Petrol Lawnmower Wont Start

Petrol Lawnmower Won’t Start Help

In this article, I’m going to cover the main reasons a petrol lawnmower won’t start.  Also, if you think you’ve got fuel and spark and compression, all the usual three things that you need to get a petrol lawnmower up and running and it’s still not starting, this could be the article for you, as I’m going to go a little more in-depth, so keep reading and let’s get started.

Why Won’t My Petrol Lawnmower Start?

A petrol lawnmower requires fuel, spark and compression to start the engine. If either of these processes are faulty your petrol lawnmower will not start.

There are lots of videos on YouTube and articles on the web showing you what to do if your petrol lawnmower won’t start. We’ll cover these basic steps very quickly, but I want to tell you some different reasons and things you can try to help you get a lawnmower running that won’t start if you’ve checked all the basic things.

Here’s a playlist of videos from my Youtube channel that covers many aspects of lawnmower repair.

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Let’s Start With The Basic Reasons For Why A Lawnmower Won’t Start

Firstly what you need to do is make sure you’ve got some fuel in your mower. If you’ve got the type of mower with a push prime carburettor on, just take the air filter box off with a flat-headed screwdriver and prime it, you can actually see if you’ve got any fuel going through.

A lot of people don’t realise that you can actually visually see if there’s any fuel going into the engine from the carburettor, this just eliminates any problem you might have with this primer bulb.

Please note you can find many parts for your needs in my online parts store here

Lawnmower Has No Spark And Won’t Start

The next thing that’s really simple to check as well is the spark.

First things first, if a lawnmower has no spark then it will not start.

What you need to do is just unscrew the spark plug whilst leaving it connected to the spark lead, just put it against something metal and see if you get a spark, and if you can’t see properly, you can always buy a spark plug tester.

Basically, once you clamp the handle which releases this brake, you can test the spark plug, you can pull the lawnmower over and put your spark plug tester on and actually see if you’ve got a spark. If you’ve got a spark, it’ll light up.

How To Check For Compression On A Lawnmower That won’t Start

The next thing that most people will tell you to check for, which is correct, is to check you’ve actually got some compression, so basically, if you pull the lawnmower over and it feels like it pulls over ok, and it’s not too easy to pull over and it doesn’t feel like it’s broken inside, you will have compression.

I have recently filmed an in-depth video on how to service a petrol lawnmower, that covers everything you will need to know if your lawnmower won’t start.

You can check it out here. Click the white triangle play button to watch the video.

You’ll be very unlucky if you didn’t have compression, and those are the three basic things that people check. Fuel Spark and Compression.

Lawnmower Has Spark, Fuel And Compression But Still Won’t Start!

Well, unfortunately, I get a lot of comments on my youtube channel now saying, “I’ve done the basic things; I’ve checked the plug – I know it’s getting fuel on it and I’ve got compression,  I’ve got all these three things working correctly and it still doesn’t start.

What are the other reasons that a gas lawnmower won’t start?” Well, I’m going to cover a few of those right now.

Check out this video I made specifically on what to do if you have these basic things working but your lawnmower still won’t start.

Please subscribe to my Youtube channel here

Check The Lawnmower Kill Switch

There are two different types of starting mechanisms on the mowers I regularly come across. One of them is a kill switch with a brake. If the kill switch doesn’t disengage, it won’t let your lawnmower get a spark. This is a safety feature on most lawnmowers in the UK. The idea behind it is that you can’t leave the lawnmower running whilst unattended.

The kill switch is going to be definitely something you need to check, and the brake usually releases by operating the cable from the handle. You will see how the cable operates a small arm, it pulls backwards and forwards against the flywheel. You can see this when you have the starter recoil cover off and remove the kill switch cover. 

If the kill switch on a lawnmower does not operate correctly then a lawnmower won’t start.

Where Is My Lawnmower Kill Switch?

The kill switch is usually around the back off the flywheel and you’ll have to strip a few covers off to find it.

On a more expensive lawnmower it will be a microswitch instead of a kill switch, if you don’t hear the microswitch clicking when you pull the safety lever you have a problem as that’s what stops you getting a spark, so if you’ve got a lawnmower that’s has a microswitch, make sure it at least clicks when you pull the lever at the top of the handle. A kill switch doesn’t make a click it.

Take a look at this video for more information on kill switches

Check The Ignition Coil And Ignition Wire

Take a good luck at the wire that runs from the ignition coil to the kill switch. It may want cleaning up where it connects to the kill switch.

