Before you can effectively use your engine without destroying it, it’s important to know it key features, their purpose and how to adjust them.
Needle Valves are tapered rods that are used to adjust the fuel mixture by restricting the fuel entering the carburetor. They come in a variety of sizes and shapes, and are designed for a specific carburetor. Needle valves that are long, with a slight taper, and fine threads, are less sensitive and easier to adjust. Others that are short, with a greater taper, and coarse threads, can be very sensitive to even a minor adjustment. Until you get a feel for your particular carburetor, restrict adjustments to 1/8 turn of the needle valve, and then evaluate the results.
Mixture Settings – The main and idle (and possibly mid-range) needle valves are used to adjust the fuel mixture in the carburetor, with the following nomenclature.
Rich – a mixture setting produced by increasing the amount of fuel to the engine. This is accomplished by turning the needle valve out (counter-clockwise) so the thinner portion of the needle valve will allow more fuel to enter the carburetor. It’s common to say the engine is running “rich”, or we can “richen” the engine.
Lean – a mixture setting produced by reducing the amount of fuel to the engine. The needle valve is turned in (clockwise) so the thicker portion of the needle valve body will increasingly restrict the incoming fuel. The engine can be running “lean” or we can “lean-out” the engine.
As a general rule, it’s better to have a slightly rich mixture for added cooling and lubrication, which will extend the life of the engine.