Most lawnmower carburettors are set to a default during manufacture, so it's not generally necessary to adjust a lawnmower carburettor. However, operating variables in fuel, temperature, altitude and load can require minor adjustments. Adjusting a carburettor can provide for richer or leaner air/fuel mixes. Adjust the carburettor through needles and screws on the carburettor.
Turn the engine off and rotate the fuel adjusting needle fully clockwise until it's finger tight. Don't put too much force on it, or it may break. Turn it out one turn.
Place the shift lever in the "Neutral" position and start the engine. Warm it up for five minutes. Place the throttle into the "Slow" or "Low" position and judge the idle. The idle shouldn't be so fast that the engine races, but it also shouldn't be too slow that the engine stalls or splutters. If there's a rev counter, use it and adjust the idle speed adjusting screw until the speed is 1200 RPM.
Set the idle needle by turning it clockwise until the engine speed slows. Then turn it counterclockwise 3/4 of a turn. This will give you optimum slow speed performance.
Test acceleration by moving the throttle from slow to fast. If the engine stutters or stops, turn the idle needle clockwise 1/8 of a turn.
Be sure the throttle cable is adjusted and install a clean air filter for best carburettor adjusting results.