All engines have a high speed needle. Thats the large one with the knurled knob. The low speed mixture is controlled by one of two methods. One method is an air bleed hole that meters the flow of air into the carb when the barrel is approaching fully closed. The second method is a low speed needle valve. The low speed mixture adjustment is typically located inside the throttle control arm attachment to the barrel of the carb. Is usually a small screw, and should only be adjusted in very small increments (1/8 turn) and should only be adjusted when the engine is not running.

Check your engine manual.

Brad