What causes a single diaphragm acceleration pump to discharge fuel?

The correct answer is that an increase in manifold pressure when the throttle is open causes a single diaphragm acceleration pump to discharge fuel. In a fuel system, particularly in carburetor designs, the acceleration pump is a critical component that helps to improve engine performance during rapid increases in throttle position.

When the throttle opens, manifold pressure increases, and this change creates a pressure differential that acts on the diaphragm of the acceleration pump. As the diaphragm moves, it forces fuel into the carburetor, which helps to prevent the engine from stalling and improves throttle response. This is especially important during acceleration, as it provides the necessary fuel to enrich the air-fuel mixture when the engine demands more power.

In contrast, conditions like a decrease in manifold pressure or a reduction in engine temperature do not directly contribute to the operation of the acceleration pump in the same way. An increase in throttle position alone does not guarantee fuel discharge without the corresponding increase in manifold pressure to actuate the diaphragm effectively. Thus, the interaction between manifold pressure and throttle position is key to the acceleration pump's function.