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How Do You Fix a Lawnmower That Dies?

So, your lawnmower is playing up, starting, but dying shortly afterward, or running for a period but suddenly stalling on you. Naturally, you don’t want to give up on the trusty machine yet, but an un-mown or half-mown lawn just isn’t a good look. You’re determined to get that thing working again, but you’re not sure where to start.

So, how do you fix a lawnmower that dies? You’ll need to check and clean the parts that help it to work, replace the old gas, or give the carburetor and the related parts a thorough clean. The problem may suggest that the carburetor isn’t doing its job, probably caused by a damaged fuel line, dirty fuel cap, or faulty choke.

That sounds straightforward, but you’ll obviously need more detail to get the job done. That’s what you’ll find below, plus some useful tips to avoid the problem in the future.

What Does the Carburetor Do?

You’ll probably be more confident in sorting out this problem if you have a basic understanding of the carburetor. It’ll also help identify the crucial parts that may be the cause or your problem. So, let’s get into it.

What Is It?

Well, you could say the carburetor is the heart of your lawnmower’s engine. It looks like a tube. It brings in gas and air, mixes the two, and then sends the mixture to the engine to combust and provide power.

The mix has to be correct for your engine to run as it should. The carburetor has to adjust the fuel-air mixture to suit the conditions. So, the mix will be different if the engine is cold than if it’s been running a while.

How Does It Work?

OK, you’ll need to draw air and fuel into the carburetor, so your lawnmower has an air intake and a float chamber containing fuel.

The Air Intake

The air intake is usually in a housing on the side of your engine. You’ll find it’s easy to spot because it’ll have vent holes or slits in it. If you remove the cover to the housing, you’ll find the air filter, which is crucial because it prevents dirt and debris from getting into the carburetor.

So, filtered air comes into the top of the carburetor, through a valve called a choke. When the choke’s closed, it’ll restrict the amount of air getting into your carburetor, resulting in more fuel in the mix. That’s just what your engine needs when it’s cold.

However, once your engine has warmed up, the choke needs to be opened to restore the air supply. With the air supply restored, you should get the correct mix of fuel to air required for the engine to run well.

The Float Chamber

The float chamber is like a small fuel tank, connected to your main fuel tank, and which contains a float. The float will rise and fall with the level of gas in the chamber. When it falls, it opens a valve so that fuel from the main tank can flow into the chamber. As the chamber fills, the float rises, and the valve to the main tank will close, stopping the flow.

The float chamber is attached to the carburetor and is the direct source of fuel for the carburetor.

Putting It Together

Hopefully, this will clarify how these pieces come together to provide the right mix of fuel to air to your engine.

So, air comes into the top of the carburetor. Inside the tube shape of the carburetor, there’s a narrowing near the center, called a venturi. As the air is forced through this narrow section, the speed at which it flows increases, resulting in a reduction in air pressure.

The reduced air pressure in the carburetor is what causes a sucking effect, drawing fuel into it from the float chamber.

So, now you have your fuel-air mix. That gets drawn into the engine through another valve located at the bottom of the carburetor. You’ll probably know this valve is called the throttle.

As you’ll also know, opening and closing the throttle affects the engine speed as it increases or reduces the amount of fuel powering the engine.

So, Where Do I Start?

If your lawnmower starts but then dies or stalls shortly after, you’ll want to try some simple solutions first. Let’s assume you’ve already established that you have gas in the tank.

Make sure you’ve turned off the engine and removed the spark plug.

Easy Steps First

Check the Fuel Line

A simple check you can start with is that the fuel line between the gas tank and the float chamber isn’t leaking or damaged.

If the line is leaking, there may be an inadequate supply of fuel reaching the carburetor. Small kinks in the line can block the flow of fuel, resulting in the wrong fuel-air mix.

Both of these issues can cause the engine to die or stall, so if the line’s leaking or badly kinked, replace it.

