Lawn mower blades lose their sharp edge during the mowing season, and the blade also can sustain damage if it hits objects in the lawn. If the blade is damaged, the motor will vibrate severely. Replacing a lawn mower blade isn't difficult and should be about a 30-minute job.
Remove the sparkplug wire from the engine. The sparkplug wire is a heavy rubber cord attached to the end of the sparkplug. Most models of lawn mowers have the sparkplug mounted on the engine, facing the push handles.
Remove the fuel filling cap. Tip the lawn mower over so it can drain the gasoline into the gasoline container. Replace the gas filling cap.
Lay the lawn mower on its side, preferably with the carburetor facing upward. This will keep the engine oil from draining into the carburetor. The push handles will stabilize the lawn mower from tipping back over, to the upright position.
Put on your leather gloves. All lawn mower blades may have extremely sharp edges, even the damaged ones.
Wedge the 10-inch-long 2-by-4 board between the blade and the inside of the lawn mower. This will keep the blade from moving when loosening the large nut at the end of the engine shaft.
Fit the end wrench or socket over the large nut that holds the blade onto the engine shaft. The size of the nut will vary depending on the manufacturer. Typically the nut will range from ¾ inch to 7/8 inch.
Turn the wrench or socket counterclockwise. You may have to replace the 2-by-4 back into position, while you attempt to loosen the nut.
Remove the nut from the shaft. Pull the large washer from the shaft that sits just below the nut. Retain both of these parts to one side. Make a note as to the direction of the sharp edges of the blade and how it fits to the engine shaft. Remove the blade from the shaft.
Replace the lawn mower blade in the reverse of removal. Allow the lawn mower to sit for 20 minutes, after it has been set upright. Check the engine oil level. Add oil if needed.
Add fresh gasoline to the fuel tank. Reattach the sparkplug wire to the sparkplug. Start the engine. The engine may smoke for a few seconds. This is normal as some of then engine oil may have entered the carburetor. The white smoke will clear in a few seconds.