Skip to Content

How to change a spark plug in a lawnmower

Who does not want a well-maintained lawn? You bought a lawnmower as you wanted to take care of your property. But maintaining the lawnmower is now an issue with your limited expertise in the technical department. One fine day, you wake up and decide to cut the excessively grown grass in your garden. When you try to start your mower, after fueling it, you realize it would not start. Starting the engine would require more than the normal pulling force on the starter rope or a more extended self from the key. Even if it started, it would either stall or run erratically.

In such a case, you might have to change the spark plug. The spark plug has probably gone bad. The reasons and effects of this defective spark plug will be discussed later. Let us first go over the main steps involved in changing a spark plug.

How to change a spark plug in a lawnmower

  • Step 1: Get the needed tools and spark plug replacement
  • Step 2: Locate the spark plug
  • Step 3: Disconnect the spark plug wire
  • Step 4: Remove the spark plug
  • Step 5: Install the new spark plug
  • Step 6: Reconnect the wire and Check by starting the engine

Reason for the required spark plug change

As suggested by the name, a spark plug is a device that produces spark inside the combustion chamber of an engine to start the combustion process. There can be various reasons for a spark plug change. Before getting into more details of malfunction reasons, it is vital to look at the spark plug’s working.

● How a Spark Plug Works

The spark plug has a tip with no insulation covering on both heads. When a high voltage current passes through the spark plug, the small gap between the two head terminals allows the current to jump and produce a spark. The high voltage current is received from the ignition coil, which steps up the battery’s 12V current. In spark-ignition engines, the spark plug is an essential component. Without a sparkplug, the machine will not start. Due to this, although it is a small part, the importance is very high.

● Reasons for a spark plug change

As we have come to know why sparkplug is such an essential component of a lawnmower, here are the reasons for the necessary spark plug change.

  • Engine health: With a lousy sparkplug, the engine health is reduced as it results in misfiring inside the engine affecting the cylinders badly.
  • Build upon Spark heads: Air flows in the engine cylinders at high speeds. Sometimes debris of dirt and oil also gets inside the cylinder. Spark plug being directly interacting components become clogged with these contaminants.
  • Wear and Tear: All components of a running machine need replacement and so do spark plugs. The lifetime of a spark plug varies from 25 – 30 running hours.
  • Overheating: If the cooling system is not performing at its peak, sparkplugs will have to operate at elevated temperatures. Due to this, a sparkplug overheats and loses its effectiveness.

Sometimes a repair in such cases might be helpful. Mostly, the spark plug needs a replacement for the lawnmower to work at its best and have an extended life. Yearly maintenance might be a popular reason for spark plug replacement.

Effects of the faulty spark plug

Knowing the effects of a faulty spark plug is essential to identify the symptoms for the necessary change.

  • High fuel consumption: Who does not like their lawnmower to spend as less of gas as it can. Well, if the sparkplug is faulty engine loses its efficiency.
  • The engine will not start: as the sparkplug is not working, the cycle of a machine can never run, and hence engine would not start
  • Erratic operation: The engine, if it does start, feels very heavy and tends to run very irregularly.
  • Misfiring: This effect is not solidly observable, but all others are generally the aftereffect of this. Engine stroke timing suffers, and power losses are massive.

You might want to know more about the complete working cycle of a spark ignition internal combustion engine with curiosity building. More knowledge will also help you understand more technicalities to help to detail, maintaining, and repairing. If technical details still do not interest you as much, you can skip to the next portion, where a step by step instruction for a spark plug change is discussed.

A Spark Ignition Engine

A spark-ignition engine is an engine that operates on gas and is used in a lot of lawnmowers. If it is a diesel engine, you are in the wrong place as diesel engines are compression ignition engines and do not have a spark plug.

The term spark-ignition internal combustion engine can be reduced to parts to explain it.

  • Spark means the jump of current from one electrode of the sparkplug to the other with air as a medium to travel
  • Ignition means the start of the combustion process of the air-fuel mixture
  • Internal means processes take place in an enclosed space
  • Combustion means the burning of fuel that generates power to drive the rotary power

The engine converts the chemical energy of fuel into mechanical energy of the crankshaft. The operating cycle is the Otto cycle, which has four processes going on.

