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How to Fix a Petrol Lawnmower That Won't Start

If you find that your petrol lawnmower won't start, there's a range of potential issues that need to be checked for in order to get it working again. In this article, we'll run you through how to fix a petrol lawnmower that won't start step by step so you can get your appliance back in action once more.

For this guide, we're using a Qualcast classic petrol 35 lawnmower, but these checks should apply to just about any model of petrol lawnmower.

What you will need:

Find your model number:

When carrying out these checks, you may discover that you need to replace certain parts of your lawnmower. If so, you'll need to know your lawnmower's model number to make sure you buy the correct parts for your particular appliance. The model number and information should be located on a sticker or rating plate on the lawnmower, either on the back, top, front or underside of the appliance.

How to fix a petrol lawnmower that won't start

Let's take a look! To learn how to diagnose and fix a petrol lawnmower that won't start, either watch the video or follow our step-by-step guide below.

Safety first!

Remember, always make sure your appliance is switched off before carrying out any work or repairs.

Check 1: The fuel

Petrol degrades and goes off over time, which can lead to your lawnmower not starting up properly. If you've left petrol in your lawnmower over winter or for a prolonged period of time, drain the petrol and top the lawnmower back up with fresh petrol.

There are two places to drain. First, drain the main fuel reservoir by uncoupling the fuel line, then proceed to drain it into a bowl.

Second, drain the fuel from the carburettor. You will see on the carburettor that there is a little release valve that will allow you to drain it.

When filling your lawnmower with petrol, it's also important to make sure you use the correct kind. For example, a standard four-stroke engine takes ordinary unleaded petrol.

Check 2: The oil

Next, check the lawnmower's oil. The dipstick on the lawnmower we're using for this video can be found on the back of the lawnmower, as pictured below. Take a look at your appliance user manual to check for the location if you're using a different type. Then, unscrew the dipstick, give it a wipe with a paper towel and then screw it back in. Remove the dipstick a second time and you will be able to see how full the oil is based on where it reaches on the dipstick.

If you are in need of more oil, top it up at the front of the lawnmower instead of the back. When topping up the oil, you should also make sure you are using the correct engine oil for your mower. For example, if you have a two-stroke lawnmower, top it up with two-stroke oil. Check your user manual for more details.

Check 3: The spark plug

Spark plugs typically have a lifespan of around 150 hours, so it's important to check that your lawnmower's spark plug is working properly and to replace it if necessary. On the lawnmower we're using for this guide, we have a standard 14mm lawnmower spark plug that can be removed with a 21mm socket which is specifically for spark plugs. However, this may vary depending on your particular mower. Using an appropriate socket, remove the spark plug.

If the spark plug is dirty or in bad shape, it will need replacing. The spark plug on the appliance we used for this guide is somewhat dirty but not in bad condition, therefore it can simply be cleaned with a wire brush and fine sandpaper between the two electrodes. However, as you can see below, there is a difference between a new spark plug and one that has seen use, so make sure you replace yours if necessary. When replacing or refitting the spark plug, make sure you don't overly tighten it.

Check 4: The belts

Next, check the belts. Start by removing the belt housing cover on your lawnmower. You'll likely need to unscrew this cover to remove it.

To check the belts, first engage the lawnmower's rear roller. As you do this, take a look and make sure that the belt is tightening when the rear roller is engaged.

Then engage the cutting blades and check that this is also putting some tension on the belt.

Next, take a close look at the belts and make sure that they aren't fraying, cracking or showing signs of damage and wear. If you find any such damage or the belts aren't tightening properly when engaged, they will need to be replaced.

Check 5: The gears

Whilst checking the belts, you should also check the gears alongside them. On the lawnmower we're using for this guide, there are three nylon gears, but this may differ on your appliance. Check the gears by giving them a quick inspection and making sure they aren't damaged or faulty. If they are, you will need to replace them with new gears.

Check 6: The throttle

Once you have checked the belts and gear and refitted the belt housing cover, check the throttle by turning it on and off.

When the throttle is in the off position, you should be able to see that the small metal lever beneath will cut the power to the spark plug.

Then, when the throttle is engaged, you should be able to see it open the carburettor.

If you find that the throttle needs adjusting at all, there should be a screw by the small metal lever that allows you to alter the position at which it opens the carburettor.

Check 7: Try starting the lawnmower

After carrying out all of these checks, you can test your lawnmower to see if it will start up properly. First, push the throttle all the way back.

If it's cold, also give the lawnmower some choke. When the air is cold it doesn't mix with the petrol quite so well, so the choke will provide a much more petrol-rich mixture to help the lawnmower start up.

Then give the cord a pull. Hopefully, your lawnmower should now start up without issue.

Check 8: The carburettor

If the lawnmower still doesn't start, however, the fault likely lies with the carburettor. Before opening up the carburettor, first uncouple the fuel line and drain the fuel into a container. Do this swiftly to spill as little fuel as possible, though you are bound to spill at least a little as you transfer the fuel line to the container.

Then proceed to remove the air filter from the lawnmower.

You then need to drain the remaining fuel from the carburettor. Hold a generous amount of paper towels beneath the release valve, then press the valve to drain the remaining fuel onto the towel.

Once drained, remove the nut from the carburettor using a socket.

As you remove the bottom screw, the float chamber will also come away. You can place the float chamber and screw to one side.

You will then be able to see the float itself. You can remove the float by pushing out the hinge pin that keeps it connected.

There are two parts to be concerned about here. The first is the small float piston that sits on top of the float and rises and falls, letting fuel into the float chamber.

The second is the bottom screw which you removed to release the float chamber, which has fuel jets.

Both of these parts will need to be cleaned thoroughly. To clean the screw, run a piece of wire through the fuel jets and then also down through the centre hole of the screw, making sure there is no debris.

Then give the piston a clean with compressed air or a specialist carburettor cleaner.

It's also a good idea to spray compressed air or a specialist carburettor cleaner around the rest of the carburettor. You should also check the gasket seal while you're there too, and replace it if it's looking damaged, worn or worse for wear.

Then, when putting the carburettor back together, you will notice that there is a shallow end and a deep end inside the float chamber. Make sure you refit the float chamber so that the shallow end goes where the float hinge is before refitting it with the screw.

Finally, don't forget to reconnect the fuel line and refit the air filter.

And there you have it! This should help you get to the bottom of why your petrol lawnmower isn't starting and get your appliance back in action once more.

While you're here, make sure you're using your lawnmower as safely as possible! Our article on important safety checks for your lawnmower is sure to get you in the know.

Before you go, make sure you also sign up to our mailing list below for more appliance repair guides, maintenance advice and all other things eSpares. Thanks for watching!

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