How to Troubleshoot a Rough-Idling Lawn Mower


The idle on a lawn mower is the natural speed at which the engine runs when the throttle has not been shifted into high gear. An idle speed that is too high causes unnecessary wear and tear and the mower's engine. An idle speed that is set too slow will make the lawn mower run sluggishly and even make it difficult to start. There are several things that can cause a mower to idle poorly, including bad gas or a dirty air filter. It is also possible that the idle-speed screw needs to be adjusted.

Things You'll Need

  • Gasoline
  • Phillips screwdriver
  • Siphon tube with hand pump
  • Open the gas cap on your push mower. Tilt the lawn mower to empty the gas tank. Set the mower upright and fill the gas tank with new gas. For riding mowers, use a siphon tube with a hand pump to drain out the gasoline. Start the mower and determine if the idle is running smoothly.

  • Remove the screw holding the air filter cover in place. Remove the mower's air filter. Replace it with a new one if it is dirty and worn.

  • Start the lawn mower and allow it to run for three minutes.

  • Turn the mower off and locate the idle-adjustment screw on the back of the engine. Turn it all the way to the right, using the Phillips screwdriver.

  • Turn the screw to the left one-half turn. Start the lawn mower. If the idle is still too high, turn the adjustment screw one-eighth turn counterclockwise. If the mower idles too low, turn the adjustment screw one-eighth turn clockwise.

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