Why Lawn Mower Starts then Dies?

Why Lawn Mower Starts then Dies Image

For a product that is meant to make your life easier, lawnmowers sure can be temperamental. The worst one is when the lawn mower starts then dies without provocation.

You then begin to run options in your mind: did I pull a lever too hard? Is the spark plug dead again? Is the carburetor busted, or is this manufacturer bonkers?

However, if you talk to other lawn mower owners, you’ll find that you are not alone.

That’s why we have created this list of the 17 most common causes and solutions. Some of these are newbie errors (we’ve all been there), while others might need you to whip out your toolbox, so let’s begin.

You’re out of fuel

If I had a penny for every time a lawnmower “emergency” was because there was no fuel in the gas tank, I’d be smiling to the bank every weekend! And this could happen with the same person more than once. So, don’t feel bad if it has happened to you.

It’s easy to forget that there is no gas in the fuel tank, especially if you haven’t used your lawnmower in a while. Unfortunately, this problem also happens with riding mowers because many of them don’t have an electric fuel gauge, making it easy to miss.

The fuel gauge might also be bad if you have owned your lawnmower for a few years – typically past warranty. So, even if it shows you have fuel, open the gas tank and see for yourself before you try a more complicated solution.

The choke is not in the right place

Depending on the type of push lawnmower you have, you usually have to  put the choke on before the lawn mower starts, then switch it back. An automatic choke will go back by itself when the engine is ready. If you forget to put the choke back, or it doesn’t return as it should, the lawn mower will die.

That is because the purpose of the choke or throttle is to control the flow of air. You need just enough air to get into the carburetor for the fuel to heat up quickly, but then too much air will switch it off.

In other instances, your lawn mower starts even when the choke is not switched on. But if you make this a habit, this could lead to more serious issues, as it forces the spark plug and carburetor to do more work in order to get started.

Bad or old fuel in the gas tank

From our experience and research, we find that the primary reason why lawn mowers start then die in spring is that they have been left idle all winter with fuel inside. Gas in a lawnmower lasts for two to four weeks before it begins to break down.

You can tell that the fuel is stale when it begins to smell almost sweet or more like kerosene. If that’s the case, here are the steps you need to rectify it

Step 1: Drain the fuel in the carburetor float bowl

If the gas has been sitting in the tank for only a few weeks, chances are you only need to drain the carburetor bowl. You can access this easily without opening up the engine. Once you’re done, try the lawn mower. If it doesn’t start, move to the next option.

Step 2: Drain the fuel tank

If this is your first use since winter, you probably would need to empty the fuel tank. You do this by pulling the fuel hose lining the gas tank to the carburetor bowl. Once you’re done, you need to pour in fresh gas.

Don’t pour in fuel you stored in your shed all winter, as this could also be bad, even if it was in an air-tight container. Go get fresh gas from the station, and everything should be just fine.

On the off chance that you haven’t used your lawn mower in over a year, it is possible that the stale gas could result in a clogged carburetor or even a faulty carburetor, and we’ll address this later.

To prevent your gas from going bad, you should do two things:

– Use fuel stabilizer

A fuel stabilizer or gas stabilizer will prevent the fuel from breaking down quickly. Instead of the gas lasting just a month, the fuel stabilizer preserves gas for up to 24 months in your gas tank. Make sure you buy the gas stabilizer recommended for your engine.

– Choose the right fuel

The best gas for lawn mowers is unleaded gasoline with a minimum of 87 octane. If there is a mix of ethanol, it shouldn’t be more than 10%. Ethanol breaks down a lot faster than pure gasoline, so you will battle with stale gas quite often.

Bad, dirty, or old spark plug

Image courtesy of YouTube

Now, we arrive at our usual suspect. When a lawn mower starts then dies, most users naturally assume it’s because of the spark plugs. Spark plugs need to be changed every 25 hours. If you mow your lawn for an hour every Saturday, the spark plug should last about six months.

If you have a bad spark plug, you might be able to get one or two starts out of it. It will be able to ignite the engine, but it won’t be able to sustain it. If the lawn mower ran for a few minutes before it died, it could be you have a dirty spark plug.

Remove the spark plug and inspect for carbon buildup. If it’s dirty, clean it with a soft wire brush and put it back. You should also check the spark plug wire to see that it isn’t loose.

If the lawn mower still doesn’t start, you need a new spark plug.

Also note that if you make repeated attempts before the lawn mower starts but doesn’t stall, it could still be a sign of defective spark plugs. This is a fairly routine procedure, so ensure you have a spark plug socket in your shop at all times and a spark plug gapping tool.

Clogged air filter

Image courtesy of Columbia Florist

One of the most important parts of your lawn mower is the air filter, and it goes through a lot of abuse every time you use it.

Air filters prevent dust and debris from entering the engine. When it accumulates too much, air can no longer enter the engine to keep the gas burning. The engine will still start with a dirty air filter, but it won’t be able to keep it going.

