What does the high-speed needle valve in a two-piece flow-jet carburetor control?

The high-speed needle valve in a two-piece flow-jet carburetor primarily controls the fuel entering the main discharge nozzle. This is a critical function because it directly influences the air-fuel mixture that the engine receives during high-speed operation. By fine-tuning the amount of fuel that reaches the nozzle, the high-speed needle ensures that the engine can perform optimally under various conditions, particularly when more power is needed.

When the engine operates at higher speeds, having the right balance of air and fuel is essential for efficient combustion and performance. An improper fuel flow can lead to issues such as poor acceleration or engine misfires. Hence, the high-speed needle is instrumental in maximizing engine power and efficiency by regulating fuel delivery in higher RPM ranges.

While the high-speed needle does affect top engine speed under no load, its primary role is more closely associated with controlling the fuel flow at the discharge nozzle, making it a more focused answer. The other options touch upon aspects related to carburetor function, but their interpretations do not fully encapsulate the primary role of the high-speed needle valve like the main discharge nozzle does.