Engine compression is the amount of internal air pressure that the combustion chamber can contain. Low engine compression means low power for any small engine. In many cases, the lawn mower engine may also be using a lot of oil. Testing a lawn mower engine for compression requires a special compression tester, which fits inside the spark plug hole. Once the tester is installed, a few pulls to the start cord will tell you the engine's condition.
Allow the lawn mower to fully cool before conducting any testing. It is easier to perform the test when the spark plug is cool to the touch.
Remove the spark plug wire from the spark plug. Generally the spark plug is located on the side of the engine that faces the handles on a push mower. Riding lawn mowers may have two spark plugs. Consult the manufacturer's engine specifications for the locations. In the case of multiple spark plugs, remove all wires from the spark plugs so the engine will not start. Test one spark plug hole at a time.
Attach the correct size socket to a ratchet. Various sizes of spark plugs are available on many models of engines. Consult the manufacturer's specifications for your particular mower and the correct size of socket.
Place the socket and ratchet assembly over the end of the spark plug. The socket must fit snugly over the metal hex head on the sparkplug. Turn the ratchet in a counterclockwise direction to loosen. Finish removing the spark plug with your hands.
Insert the threaded end of the compression tester inside the spark plug hole. Turn the threaded plug in a clockwise direction. Tighten with your hands. Many compression testers for small engines only require hand tightening and no tools are needed.
Face the dial gauge of the compression tester towards you. Pull on the lawn mower's starter cord a few times. Read the gauge dial. Most small engines have a broad range of engine compression. A good reading will fall between 80 and 140 PSI (pounds per square inch). A pressure reading below 80 PSI may indicate a problem with the engine. Consult the owner's manual or engine specifications for your particular model.