism. Hence, it is the reduction of fuel level that effectively opens the float and inlet needle.