How To Tell If A Lawn Mower Spark Plug Is Bad

It can ruin your day when you try to mow your lawn, but your mower is difficult or won’t start at all. But there is frequently a simple solution if you know the cause of the issue. Bad spark plugs are a likely reason your engine is having trouble starting.

A center electrode and a side-firing electrode are the two electrodes on a spark plug. The air-fuel mixture in the cylinder is ignited by a spark created to flow through the tiny gap between the two electrodes.

Spark plugs are required for a gasoline-riding lawnmower, chainsaws, or other gasoline-powered devices or vehicles to ignite fuel in the combustion chamber to function properly.

Our guide will focus on a spark plug on a lawn mower. By the end, you’ll know all the reasons why your spark plugs are bad and whether you need to change spark plugs, or it just needs cleaning, or you need a spark plug gauge to test the spark plug gap. (Learn How Long Does Ethanol Free Gas Last)

lawnmower spark plug

What A Spark Plug Does

A spark plug is an essential component of a gasoline-powered engine despite its diminutive size. The spark plug in a lawnmower’s engine does the same job as in a car engine.

A spark plug is an electrical device that creates a spark as the magneto or the ignition coil delivers a high voltage to the central electrode.

The spark ignites the fuel and air mixture inside the cylinder head. There will be no ignition or improper ignition if the spark plug cannot perform its job properly, which can cause a variety of issues. The worst-case scenario is that your engine might not even turn on.

How to Tell If a Lawnmower Spark Plug is Bad

You know the major symptoms now; let’s get into their details so that judging becomes more comfortable for you.

1. Engine won’t start easily:

This is the most common symptom of a bad spark plug. A damaged spark plug prevents a bad engine start. It will take several pulls on the starter rope when using a push mower before the engine turns on. Similarly, you won’t be able to start a lawn tractor’s engine with just one turn of the key.

Knowing what a spark plug does helps to make the explanation of this symptom fairly simple. The spark produced by a malfunctioning spark plug does not ignite the air-fuel mixture from the carburetor via the fuel tank.

The mixture floods the engine as it enters the cylinder, and only a small portion ignites.

2. Poor Performance:

Even if the engine starts with a bad spark plug, it can take many attempts; it won’t be able to continue running for long. The engine will shut off or stop mowing because the spark plug is not functioning correctly, and you’ll find the engine sputtering before it dies.

The difficulty of starting the engine increases after each attempt as the engine warms up. This is because heat expands metals, which widens the space between the two electrodes and reduces the intensity of the ignition spark.

The problem can be worse on a riding mower as you go about lawn care, you may be stranded, and a lawn tractor can’t be pushed. Another reason you can have a lawn mower bad spark plug isn’t the gap is incorrect; it is the carbon buildup on the electrodes.

You get this from unburned oil and fuel pumped into the combustion chamber or seeps past the piston rings, and the electrodes can’t create sparks. (Read Spark Plug Socket Size Guide)

fuel consumption

3. High Fuel Consumption:

A bad spark plug could cause your lawnmower to use more fuel than usual to complete the same job. A bad spark plug causes inefficient ignition and improper fuel combustion. This makes the mower less fuel-efficient, raising the fuel used.

The odor of your mower is another indicator that it is not burning and consuming fuel efficiently. When the mower runs, raw gasoline is produced because a bad spark plug prevents the fuel from being burned properly.

How To Diagnose a Spark Plug Problem?

You won’t have to wait for symptoms to appear because a bad spark plug will give you a sufficient warning. Periodically inspecting the spark plug will keep you informed about the condition of the spark plug in your mower.

You can assess the condition of your spark plug based on several factors. You can fit a new spark plug or clean your spark plug and adjust the gap from there.

Understanding Spark Plug Problems:

Know the engine problems a bad spark plug can cause. These problems are explained in the manual or online.

  • Check the spark plug if the engine won’t start.
  • Check the spark plug to see if the engine cuts out.
  • Check the spark plug if you keep adding fuel to lawn mowers gas tank.

Spark Plug Gap:

The spark plug gap is between the electrodes. The spark plug gap is crucial. Deviations from the specified width can cause problems. As the engine heats, the gap widens. Cold engine contraction reduces the gap.