That’s why you are to change the air filter regularly, usually every 3 months. Ensure that you use the air filter recommended for your lawn mower.

Clogged fuel filter

Another part of the lawn mower you need to replace often is the fuel filter. The fuel filter does as the name suggests – it keeps particles in the gasoline from entering the carburetor. A dirty air filter will cause the engine to stall, sputter or run unevenly.

Most manuals will tell you to change the fuel filter every 200 hours, but to keep it simple, you should change it once a year. Even if you use a fuel stabilizer, you should still change the fuel filter because they don’t do the same job.

Low oil level

There are a lot of moving parts in the tiny engine of your lawn mower or the big engine of your riding mower. In both cases, you need engine oil to provide lubrication so that the grinding parts don’t overheat or melt.

If the engine oil level is too low, your engine will stall, and you should be very concerned because this might mean something in there has gone bad or needs to be replaced. If all you need is fresh oil, you’re in luck.

However, if after you put in new engine oil, the lawn mower still won’t start, it could mean an expensive repair job. That is why you are to check the oil level before every use.

What we tend to do is estimate how long the oil will last and only check when we think it’s due. However, some types of oil drain quicker in certain temperatures, so you shouldn’t take this for granted. Take a few minutes to check the oil, so you don’t spend a few hundred dollars trying to repair it.

Too much oil

Some of us don’t like the idea of constantly checking the oil before using the lawn mower, so we came up with the ingenious idea of pouring in more oil than recommended. Or you might assume you are meant to fill up the oil tank just like you would the gas tank.

The problem is that too much oil can cause the engine to work too well, so they begin to overheat. Not only that, but excess oil can also lead to leaks, which can damage the motor.

If you have poured excess oil by accident, use an oil siphon to bring it down to the recommended level.

Bad or damaged fuel pump

If you have a push mower, you can skip this one. As the name implies, the role of a fuel pump is to move fuel from the gas tank to the carburetor. In a push mower, the carburetor tends to sit below the tank, so gravity will make the fuel flow.

However, in a riding mower, you need a pump to ensure consistent gas flow, otherwise, the engine will start and then die and when it gets really bad, it won’t start at all. The good thing is diagnosing a bad or faulty fuel pump doesn’t require a mechanic.

The first thing you might want to check is the fuel line. Disconnect the line from the pump and place a bowl under it. Switch on the lawn mower and see if fuel comes out. If gas comes out, then it means the fuel line isn’t clogged, so you can then inspect the fuel pump.

Next, you need to disconnect the fuel line from the fuel pump to the carburetor. Replace that line with a hose that will drain into a bucket, then turn the lawn mower on. If the fuel pump is good, it should yield a consistent flow of fuel. If there are delays in the gas flow, you need a new fuel pump.

Clogged fuel cap

The fuel cap for your lawn mower is designed to vent by itself, which means it should allow some air to come in as the fuel goes out. When the fuel cap is clogged, and it’s not venting, it will create a vacuum that will prevent fuel from going into the carburetor.

One sign that this might be the issue is if your lawn mower runs for about ten to fifteen minutes and then begins to sputter as if it’s out of fuel. When you open the tank and see that there is still gas, it means the fuel cap isn’t venting.

If you suspect this might be the case, you can perform a simple test.

Step 1: Fill the tank with fuel

You are going to empty the fuel tank, but first, you need to fill it up close to the brim.

Step 2: Empty out the tank

Place a bowl beneath the fuel line connecting to the fuel tank, then pull it out. Leave it open until the gas stops coming out.

Step 3: Open the fuel tank

If the tank is empty, it means something else is wrong, and the troubleshooting continues. However, if there is still fuel inside, even though it stopped coming out, it means there is a problem with the fuel cap.

All you need to do is replace the fuel cap and take the lawn mower for a test drive.

Clogged cooling fins

Image courtesy of YouTube

When you have a fan-cooled lawn mower, you might experience this problem every few years. If you ever notice your lawn mower smells hot before when it’s on, there might be a problem with the cooling system.

The fans pull air into the mower, but often grass and mud come in too. Eventually, this will build up to large chunks. Admittedly, this is not the easiest part of the lawn mower to get to, but it literally only takes a few minutes.

Once you get to the fins, you can hose it down or use a soft steel brush to clean it.

Grass buildup in the mower deck

Image courtesy of Neighbor

Keeping your mower deck clean is one of the most important routines you should do. Sure, it looks weird to clean something no one ever looks at, but there are tremendous benefits.

But then again, why does grass buildup? If you wait until your grass grows too long before you mow, chances are you will get a lot of buildup in the mower deck. You will also get a lot of debris if you cut the grass too low. When this happens, the airflow is restricted, forcing the engine to work harder.

Not only that but the chute will get blocked, which also forces the engine to exceed its capacity. As a result, the engine can get hot, and it will stall.

