Adjusting the air-fuel ratio on a lawnmower carburetor is a complex process and should be done by a qualified mechanic. generally, it involves adjusting the carburetor's idle speed, air screw, and main jet. if you are familiar with carburetor tuning, you will need a tachometer to measure the rpm of the engine and a vacuum gauge to measure the vacuum of the carburetor. other tools may also be required.