Engine tuning is the process of adjusting the carburetor mixture for optimum performance. Engine tuning is adjusting the fuel and air mixture entering the engine. The High speed needle(HSN) and low speed needle (LSN) are what determine fuel mixture settings. The low speed needle is used to meter the fuel burnt by the engine at idle and low RPM. The high speed needle is used to meter fuel burnt at moderate RPM to wide open throttle (WOT). The two needles in conjunction provide precise fuel/air mixture throughout the engine's power band.