How to Tell if a Lawn Mower Spark Plug is Bad

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Due to carbon build-up and a weakened electrode, a small engine spark plug will degrade over time. If the engine on your lawn and garden equipment won’t start, or it runs poorly, chances are the spark plug is worn out. Fortunately, the spark plug is one of the easiest engine components to replace. This step-by-step guide will teach you how to tell if a lawn mower spark plug is bad along with tips for testing other garden equipment.

How does the spark plug work?

If you’re wondering how to check power equipment spark plugs, first you need to know how they work. All gas-powered lawn mowers and snowblowers, along with hand-held products like chainsaws, string trimmers, and leaf blowers, use a spark plug to ignite a mixture of fuel and air to start and run the equipment’s engine. The ignition process relies on a suitable balance of fuel and air being drawn through the carburetor into the engine, a properly operating ignition coil, and a functioning spark plug. When the engine’s starter rope is pulled, the starter engages a drive cup on a flywheel and rotates a crankshaft which moves a piston up and down within the engine cylinder. As the piston travels downward, it draws the fuel and air into the engine’s combustion chamber. The flywheel has permanent magnets built into it, and as it moves past the ignition coil a magnetic field is created which induces electricity, allowing the ignition coil to send a high-voltage pulse to the spark plug. When the piston reaches the top of the cylinder, the spark plug ignites the compressed fuel and air mixture, forcing the piston back down. The spent fuel exits an exhaust port, fresh fuel enters the intake port, and the combustion process is repeated to keep the engine running.

Garden equipment & lawn mower bad spark plug symptoms

  • The engine requires repeated attempts to start or the engine won’t start at all.
  • The engine misfires or runs rough.
  • The engine starts, but stalls shortly after.
  • There is a noticeable increase in fuel consumption during normal equipment use.
  • The spark plug itself appears damaged or corroded with carbon build-up or rust.

Use an ignition tester to test a bad lawn mower spark plug

If you’re uncertain whether the spark plug, the ignition coil, or another part of the engine’s electrical system is at fault, you can use a small engine ignition tester to test the components:

  • Remove the ignition coil wire and boot from the spark plug.
  • Attach the ignition tester clip to the spark plug and insert the opposite end of the tester into the boot.
  • Attempt to start the engine but stand clear of the tester or any other electrical component to avoid a shock.
  • If a spark jumps across the terminals in the tester window (or the engine starts), the spark plug and ignition coil are working properly.
  • If you do not see a spark, unclip the tester from the spark plug and attach it to a good grounding location like a bolt on the engine valve cover.
  • Attempt to start the engine again.
  • If there is now a spark visible in the tester window, the spark plug is likely defective and will need to be replaced.
  • If there is still no spark in the tester window, the ignition coil may have failed or there may be another problem with the equipment’s electrical system.

Spark plug installation tips

  • You should use an appropriate-sized socket or a dedicated spark plug wrench to help loosen an old spark plug before unthreading it from the engine cylinder.
  • Before you install a new spark plug, confirm that the “spark gap”, the space between the end of the side ground electrode and the tip of the center electrode, is at manufacturer’s specifications. You may need to adjust the side ground electrode to decrease or increase the gap.
  • After threading the new spark plug into the engine cylinder, avoid damaging the cylinder threading by over-tightening.
  • As a general guideline, it is recommended to replace the spark plug once a year.

Replace your power equipment spark plugs at Repair Clinic

If you are observing lawn mower bad spark plug symptoms, or you’re having issues with any other lawn equipment, we’ve got you covered. Repair Clinic stocks the appropriate spark plugs that fit the engines on all outdoor power equipment, including Craftsman, Honda, Toro, Echo, Poulan Pro, Ryobi, and Troy-bilt models. To find the right spark plug for your equipment, enter the full model number of the equipment, or the equipment’s engine, in the Repair Clinic website search bar, then select “Spark Plug” from the part category filter. While Repair Clinic carries Champion, NGK, Briggs & Stratton, MTD, and Kohler brand spark plugs, you’re going to want to make sure you’re selecting the exact spark plug that matches the engine used by your lawn and garden equipment.

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