If the line is in good condition, try detaching it from the carburetor, after clamping it off to stop gas coming out of the tank. Spraying the inside of the line with a carburetor cleaner should clear any debris or residues that may have come from the tank. Clearing blockages in the fuel line will restore the normal flow of gas to the carburetor.

If your lawnmower has a fuel filter inside the fuel line, you should also check that it’s clean and clear of debris. If it’s clogged, you should either clean it with a carburetor cleaner or replace it so that gas can pass through it freely.

Check the Fuel Cap

You may have noticed that the cap on the main gas tank has a vent in it. The vent is vital for the operation of the lawnmower as the gas tank needs to allow air in when the level of fuel drops.

Without air replacing the fuel used, you’ll get a vacuum in the tank, which will prevent fuel from leaving the tank and filling the float chamber. So, your lawnmower will run for a while but will stall because fuel has stopped reaching the carburetor.

You can do a quick check to see if this is the reason why the engine dies or stalls, by loosening the cap and starting the engine. If it runs normally, you’ll know that the vent in the cap is blocked.

Just clean the cap and ensure that the vent is clear, using a needle or thin wire.

Check the Air Supply

Since air is one of the vital ingredients to run your motor, you need to ensure that it’s getting to the carburetor.

As mentioned above, the air intake to your carburetor is in a vented housing attached to your carburetor. Within the housing, you’ll find a filter.

So, with the engine stopped, remove the housing cover and check that the filter is clean. If it’s not, it won’t be doing its job of ensuring that the air going into your carburetor is debris-free.

Debris can clog your carburetor, but also, if the filter is clogged, it may restrict the flow of air to the carburetor. So, the carburetor won’t get the air it needs for the correct fuel-air mix to keep the engine running.

This may be why the engine keeps dying or stalling, and cleaning or replacing the filter will restore proper airflow.

Check the Choke

You’ll recall from above that when you start an engine from cold, it’ll need a richer fuel-air mix than is required once the engine has warmed up. You get this by switching off the choke at start-up, so the choke valve, which is like a swiveling plate, closes. The closed valve reduces the air in the air-fuel mix.

However, if you leave it off once the engine has run for a bit, the fuel-air mix will be wrong, so the engine won’t run as it should or will stall.

So, once the engine has warmed up, make sure you turn the choke on, so the choke plate is open, allowing air to flow into the carburetor.

If the engine stalls even though you set the choke lever to on, you’ll need to check the choke valve is working.

You’ll find the valve behind the air filter, which, as you’ve seen above, is in a housing attached to the carburetor.

With the engine off, remove the housing cover and the filter, followed by the base of the housing, so that you can see the choke valve.

Watch if the choke valve opens when you turn the choke on and closes when you turn it off.

If not, you’ll need to look at the cable, which connects the choke to the choke lever. You should see a screw that you can adjust to set the position of the cable.

You’ll need to loosen the screw first. Then, move the cable until you see that the choke valve is opening and closing fully as you turn the choke on and off. Once you’re happy with it, tighten the screw.

Check the Choke Shaft

It’s also worth checking the choke shaft, which is what moves the choke valve in response to movements of the throttle cable. If it’s not rotating freely, it may be due to grit that can collect during use. Spray it with a carburetor cleaner to get rid of that grit.

Change the Gas

If you’ve left gas in the tank for an extended period, for example, over the winter, don’t go thinking you’ll be able to use it come spring.

The problem with gas is that it can deteriorate over time as lighter chemicals evaporate, leaving heavier ones behind. This affects combustibility, so compromises the performance of your mower.

Also, oxidation causes chemical reactions, which result in a gummy residue that can clog the carburetor, preventing it from getting the fuel it needs.

So, drain the old gas, clean the tank to remove the residues, and replace the gas with fresh gas. Once done, fire up your lawnmower, and it should now run correctly.

