6 Common Reasons Why Your Lawn Mower Won't Start

It’s a beautiful late summer morning. The gentle hum of the neighbor’s lawn mower reminds you that it’s a perfect time to mow your lawn too. You pull your mower out of the garage and prepare for a routine you’ve done a million times before. You pump the primer bulb and pull the starter cord like you always do…but this time, something is different. The mower engine lets out a grumbling gurgle and refuses to turn, even after multiple attempts. You’re suddenly faced with the harsh reality that you have no idea what to do next.

There are a number of possible causes for a lifeless mower, but ultimately only two things are required for an engine to start: fuel and a spark. If the engine isn’t getting a spark at the plug or fuel isn’t reaching the engine, your lawn mower is never going to start.

The important thing to remember is that like any other machine, lawn mowers require routine care and proper maintenance to continue running properly. Some of the most common problems that mower owners encounter are the direct consequences of improper or neglected regular maintenance from season to season. The lawn mower not starting problem can develop just as easily when the mower sits in storage all winter as when it’s in heavy use during the spring and summer.

We’ve been repairing lawn mowers for over 20 years, servicing nearly every mower brand on the market. Our shop has seen it all, but there are certain issues that we deal with constantly. If you’ve ever stared helplessly at your lifeless lawn mower, this troubleshooting guide discusses the 6 most common mower problems that owners face based on personal experience.

Firstly, your engine needs to be lubricated well to prevent your lawn mower from overheating and smoking. If your mower runs out of oil and is run anyway while you do yard work, you WILL be either paying for engine repairs or a new lawn mower. CHECK YOUR OIL BEFORE EVERY USE!

Before you attempt to work on any lawn mower, the following precautionary measures should be taken:

Remove the spark plug or disconnect the lead. All controls should be switched off and the spark plug lead should be disconnected before working on the underside of an engine. Allow the engine to cool down before attempting any troubleshooting.
Check the owner’s manual for the proper way to tip your mower. Your mower’s manufacturer will have specific recommendations about whether to keep the carburetor side or the spark plug upright. However, no equipment should be turned so that its carburetor and air filter are facing downwards: doing so can cause engine oil to damage them.
Run the engine to drain the fuel. If the mower needs to be placed on its side or plug side up, drain fuel from the mower’s tank and bowl by running the engine until it cuts out. If the mower’s tank is full of fuel, it will leak out the vent in the cap of the tank and can potentially start a fire.

You should also note that gasoline is flammable and blades are sharp: never attempt repairs while smoking or near any potential sources of ignition, and never lift the equipment by the bottom to transport.

Embarrassing? Yes. However, forgetting to confirm whether there’s gas in the tank is actually a very common oversight, perhaps on account of its simplicity. The resolution for this particular issue should be fairly straight forward: check your mower’s fuel level, every time, before starting it up.

On a related note, gasoline has a shorter shelf life than you may realize. Stale, untreated gas is one of the most common types of fuel problems as it begins to break down after about a month. If left in the tank long enough, old, stale fuel will eventually lead to engine damage.

If the gas in your mower’s tank is older than 30 days, you’ll need to empty the tank and refill it with fresh fuel. Depending on the equipment, it may be possible to tip the mower and drain the tank. If not, the fuel will need to be siphoned out. Stale fuel is highly noxious, so always work in a well-ventilated area. Once the stale fuel has been removed, replace it with fresh fuel and fuel stabilizer.

PRO TIP: Always add fuel stabilizer to fresh fuel, since it slows the buildup in carburetors. Using a quality gasoline stabilizer allows better fuel flow and can help to prevent many of the problems with a mower that won’t start. Star Tron Enzyme Fuel Treatment Gasoline Stabilizer, for example, helps to prevent the lighter hydrocarbons from evaporating – this reduces gum and varnish and keeps the fuel flowing. It also offers additional protection by containing corrosion inhibitors. This will keep gasoline fresh for up to a year.

This is one of the countless lessons for understanding the importance of routine maintenance: neglected mower maintenance will always lead to future headaches.

On that note, get into the habit of always draining the fuel at the end of the mowing season. You’ll thank yourself when next year comes around.

NOTE: Ethanol fuel can cause serious issues with your mower’s carburetor, fuel line, and fuel filter. Ethanol that has absorbed enough moisture and has sat long enough can foul the fuel system and prevent the engine from starting. It eats the inner lining of the fuel line, which carries the debris through the fuel filter and into the carburetor. Using a fuel stabilizer formulated to combat ethanol or selecting non-ethanol at the pump can help. However, the first few gallons from the pump may contain ethanol. For that reason, we recommend filling your car before filling the gas can.

A clean carburetor is the lifeblood of your mower’s small engine. A corroded, clogged or dirty carburetor can bring the entire engine to a halt. Your mower’s carburetor has a tough job to do: it’s responsible for combining air and fuel together in the proportion needed to maximize power and minimize fuel consumption. Deposits inside the carburetor will clog fuel and air passages and stop the entire engine.

