How to Tell If a Lawn Mower Spark Plug is Bad -

How to Tell If a Lawn Mower Spark Plug is Bad

Among the many responsibilities of maintaining a garden, the loathed and never-ending task of keeping the grass at an appropriate length is perhaps the most important. While it is widely known that gas driven lawn mowers are the most convenient to have, not many who keep these machines know how to maintain them properly.

Appreciated for their cordless design, high power, and quick employment, gas-powered lawnmowers hold one flaw. Quite often, they require as much care and maintenance as your car would, and for some, this is an inconvenience that can be troublesome. Here we will be discussing how to tell if a lawnmower spark plug is bad.

What Does a Spark Plug Do?

The spark plug is a very important component. Without it, your lawnmower will not function at all. In any internal combustion engine that runs on gas, the spark plug is responsible for the ignition of the fuel. Once the piston reaches the height of its compression cycle, an electric current is passed through the spark plug, causing a spark to form in the gap between the firing electrode and the center pin.

In basic terms, a spark plug is nothing more than a conductor sealed, most often, in a ceramic casing, with an electrode firing end that is spaced away from an earthing point, or the center pin.

When current travels through the conductor and passes out from the electrode point, it earths itself on the pin. The product of the process is a spark that ignites the compressed fuel. The electric current is always supplied by some form of battery. In lawnmowers, this would most likely be a small 9-volt battery.

However, for enough current to pass through the spark plug and produce a spark, the current is passed through a step-up coil before reaching the plug. Any faults in the spark plug will result in a massive loss of power, or could even cause the engine to fail completely.

How To Remove a Lawn Mower Spark Plug

In all engines, the spark plug is hosted within the blockhead, or for common reference, the cylinder top. Some designs could have it entering the combustion chamber, the dome in which the fuel is compressed and ignited, from the side or from straight atop. Locate the spark plug on your lawnmower.

An easy method is to trace the cable lead from the coil to where it connects with the spark plug. Every spark plug will have a nut ring towards the center of its body. Always loosen the spark plug by the provided nut ring with either a socket or wrench of the appropriate size. Never twist or pull on any other part of the body, as this could cause unwanted damage to the spark plug. The sizes of the nut ring vary with the size of the spark plug.

How to Tell If a Lawn Mower Spark Plug is Bad

How To Check a Spark Plug for Faults

With the spark plug removed, check that it receives the correct amount of electric current. To do this, keep the spark plug connected to the coil and place the electrode end close to any earthed metallic surface. A piece of tin foil wrapped onto a wooden block would be good enough.

Then allow the engine to run through its cycle until it reaches the combustion point. Either press the ignition button long enough or if it uses a pull-rope starter, pull on the rope until it is supposed to ignite.

A spark should jump from the electrode to the tin foil. It should be a visible, powerful spark. If the spark is faint, or it does not spark, then it is an electrical fault, most likely because of a defective coil. When examining the electrode end, keep an eye out for carbon build-up.

This would be thick, black soot that forms due to fuel combustion. This can be remedied by simply cleaning the electrode and center pin. Soot build-up can cause the spark to reduce and result in a loss of power.

Also, examine for any moisture, of either fuel or oil. This is caused by a defective engine and should be treated before serious damage occurs. It could be because of many problems, including but not limited to defective piston rings, or inappropriate combustion likely because of a reduced spark.

If the spark plug has whitening at the electrode end, it is a sign that it becomes too hot. It could be because of an over-productive coil, or even because of too much air mixed with the fuel. Check the gap and ensure it is spaced correctly. The lawn mower’s manual should contain information on the correct gap spacing. If the electrode is damaged or corroded, it will require fhglawnmowers-20nt.


There are plenty of faults that can occur with spark plugs. If you encounter a problem that seems unfamiliar or is beyond your expertise, consult a technician. Proper care and maintenance of your lawnmower is important, so always read the user manual for safety and care instructions.

Never operate the machine beyond the conditions described in the manual. If you do not have the manual, visit the manufacturer’s website where you should gain access to this important information and could even download a digital copy of the manual.

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