I’ve Tried All of the Above, but the Problem Persists

OK, those were the possible quick fixes. If they didn’t work, you’re going to have to try a couple of trickier, but achievable things.

The Next Stage

Clean the Float Chamber, the Float, and the Bolt Feed

Underneath the carburetor, you’ll see the bowl-shaped attachment called the float chamber. It’s connected to the carburetor by a bolt. However, the bolt isn’t just a fixing. It’s also a feed for the fuel jet that delivers gas to the carburetor.

You’ll need to have an old rag handy to catch the gas that’ll drain from the chamber as you remove the bolt.

Unscrew the bolt and remove the float chamber. Make sure you clean the float chamber thoroughly, including the outlet pipe, to remove dirt and debris. This will ensure the float can function properly, and there’s nothing to block the flow of gas into and out of the chamber.

Don’t forget to clean the bolt feed, because if it’s clogged, fuel won’t be able to pass through its tiny ports. So, you should clean them by pushing a small wire through the ports.

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Finally, check the float, because if there’s gas inside, it won’t function as it should. Gas inside the float means the float is damaged, so you’ll need to replace it.

You’ll also need to check the needle seal that shuts off the gas when the float is raised by fuel filling the chamber. Make sure it’s in good condition because if not, you’ll need to replace it.

Also, if the seat for the needle seal is clogged, fuel won’t be able to flow freely from the gas tank. So, clean it with a thin piece of wire and spray carburetor cleaner up into it.

Once you’ve done all this, try running the mower again.

Hopefully, that’s solved the problem, but if not, there’s still more you can do.

Clean the Carburetor

You’ll have to bite the bullet now and detach the carburetor so that you can take it apart for a more thorough clean. This will involve removing and cleaning all of the parts referred to above and the inside of the carburetor.

To clean, you should soak all the parts in the carburetor cleaner for the time recommended for that cleaner. So, check the instructions, but soaking time maybe around an hour or so.

After that, you’ll need to rinse thoroughly with water and allow the parts to dry completely before reassembling them.

If that’s too daunting, or doesn’t solve the problem, your next option is buying a new carburetor for your mower. If you’ve carried out all the steps above, fitting a new carburetor will be a breeze.

How to Avoid Recurrence

Regular Cleaning

Once you’ve got your mower running correctly again, make sure you clean the carburetor regularly. It’s a relatively straightforward task using a spray carburetor cleaner.

You’ll need to start with a cool engine. Then, remove the air filter housing and the choke linkage to expose the side of the carburetor.

It’ll be damp at this stage, so let it dry out before you start the mower. Once the motor’s running, spray the carburetor cleaner into the center of the carburetor via the air inlet. So, you’ll need to make sure the choke plate is open.

This gets the cleaner into the body of the carburetor and down to the throttle plate at the bottom.

It should go without saying, but while the engine’s running, don’t touch the carburetor.

Now, turn off the engine and spray the choke shaft, which you read about above.

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Make sure you spray the cleaner into the fuel line to remove any dirt or debris that may have built up during use.

Empty the Gas Tank at the End of the Mowing Season

After everything you’ve read here, you’d be mad to even consider leaving gas in your mower if you’re not going to use it for an extended period.

In particular, at the end of the mowing season, you should drain the gas tank and clean it thoroughly, as described above.

Don’t forget about the float chamber because there’ll still be gas in there even once the main gas tank is drained. Make sure you empty it also, by following the steps you’ve learned here.

Don’t even think of saving this gas or any other gas you still have left, for use next season.

Final Thoughts

So, armed with the information above, you should be able to solve the problem of your mower starting, but then dying on you or stalling.

Once you’ve got it running again, the important thing is not to neglect it and end up in the same situation.

So, regular checking and cleaning, as described above, is vital, especially at the end and the start of the mowing season. But mid-season maintenance will ensure your mower is in tip-top condition throughout the season.

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ReadyToDIY is the owner of this article. This post was published on May 22, 2020.

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