Since your mower is exposed to a constant barrage of debris, grass and twigs, it’s easy for the main fuel jet to become blocked. The vast majority of carburetor problems are caused by blockages from things like dirt, varnish, and gasoline deposits.

The good news is that there’s a fairly easy fix for a dirty carburetor: it simply needs a good cleaning with a commercially available carburetor cleaner. Mower owners are often intimidated by this process since it sometimes requires removing the carburetor from the engine.

Periodic cleaning of both the inside and outside of the carburetor will help you avoid headaches in the future, such as replacing the entire carburetor. Check the carburetor regularly: it should always be firmly bolted to the engine and clear of any blockages or ‘gummy’ accumulations.

Your mower’s spark plug is the ignition source for its engine. It facilitates the spark, which is one of the two critical components required to start any small engine. Your mower’s spark plug must have clean and sharp electrodes in order to produce a spark that’s powerful enough for ignition. A worn or dirty spark plug covered in carbon deposits or oil residue will lead to a greater tug on the rewind, and more effort will be required to produce an adequate spark. If you have to tug repeatedly on the rewind to start the engine, a damaged or dirty spark plug is likely to blame.

There are several key indications that a mower’s spark plug is bad. You may notice these performance glitches in a mower with a bad spark plug:

Stuttering and difficulty starting
Loss of power
Slower acceleration
Excessive fuel consumption

Although these issues can be subtle at first, they will progressively become more pronounced over time.

You can confirm your suspicions by looking at the plug itself. Use a socket wrench to unscrew the plug so you can inspect the electrode and insulator. (While it’s removed, take an extra couple of minutes to wipe away any debris surrounding the plug with a paper towel or shop rag).

Look closely at the spark plug. The center of the electrode should have a flat top. If it appears rounded, cracked or appears black from carbon or degraded excess fuel, it would be wise to replace it. Spark plugs are designed to be very durable, but they’re certainly not indestructible and their lifespan is finite. Fortunately, spark plugs are inexpensive and easy to replace.

The purpose of your mower’s air filter is pretty straight forward: it filters the air in your mower and guards the engine and carburetor from grass clippings, dirt, and other harmful debris. This is an important role because the air must be clean before it mixes with the fuel and burns in the cylinders of an engine. If your mower’s air filter is not performing properly, the incoming air will be contaminated with all of the dirt and debris that flies around when you mow your lawn. It goes without saying that those contaminants shouldn’t be entering the internal parts of your engine.

A dirty air filter will result in a noticeable decline in the performance of your mower, with operational issues that include:

Less horsepower and inconsistent power
Decreased running time and increased fuel consumption
Sputtering and starting trouble
A clogged air filter can also cause black smoke to emit from the exhaust.

Replacing the air filter is inexpensive and easy to do by yourself. The process for removing the filter from a push mower is different than it is for a riding mower. For a riding mower, you can start by shutting off the engine and raising the parking brake; for a push mower, pull the spark plug wire from the plug. You will then be able to remove it from its air filter housing.

Paper filters will generally require replacement. A foam filter can be washed in a solution of hot water and detergent, which will loosen any grime. Let the filter dry completely, then wipe fresh motor oil over its surface and replace it in its housing. When you power up the mower this time, you should be hearing the familiar whirring of an engine in prime condition.

Clean fuel is essential for your mower to be able to operate properly. Located either in the fuel tank or the fuel line, a lawn mower’s fuel filter prevents unwanted particles from entering the carburetor.

When the fuel filter becomes clogged or damaged, it’s unable to perform its critical role of protecting the carburetor. Dirty fuel filters will eventually create serious problems in the performance of your mower.

A clogged fuel filter should be considered if you notice decreased performance or difficulty in starting the mower. You’ll need to remove and examine the filter to determine whether it’s the culprit and if it should be replaced.

The sign that a fuel filter should be replaced involves a visual inspection. If you notice debris on the filter or you’re unable to see light through the filter when its held against natural light, it would be wise to replace it.

Your mower’s ignition coil – often referred to as the starter coil – is responsible for transferring voltage from the battery to the spark plug. This creates the spark igniting the fuel mixture that starts the mower. So if the spark plug fails to receive current via the ignition system, the ignition coil may need to be replaced. Due to the nature of its role, problems with the ignition coil will always result in problems with starting the mower.

One of the unique symptoms of a bad lawn mower ignition coil is evident when the mower is in use; the mower will often overheat and shut down, requiring time to cool down before starting again briefly then overheating again.

If you suspect something is wrong with the ignition coil, test the starter using an in-line spark tester. If the spark tester successfully produces a spark when it’s connected to the spark plug, the starter coil is not the source of your lawn mower’s problems.