Needle valves are like a garden hose spigot. Open the spigot for more water or close it for less.

The engine will continue to hurry up as you close the needle valve till you go the optimum gasoline setting.

using a plumber in needle valves Gauge for work

Needle valve allows precise regulation of flow, although it is generally only capable of relatively low flow rates

A way to remove some of the danger of tuning the engine is to buy a relocating kit for the carburetor needle.

The needle valves can be supplied with a number of different operators

Needle valves are often designed with a metal needle (generally brass, bronze, or stainless or other alloys of steel) and an elastomeric seat (generally PVC, CPVC, PTFE, or a wide range of brand name

The needle valve is also placed in the bypass of the turbine inlet valve. That valve is normally butterfly or spherical type and not prepared to open against all column water pressure.