The carburetor on a Ryobi grass trimmer includes three adjustments. The low speed adjustment controls the highest possible idle speed. The idle adjustment controls the idle speed while not turning the trimmer head. The high speed adjustment controls fuel consumption and overall engine performance. When making adjustments, always turn the knobs in small increments, and do not force them past their internal stops.
Locate the three adjustment knobs for the Ryobi carburetor. The three knobs are to the left of the pull rope and include a high speed, low speed and idle adjustment knob. Each knob has a letter designation near the side to identify the knob: "H" for the high speed, "L" for low speed and "T" for idle adjustment.
Turn both the high speed and the low speed adjustment knobs to the midway point between the left and right stops with a flathead screwdriver. Rotate each one as far as it will go left, and then rotate as far right as possible. Determine the midway point, and adjust the knob to the midway point.
Start the Ryobi grass trimmer, and let the engine warm up for 5 minutes. If the trimmer head is rotating, turn the idle screw counterclockwise until the trimmer head stops.
Find the highest possible idle speed with the trimmer running. Rotate the low speed adjustment knob clockwise and then counterclockwise. Listen and feel for the highest speed possible without choking and cutting off the engine. Once you find the highest idle speed, turn the low speed adjustment screw ¼ turn counterclockwise.
Adjust the idle speed adjustment until the trimmer head does not rotate and the trimmer engine still runs. The engine may stop while you are making this fine tune adjustment. Start the engine again, and continue tuning the idle adjustment screw as necessary.
Press the throttle trigger, and turn the high speed knob counterclockwise until the engine begins to run unevenly. Let go of the trigger, and allow the engine to idle. Press the trigger again, and slowly turn the high speed adjustment until the engine starts to run evenly.
Make any necessary fine tune adjustments to the idle speed with the idle adjustment knob.