Over time, electrodes develop residual stresses and strain, distorting the gap, and an incorrect gap weakens ignition. Spark plug gaps can change during shipping, and handling, if the plug falls or the anode thins, so before installing a spark plug, check the gap. You often need feeler gauges to do this, although a quick way is using the cardboard from a box of matches.

The gap should be the same thickness as this, so you shouldn’t need your user manual unless you are using gauges and need to know the proper gap.

Cleaning your Spark Plug:

If the spark plug is badly corroded or has thick burnt deposits, you must replace it. If a spark plug is wet from gas or oil, it can be cleaned using a wire brush. The central electrode should be flat on top. If its top is damaged, replace the spark plug. If you see cracks in the spark plugs porcelain sheath, this needs to be changed immediately.

Only a wet gasoline or oil plug with no visible damage can be cleaned. Oil-soaked plugs mean your engine is leaking oil, and most often, this is from the engine cylinder head and pistons.

Replacing a Spark Plug:

The lawnmower spark plug is under the black spark plug wire. Remove the wire and plug to inspect. You need the right-sized socket and a ratcheting socket driver to remove the plug.

Follow these steps to replace a spark plug in your lawn mower’s engine:

  1. Get a Spark Plug: Ensure the replacement plug is the same size and has the same specs.
  2. Disconnect the Spark Plug: Remove the lead and clean around the spark plug wire.
  3. Remove the Spark Plug lead: Take the faulty spark plug using your wrench and spark plug socket, and check the electrode for signs of damage. Too wet could signal a blocked air filter.
  4. Put the new Spark Plug in: Put the new plugin by first turning it in by hand to ensure that you don’t cross-thread. When it can’t be turned by hand, use the wrench to tighten the new plug, and don’t over-tighten.
  5. Reconnect the Spark Plug wires: Connect the lawnmower’s spark plug lead.

Depending on your mower manufacturer, they may sell tune-up kits for end-of-season storage.

Briggs & Stratton offers a small engine kit that includes:

  • Spark plugs
  • Oil and air filter
  • Lawn mower oil, fuel stabilizer, etc.

Where does a spark plug get power from?

A magneto or ignition coil’s high voltage output is connected to the spark plug. As the electrons exit the ignition coil, a voltage forms between the center and side electrodes.

Since the fuel and air in the gap are insulators, no current can flow there. However, as the voltage increases, the structure of the gases between the electrodes starts to change.

Gases become ionized when the voltage rises above their dielectric limit. The ionized gas transforms into a conductor, enabling the flow of electrons across the gap. Small Engine spark plugs typically need a voltage of between 5 and 15,000 volts. (Read Lawn Mower Won’t Stay Running)

Other Signs That You Have A Bad Lawn Mower Spark Plug

The job of the lawn mower spark plug was briefly mentioned, and knowing this will help you understand why you have some of the common symptoms of a faulty spark plug.

Our lawnmower may have appeared to be having engine issues, which could have cost you hundreds of dollars or caused purchasing a new mower, but it may only result from a cracked or defective spark plug.

By the way, the information on recognizing a bad spark plug also applies to examining the spark plug condition in any gas-powered weed eater or brush cutter.

Listen to the spark plug socket.

As we previously mentioned, once the lawnmower engine runs, it may shut off suddenly and seemingly for no reason. It might be simpler to comprehend what is happening now that we know a spark plug’s job.

The spark produced by the spark plug frequently does not ignite the fuel that mixes with the air in the carburetor, which is the major cause of engine failure. The fuel not being burned off properly can frequently cause the engine to flood as more fuel is added to the cylinder.

Lengthy periods of engine operation cause the spark plug to heat, which can widen the gap between the firing electrodes and the spark plug center, weakening the ignition spark even more.

If you’ve been trying to start the lawnmower for a while, but it keeps cutting out, this may be the reason you need to let it cool down.

Excessive Gas Consumption

Excessive fuel consumption is another red flag that indicates a bad spark plug in your lawn mower. The need to fill the engine with gas more frequently than usual may have become apparent. Faulty spark plugs are frequently to blame for this. (Read Too Much Oil In Lawn Mower)

Your fuel consumption will rise, and your fuel efficiency will fall if the gasoline is not burned completely or efficiently. The air may smell like fuel, frequently like raw gasoline, as the gasoline isn’t being burned off as it should be. To clean your engine, use a soft wire brush.

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