How to Change the Spark Plug in a Lawnmower (Example: Briggs & Stratton Engines in Craftsman and Murray Mowers)

August 5, 2008 | by Fred (email) |

Performing full mower maintenance once per cutting season is important to keep your mower in top running shape.  This article is the fourth in a series of maintenance articles that cover complete mower care.  While this post is focused on replacing the spark plug in a push-type lawnmower, the basic principles can be applied to all 4-cycle gas powered equipment (including such things as a pressure washer or chipper shredder, and even an automobile).

For the entire series, we’re performing maintenance on both a Craftsman 6.5 horsepower self-propelled mower, and a 4.5 horsepower Murray mower.  Both push mowers are powered by Briggs & Stratton small engines.

One of the top reasons a lawnmower won’t start is a faulty or worn out spark plug.  The spark plug ignites the repeated explosions inside the cylinder of a lawn mower’s engine.  Since it sits inside the cylinder, it is subject to very high heat, corrosion, and wear.

When a spark plug detoriates too much, it loses its ability to generate a full spark, and two things can result:  (1) the spark plug cannot produce sufficient spark to ignite the fuel in the chamber, and the lawnmower won’t turn over at all, or (2) the spark plug produces a sub-optimal spark, and the explosion inside the chamber is uneven.  The second can result in an engine that rattles or vibrates, and occasionally stalls.

Fortunately, changing a spark plug is both simple and inexpensive.  Most lawnmower spark plugs are $2.00 or less at the local hardware store.   Here’s the few steps required to change the plug:

How to Change a Lawnmower Spark Plug

Step 1: Locate the spark plug on the engine.  On a lawnmower engine, the plug location is usually obvious, as the wire that provides the electricty for the spark connects to the end.  On both our Briggs and Stratton engines, the plug is located in roughly the same spot.

Step 2: Remove the spark plug wire and unscrew the plug from the engine.  The tool required to unscrew the plug depends on the plug’s location.  If the plug is embedded deep inside a hole, you may need to use a spark plug socket to grip and remove it.  This special socket has a deep well and a hard foam insert that grips the plug during installation and removal.  If you’re planning to do car tune ups in the future, this type of socket is essential.  You can find them at most major hardware & auto parts stores.  If the plug is in an easy-to-access location as it is on both of our mowers, a pair of channel locks will work great.

Step 3: Determine whether the plug needs to be replaced [optional].  We recommend replacing the spark plug every year because it is so simple and inexpensive; however, depending on your lawnmower use this may not be necessary.  If the metal hook that sits over the pin looks solid and uncorroded, you may not need to replace the plug.

Step 4: Purchase a compatible spark plug at the local hardware store.  Most hardware stores have part look up systems to identify the right spark plug for your mower/engine.  Note that even the same manufacturer will occasionally change the designations on plugs, and sometimes multiple plugs will work with the same engine, even if the plugs don’t look 100% identical.

Step 5: Gap the spark plug if necessary.  Gapping a spark plug means ensuring that the exact right gap exists between the sparking pin and the L-shaped bracket that covers the pin.  Many plugs now come “pre-gapped” and don’t require the user to gap the plug.  If your plug does require gapping, all you need is a simple spark plug gap tool (shown on the right).  The tool sounds fancy, but it isn’t.  It’s a simple tool that has different width metal wires.  Insert the correct wire in between the pin and the L-bracket on the plug and bend the L-bracket so it fits snugly around the key.  The right spark plug gap is usually listed in the engine manual for the mower.

Step 6: Insert the spark plug back into the mower.   Hand tighten the plug at first, ensuring there is no significant resistance.  If there is significant resistance, stop immediately to avoid cross-threading.  Remove the plug and begin again.  Once the plug is hand-tightened, use a rachet/channel locks to tighten the spark plug in place.  Do not over-tighten as this could cause the threads to strip.  If you have a calibrated torque wrench on hand, you can set the torque to the exact amount suggested by the manufacturer.

Step 7: Reattach the spark plug wire, start the mower, and listen.  If the engine is running smooth, you’re successful!  If not, recheck these steps to ensure you’ve done everything correctly.

What do you think? Let us know if this post helped, or if you add/remove anything from our instructions.