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Servicing the Spark Plug

How is the engine's spark plug serviced?

The electrodes on a spark plug must be clean and sharp to produce the powerful spark required for ignition. The more worn or dirty a spark plug, the more voltage - and the greater the tug on the rewind - required to produce an adequate spark.

If you haven't tuned your engine recently and have to tug repeatedly on the rewind to start the engine, a damaged spark plug may be the culprit. Inconsistent firing, known as spark "miss," can result in sluggish engine operation and poor acceleration. A damage spark plug may also cause excessive fuel consumption, deposits, on the cylinder head and oil dilution.

Luckily, a spark plug is one of the easiest engine components to repair and an inexpensive one to replace. And your standard socket set may already include the most important tool - a spark plug socket.

Many engine performance problems can be linked to maintenance issues such as stale fuel, dirty air filter, fouled spark plug, and deteriorated oil.

A great way to help avoid these problems would be to perform an annual tune-up using a Briggs & Stratton engine Maintenance Kit.


This FAQ covers the essentials of spark plug inspection and replacement. It shows you how to use a spark tester and how to adjust and clean a spark plug that is worn but still serviceable. Just remember, you can't go wrong by replacing it.

Cleaning And Inspecting A Spark Plug

  1. Disconnect the spark plug lead. Then, clean the area around the spark plug to avoid getting debris in the combustion chamber when you remove the plug.

  2. Remove the spark plug using a spark plug socket.

  3. Clean light deposits from the plug with a wire brush and spray-on plug cleaner. Then, use a sturdy knife if necessary to scrape off tough deposits. NOTE: Never clean a spark plug with a shot blaster or abrasives.

  4. Inspect the spark plug for very stubborn deposits, or for cracked porcelain or electrodes that have been burned away. If any of these conditions exists, replace the spark plug.

  5. Use a spark plug gauge to measure the gap between the two electrodes (one straight, one curved) at the tip of your spark plug (image A)Spark Plug Gap 51A
    Image A
    . Many small engines require a .030" gap. Check our Identifying the Correct Spark Plug and Gap FAQ for the specifications for your model or consult with your local Briggs & Stratton Authorized Dealer. If necessary, use a spark plug gauge to adjust the gap by gently bending the curved electrode. When the gap is correct, the gauge will drag slightly as you pull it through the gap.

  6. Reinstall the plug, taking care not to over-tighten (15 ft. lbs. MAXIMUM ). Then, attach the spark plug lead.

Checking Ignition With A Spark Tester

A spark tester offers an inexpensive, easy way to diagnose ignition problems. If you find a problem, remove and inspect the spark plug. Replace the spark plug if you find evidence of wear or burning at the spark plug tip. Spark plugs are inexpensive and a new one may solve the problem.
  1. Connect the spark plug lead to the long terminal of your tester and ground the tester to the engine with the tester's alligator clip (image B)Checking Spark Plug 51B
    Image B
    .

  2. Use the rewind or electric starter to crank the engine, and look for a spark in the tester's window.

  3. If you see a spark jump the gap in the tester, the ignition is functioning. The absence of a visible spark indicates a problem in the ignition system.

Checking For Spark Miss

A spark plug that is fouled or improperly gapped may not allow sparks to jump the gap between electrodes consistently. The spark plug will fire erratically or may occasionally fail to spark. Test for this problem - known as spark "miss" - if your engine stumbles, with a noticeable decrease in engine sound. Spark miss can also cause the engine to emit black smoke or a popping sound, as unburned fuel exits with the exhaust and ignites inside the muffler.
  1. With the spark plug screwed into the cylinder head, attach the spark plug lead to the long terminal of the spark testerIgnition Tester 19368
    Spark Tester
    (service part number 19368). Attach the tester's alligator clip to the spark plug (image C)Checking Spark Plug 51C
    Image C
    .

  2. Start the engine and watch the tester's spark gap. You'll recognize spark miss by the uneven timing of the sparks in the tester.

For additional information, please visit our Ignition Troubleshooting FAQ and view our Spark Plug Maintenance tutorial.

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