How to Tell When Spark Plugs Are Bad in Riding Lawnmowers

Check your spark plugs for a smoother ride on the lawnmower.

Checking your riding lawnmower's engine spark plugs at the start of the season and replacing them after every 100 hours of operation helps keep the equipment running smoothly. A rough-running or hesitant engine could be caused by a misfiring spark plug, which can quickly fail. The engine likely will not start at all if the spark plug is dead. Remove the plug with a spark plug socket wrench to determine if it should be replaced.

Things You'll Need

• Spark plug socket wrench
• Plug spray cleaner
• Wire brush
• Plug-gap tool

Instructions

1. Disconnect the wire lead from the spark plug. The round clip on the end of the wire pulls straight off the plug's terminal stud.

2. Unscrew the plug with the socket wrench by turning counterclockwise.

3. Spray plug cleaner on the L-shaped ground wire and center wire, which are at the opposite end of the terminal stud.

4. Scrub the end of the plug with a wire brush and inspect the exterior for cracks or other obvious damage in the ceramic insulator. If any part of the plug is cracked, scorched or blackened with burn marks, the plug is bad and must be replaced.

5. Adjust the plug gap to .03 inches, using the plug-gap tool. This is a standard plug gap for many riding mower engines. Consult the owner's manual for your particular mower to confirm the recommended gap.

6. Screw the cleaned plug into the socket and reconnect the lead wire to test operation. If the engine still hesitates or misfires, replace the old plug with a new one.

Tips & Warnings

• Briggs & Stratton, which is one of the world's largest manufacturers of small engines, recommends spark plug replacement after every 100 hours of engine operation.

• Allow the mower to cool down before touching the hot spark plug.

References

• Briggs & Stratton: Choosing Spark Plugs
• The Grayline: Change Your Sparkplugs Regularly
• Briggs & Stratton: Cleaning and Inspecting a Spark Plug