How to Tell If a Lawnmower Spark Plug is Bad?

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It’s a real mood destroyer when you are mowing your lawn, but your mower won’t start easily or won’t start at all. But if you know the reason behind the problem, there is often an easy fix. A highly likely cause of your engine having trouble starting could be a bad spark plug. Lawnmowers with gasoline engines need spark plugs to provide ignition to the engine. A spark plug is a small, simple device with a center electrode and a side electrode. A spark made to flow through the small gap between the two electrodes ignites the cylinder’s air-fuel mixture. Mowing becomes problematic when the spark plug is bad. You can judge symptoms if your spark plus is having problems and needs a change.

How to Tell If a Lawnmower Spark Plug is Bad:

  • The engine is difficult to start. This is the most common symptom of a bad spark plug.
  • The engine shows poor performance and keeps stopping while mowing.
  • Unusually high fuel consumption.

Now that you know the most common symptoms associated with a bad spark plug let’s go into the details of why a bad spark plug may cause these problems and what to do when these symptoms emerge.

What does a Spark Plug do

Though a spark plug is relatively small in size, it plays a vital role in a gasoline-powered engine. The spark plug in a lawnmower’s engine performs the same job as in an automobile engine.

Unlike most parts of an engine that perform mechanical roles, a spark plug is an electrical device. Its central electrode receives a high voltage from the ignition coil or the magneto. That’s why it is the live electrode. The side electrode doesn’t receive voltage from any source and is a ground electrode. When a high voltage is provided to the central electrode, a high potential difference is created between the two electrodes, which allows a spark of current to flow between the electrodes. The spark plug is fitted in the engine cylinder head, and the spark ignites the air-fuel mixture. Combustion of the air-fuel mixture provides energy for the engine to run.

If the spark plug cannot do its job correctly, there will be no ignition or improper ignition, causing all sorts of problems. In the worst case, your engine might not start at all.

Symptoms of a Bad Spark Plug

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

● Engine Has Trouble Starting:

Like said before, it is the most common symptom associated with a faulty spark plug. An engine with a bad spark plug won’t start normally. If you are using a push mower, it will take many pulls on the starter rope before the engine starts. Likewise, if you are using a lawn tractor, a single turn of the key won’t be enough to start the engine.

Now that you know what a spark plug does, the explanation of this symptom becomes relatively easy. A faulty spark plug generates a spark too weak to ignite the air-fuel mixture coming from the carburetor. As more mixture enters the cylinder and only a tiny portion gets ignited, the engine gets flooded by the entering mixture.

● Bad Performance:

Even if the engine with a faulty spark plug finally starts after many tries, it will fail to keep running for a normal period. Because the spark plug is not functioning correctly, the engine will die out immediately or stop mowing.

When the engine keeps dying out, it needs to be started again and again. After each try at starting the engine, starting becomes more difficult as the engine is getting warmer. This is because heat causes expansion in metals, which widens the gap between the two electrodes, and the strength of the ignition spark further decreases.

Other performance-related problems associated with a faulty spark plug may include the engine sputtering, popping, or missing. Misfires cause these problems.

● Unusually High Fuel Consumption:

If your lawnmower is taking more fuel for the same job than usual, one reason might be a bad spark plug.

With a bad spark plug, ignition is not efficient, and the fuel is not combusted correctly. This decreases the fuel efficiency of the mower, thereby increasing fuel consumption.

Another way to judge that your mower is not burning and consuming fuel efficiently is odor. Because of a bad spark plug, the fuel is not burned correctly, and raw gasoline is created as the mower is in operation.

Checking the Spark Plug:

A faulty spark plug will give you clear enough symptoms of the problem, but you don’t necessarily need to wait for the signs. You can check the spark plug periodically so that you are always aware of your mower’s spark plug health. There are several criteria on which you can judge the health of your spark plug. Once you know your spark plug is not in perfect condition, you can either fix it or change it.

● Understanding Problems:

Stay well informed about the problems an engine might have if the spark plug is bad. You can learn about these problems from the user manual or easily from the internet.

If the engine is not starting at all or not starting correctly, immediately check the spark plug. If the engine shuts down during operation, check the spark plug. If you are forced to add fuel, again and again, check the spark plug.

● Spark Plug Gap:

The spark plug gap is the gap between the two electrodes. The gap of the spark plug must be right. Even a small deviation from the designated width can cause problems.

During operation, the engine becomes hot, which causes expansion, and the gap increases. When the engine is cold, contraction occurs, and the gap reduces. Over time, the electrodes develop residual stresses and strain the gap is permanently distorted from its right width. The wrong gap reduces ignition strength. Due to this reason, you should let the engine cool down a bit if you’ve been trying to start it again and again and let the gap.

The spark plug gap can also change during shipping or handling or if the plug falls on the floor, or if the anode gets thinner. So, always check the gap before installing a spark plug.

The spark plug gap is something that can be adjusted. If your engine has a problem starting, check if the spark plug gap is right. If it’s not, you can adjust it to match the spark plug packaging or engine manual’s gap specification.

When to Change Spark Plug?

According to experts, a home-owned lawnmower’s spark plug needs to be changed once per season or after 25 hours of operation.

An acceptable practice can be to be cautious about the problems and change the spark plug as soon as any problem appears.

To stay safe, though, and avoid any problems, it’s good to change the spark plug once a year.

Cleaning the Spark Plug:

If the spark plug is corroded too badly or has carbon burns or burnt deposits, all you can do is change it. But if the spark plug is just wet from too much gas or oil buildup, it can be cleaned.

The live electrode (central electrode) should have an even and flat surface at the top. If its top surface is damaged, the spark plug should be changed. If there are any cracks on the spark plug’s porcelain sheath, the spark plug needs to be changed.

The plug can only be cleaned if it is wet from gasoline or oil, and there is no visible physical damage.

Replacing the Spark Plug:

The spark plug in a lawnmower is located under the black spark plug wire. Disconnect the wire and take the plug out for inspection.

Follow these steps for replacing a spark plug:

● Step 1: Get a similar Spark Plug

Ensure the replacement plug is the same size and with the same specifications as the one being replaced.

● Step 2: Disconnect the Spark Plug

Disconnect and clean around the spark plug wire.

● Step 3: Remove the Spark Plug

Take the plug using a wrench out and inspect the electrode for any signs of engine problems. If it’s too dry or wet, there might be a problem with the carburetor.

● Step 4: Put the new Spark Plugin

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 anymore, use the wrench for further turning.

● Step 5: Reconnect the Spark Plug wires

Connect the spark plug wire back.

Final Remarks:

A faulty spark plug can cause problems to the lawnmower engine, making it almost impossible to operate. But if you have basic knowledge of a spark plug’s function and how it works, and the symptoms of a bad spark plug, the problem can be easily solved. Problems like distorted gap and the electrode being wet can be fixed, while if there is any physical damage to the plug, it has to be replaced.