Follow the accelerator cable and see where it's connected to the carburettor; a screw adjuster near this point is the slow running adjustment, and winding this screw in increases the tick-over speed and vice versa. Further away from this screw, lower down on the carburettor is another screw, and this is the mixture screw; turning this screw outwards (anti-clockwise) enriches the mixture and (again) vice versa.
As for tuning the engine, assuming the ignition system is correctly set (points gap, plug gaps, timing correctly set & condenser in good order) and the engine's warmed up, use the tick-over adjustment screw to raise the tick-over speed to about 1,500 RPM. Now turn the mixture screw in (not forcing it) and then unscrew it by about 2 to 2 1/2 turns. Carefully move the mixture screw in and out until the engine sounds smooth and is not "hunting" (revving up and then slowing down) then reduce the tick-over speed by unscrewing the tick-over adjusting screw until the engine idles happily at the correct speed. Use a tachometer to check this.
