One of the most common problems on a self-propelled lawnmower is the cable which runs from the handlebar down to the transmission box on the deck of the lawnmower. This cable simply succumbs to wear and tear over time, and can stretch out or warp. The hook mechanism at the end of the cable can also break or become unattached, making the self-propelled system unusable.
Find the end of the cable where it attaches to the self-propel control system on the handle bars. Follow it all the way down to the transmission, checking for tears, stretching, or a broken spring or hook at the end of the cable.
Remove the transmission cover and check for debris, dirt, wood chips, packed grass clippings, or other random items that may be clogging up the operation of the transmission. Also check for adequate lubrication. Transmissions needs to be lubricated in order to operate properly, and it's an often-overlooked maintenance item that can lead to a failure of the whole self-propelled system.
Wheels and Gears
Check for any items stuck in the wheels or gears that may block them from turning. Check for proper lubrication of the gears, as well, especially on the drive wheels. Note the condition of the wheels. If they are worn thin or broken, it will affect your overall cut. Wheels usually need to be replaced every 2 to 3 years.
Under the Deck
Turn the mower on its side and check for anything blocking the blade from turning correctly, and be sure the blade itself is sharp and attached to the proper torque ratings for your lawnmower. A loose blade will wobble during operation, which will affect your mower's performance. It holds a serious danger of becoming unattached and flying off the machine.
Other Common Mower Problems
Check the battery for full voltage and the starter for correct voltage as well. As with any lawnmower, a burned-out spark plug or drained battery will prevent a self-propelled mower from even starting. If your machine starts but seems to die out, sputter, or surge, then look into the fuel system. Check the line and the fuel filter for clogs or leaks. If you've used bad gas (it will have an off smell) then you may have varnish built up on the carburetor from the bad gas. Run a product such as SeaFoam through the system; if that doesn't work, you'll need to clean out the carburetor manually.