Adjustment Needles

The adjustment needle screws are very simple in their design, they are very much like standard screws. When you tighten a needle screw it restricts the amount of fuel that is allowed to flow into the carburetor. When you loosen the screw you allow more fuel to flow into the carb.

Most nitro RC engine have two needle adjustment screws, one to control the ait/fuel mixture at low RPM and one to control it at high RPM, it's possible that a lot of engines only have one needle adjustment screw for simplicity's sake, but it's difficult to get an accurate engine tune using these.

Be careful not to tighten the screw too much as this can damage both the carburetor and the needle adjustment screw.

Typically you made adjustments in small measurable increments, then take the car out for a lap and see how it feels, bring the RC car back in and make at most a 1/8th adjustment and go out again. Using this method you can quickly see if your settings are improving the tune or making it worse.