If your Lawn-Boy mower won't drive when you press the self-propel lever on the handle, you need to troubleshoot why the mower doesn't engage into the self-propel function. In most cases, you can complete half the repair work while you investigate the issue. This means you will disassemble the necessary parts to see if the drive belt is broken. This makes replacing the belt simple, as the belt is likely the cause.
Disconnect the wire from the spark plug to keep the Lawn-Boy engine from starting while you work.
Place the mower on the side that doesn't have the air filter. This will prevent gas from flowing back into the filter.
Locate the screws on the transmission pulley cover and remove them with a screwdriver. Remove the cover and set it aside.
Inspect the belt that connects to the pulley on the rear axle. If the belt is missing, it broke and you must replace it. If the belt is frayed or has a lot of dry rot, the belt is slipping and won't create propulsion.