  • Intake of the air-fuel mixture
  • Compression of the mixture
  • A spark from the sparkplug
  • Combustion of fuel
  • Power stroke on piston head driving it down
  • The exhaust of burnt gases

The piston moves linearly and transfers power as rotational energy via a connecting rod.

Step by step approach to change the spark plug

● Step 1: Get the needed tools and spark plug replacement

It is crucial to make sure you have all the necessary tools and a new spark plug before working.

The list of all the tools required is as follows

  • gap gauge
  • Needle-nose pliers
  • Rags
  • Socket/ratchet set
  • spark plug wire puller
  • swivel socket.
  • Torque wrench
  • Safety items: Gloves, safety goggles, coverall

Some other materials are also required

  • Spark plug
  • Anti-seize Compound
  • Lubricant grease

● Step 2: Locate the spark plug

After arranging all of the mentioned equipment, the next step is to locate the spark plug. It is one of the accessible components to discover on a lawnmower. Most lawnmowers have a spark on the side of the engine outside the body casing.

Locating the plug can be approached in two ways

– Method 1:

 Start by using the instructions manual that was shipped along with the lawnmower. Most manuals have labeled diagrams at the very start or the end. The sparkplug can be easily located using that.

– Method 2:

Suppose you do not possess an owner’s manual and have little to no component know-how of your lawnmower. There is nothing to worry about. To locate the only heavily insulated wire of orange or black color extending out from the engine. This wire has mostly an L shaped socket/ connector inserted in the engine compartment. Here is the sparkplug wire. The insertion point on the machine is the spark plug. By outside appearance, the sparkplug is like a headphone jack but larger.   

● Step 3: Disconnect the spark plug wire

After locating the plug, removal of the plug wire is performed. The wire can be pulled off using hands. It is preferred to use a sparkplug wire puller, ensuring the insulation does not get damaged. 

● Step 4: Remove the spark plug

If you want to remove the sparkplug, either a socket wrench or a sparkplug wrench tool can be used. The specialized tool has an extended neck of the socket and makes the job much more comfortable. Follow these stepwise instructions

  • Fit the wrench in place: Connect the wrench to the base of the plug
  • Loosen and unscrew the plug: Apply firm force to loosen up the sparkplug and then unscrew with ease with normal hand force
  • Removing old sparkplug: When unthreaded completely move the old sparkplug out

● Step 5: Install the new spark plug

  • Mind the electrode gap:
  • Before installing the new sparkplug, it is imperative to gap it. Using the gap gauge or feeler gauge, choose the specific gap dimension a push between the electrodes.
  • Place the sparkplug in place: After gaping is completed, gently screw the new plug into the initial thread. Be as gentle as possible so that threads are not damaged.
  • Tighten the new sparkplug: Screw with hand force and tighten using wrench tool. Make sure not to overtighten because it might damage the plug, its seal, and threads

● Step 6: Reconnect the wire and Check by starting the engine

After the new spark plug is in place and set, connect the wire back again. Be gentle while pushing, not to damage the tightness of connection. Also, check the ignition coil side of the wire to be in place.

If you feel necessary, relubricate the ignition coil boot. Now pull the starter rope to check the lawnmower. Hopefully, all problem of starting and irregular operation of the engine is now resolved.

Precautionary Measures

  • Let the engine to cool down if it was already running
  • Choose the sparkplug which was already in use or guided by the company in manual
  • Sparkplug specifications are mentioned on the ceramic part of the plug. Choose wisely.

Common problems and their solutions

A few problems that might arise when changing the sparkplug are enlisted along with their suitable solutions.

  • Stuck old sparkplug: Unscrewing the sparkplug might require more than usual force. It can be solved using CRC or WD40 to clean up any rust or debris stuck in the threads.
  • Wrench unable to reach: In such case, a better option is to use a socket screwdriver, which is rare to occur, but it may.   

Related Question

1. Even all connections are made and the sparkplug is changed, a bit of irregular behavior persists. Why?

This issue might be due to the sparkplug wire. Check the resistance using a multimeter; if higher than standard, replace the wire as well.

2. What to do to make sure spark plug does not stick when changing next time?

Using an anti-seize compound around the thread of the sparkplug before threading is essential.

Final Remarks

Although small yet essential lawnmower, sparkplug, needs repair and replacement for good running and extended life of lawnmower. Replacement of sparkplug is necessary as a maintenance process but also in case the engine fails to start. This article has summarized the process for readers’ help.