Make sure you clean the mower deck at the end of every month. Sure, manufacturers will tell you to do it twice a year, but the cleaner, the better.

Plugged fuel line 

If you often pour fuel into your lawn mower without a strainer, chances are large particles will sneak into it. The debris can eventually damage either the fuel filter or clog the fuel line.

Another thing that can clog the fuel line is old fuel. Old gas gums up as the chemicals break down. When this happens, fuel flow is cut off or restricted, which then causes the lawn mower to die after it’s been on for a while.

If you had to get rid of old gas, you should inspect the fuel line too. Check the body for cracks or kinks and if found, get a new line.

If the fuel line is clogged, you need to spray carburetor cleaner into it and then flush it out with compressed air. Repeat this process until all the debris is gone.

Faulty ignition coil

The ignition coil is responsible for producing the spark that gets your spark plug going, which then ignites the fuel and so on. Diagnosing a faulty ignition coil can be a bit tricky, which is why this is further down the list.

Signs that you have a faulty ignition coil are: the engine takes a while before it starts; the lawn mower starts then dies when it gets hot; you can smell fuel when the lawn mower is running.

If you notice any of these, you should first check the spark plugs because this is the more likely cause. If you have already checked the spark plugs, you might need to get an ohm meter.

If the coil is fine, the resistance will read between 2,500 to 5,000 ohms. Anything outside of this suggests a fault, and you need to change the coil.

Ignition coils are pretty sturdy and should last you five to ten years on average before breaking down. However, if the coil is exposed to too much heat, it will wear out faster. A new one could cost as much as $300 besides labor, which is why it is important to keep an eye on the oil level, the air filter, and the type of fuel in order to prevent overheating.

Dirty carburetor

This is one of the most common causes of a dying lawn mower and deserves to be higher up on our list. However, we put it here because it takes a bit of effort to clean. There are many things that could cause a dirty carburetor, but old fuel is the usual cause. Debris from grass, dirt, and mud is another culprit.

Whatever the cause, a good clean should do the trick, as long as the carburetor isn’t damaged.

There are two main ways to clean a carburetor. You can take it off completely or clean it on the mower.

The safer option is to remove it completely and give it a thorough clean.

However, if you don’t have the time, tools, or inclination, here is a shortcut that works with most mowers when the carburetor isn’t too dirty.

Summary of how to clean a dirty carburetor

  1. Get your tools ready

You’ll need pliers, a wrench, screwdrivers, a ratchet, carburetor cleaner, clean blows, and compressed air.

  1. Clean the air intake

Before you go too far, take out the air filter and spray carburetor cleaner into the intake. If this solves the dying problem, then you’ve just saved yourself a lot of work. Otherwise, we need to push ahead.

  1. Take off the fuel tank

Chances are the dirt came from here, so you’re going to empty that out and replace it with fresh fuel when you’re ready.

  1. Clean the float bowl

Before you remove the carburetor, you should clean the carburetor bowl with carb cleaner. You should do this before to prevent dirt from falling into the carburetor when you take it off.

  1. Remove the jet

You then need to remove the jet from the bowl and clean it. Use carb cleaner or a brush to take off any debris from dirt or the gasket. You should also clean the hole in the jet with a small piece of wire.

  1. Remove the float bowl

If you have a clogged carburetor bowl, you probably should just replace it instead of trying to clean it. If it looks decent, spray carb cleaner and give it a nice polish.

  1. Change the gasket

The gasket will most likely be in bad shape, so you need to replace it. Ensure you don’t use carb cleaner on the o ring because it will cause it to slack. Instead, use compressed air to get rid of any fragments.

  1. Reassemble the carburetor

Put everything back in place and give the lawn mower another turn. If, after all this, the carburetor isn’t working, check out the next option.

Worn out carburetor

If you have noticed black smoke coming out of the exhaust or that your lawn mower backfires, it is possible that the carburetor is worn out. Unfortunately, you can’t know for certain until you open it up.

If, after a thorough clean, the carburetor still isn’t working, you should replace it.

Loose parts

This is one of the most frustrating issues that can cause your lawn mower to stall. There are loads of tiny bits in the engine, including in the fuel cap, carburetor, ignition coil, and elsewhere. If any of these bits is loose, it could prevent your engine from running properly.

You will then need to take it apart until you find where the problem lies. This can be time-consuming because you have to check the tank, carburetor, cooling fins, and everything else.

However, this is the last resort, and if you get to this stage, you are better off calling a professional.

Don’t Stop Mowing

There you have it: almost all the things that could go wrong. If your lawn mower starts then dies, work your way through the usual suspects first: fuel supply, spark plug, carburetor, and oil level. Most of these fixes only take about half an hour and require tools you should have at home.

However, a lawn mower is a pretty sizeable investment, so if you think any repair is beyond your skill level, take your baby to a lawn mower repair shop.

If the repair bill ends up being too hefty and you want to buy a new mower, check out our review of the best lawn mower under 500.

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