This wire will run right around the side of the flywheel and actually runs into the ignition coil. I get a lot of questions about ignition coils saying, “Could it be the ignition coil, or could a faulty ignition coil  be the reason my lawnmower won’t start?”

Well, I’ve repaired hundreds of lawnmowers for profit and I’ve rarely ever had one with an ignition coil that doesn’t work, but one reason I have found that will stop your lawnmower starting properly, is if the little ignition wire gets damaged, because this wire runs around the flywheel then runs right around the back and it goes to the kill switch. If this wire is damaged or has become disconnected your lawnmower won’t get a spark and therefore won’t start.

I’ve seen lawnmowers when people have had a play about and whilst taking the recoil cover off and putting the starter recoil cover back on have either snapped or damaged the ignition wire.

So, if your lawnmower won’t start, you’ve done the usual things, there are two things to check:

You need to make sure that the kill switch is clean and the brake’s actually moving out of the way correctly, and there’s not a problem with the cable or wire, you need to make sure the wire doesn’t have a split in it or it hasn’t actually pulled off the kill switch  or microswitch, and you need to make sure that it’s connected under the ignition coil properly.

When you strip a  lawnmower down and remove the ignition coil, turn the coil over and take a good look at exactly where the wire runs and all the housing clips are. You will see the wire connects to the underside of the ignition coil. 

  • Note I’ll just mention this as well as I do on most of my videos, whatever you do on a lawnmower, even if you’re not working underneath it, it’s always a good idea to remove the spark plug.


How To Remove The Ignition Coil On A Lawnmower Won’t Start

Now, all you need to do is remove the ignition coil with a quarter-inch socket. Basically, just take the two bolts out and you can move it out of the way. These two bolts/ screws on either side of the coil hold the ignition coil in. From here you can set the correct gap when re-installing. Remove these parts, put them in a magnetic tray so you don’t lose any of them, now you will see the underneath this ignition coil exactly where the ignition wire connects to.

Move the governor flap out the way and don’t disturb any of the spring linkages on the carburetor, you actually see that you’ve got the ignition coil off and the wire is really easy to connect – it just pushes on from underneath – that’s all you need to do if you want to replace the wire from the ignition coil.

Here’s a video on how to set the ignition coil. Skip to 30 mins and 40 seconds.

This video is actually a full-service tutorial.

If you want an in-depth look at everything you will need to know to service and repair please check out my online training products here.

Currently, this package includes 2 extra pieces of training that includes Mountfield, Briggs and Stratton and Honda Lawnmowers

Here’s a sneak peek!


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So, if you’re having trouble with starting your lawnmower, I would suggest having a good look at the kill switch wire and definitely a good look at the kill switch. Now, I’ve had loads and loads of these types of lawnmowers , I’ve put loads of videos on youtube, and a lot of people on YouTube are talking about three things: they’re talking about getting fuel, getting spark, and also having compression, but the fourth thing that I think is a massive thing to look at is timing.

How To Check The Timing Of A Lawnmower Engine That Won’t Start

If a lawnmower doesn’t spark at the correct time, you’ll have fuel going into the mower exactly as it should. You’ll have spark exactly as you should, then you’ll have compression exactly as you should, but if everything’s at the wrong timing, it will spark at the wrong time and your lawnmower won’t start. I’m going to teach you in the next part of this article how to check the timing on your petrol lawnmower.

So, to check the timing on a petrol lawnmower, what you need to do is remove the flywheel. I have an impact tool for this so I can just zip the top nut off and get the flywheel off.

If you can’t do that, if you don’t have the equipment, maybe take it to somebody who’s got the equipment and ask them to take it off for you, I have used my local car garage mechanic in the past. Sometimes, they’re happy to do that for a few pounds.

If not, what you can do is put a piece of wood under the blade that stops it from turning, then you can get your socket on the flywheel nut and undo it, but it’s a bit of a fiddly job. I do recommend doing this with an impact tool.

With the flywheel still in place and looking down from the top of the crankshaft, you can actually see where the flywheel key sits and if this is damaged in any way, it has a knock-on effect over the whole timing of the lawnmower ignition. Take a very good luck at the flywheel key and if it looks damaged in any way replace it.

The flywheel has magnets on it, you can see them all the way around the flywheel and this is what makes the spark actually come through the ignition coil.

If the flywheel key is damaged and therefore the flywheel is set in the incorrect position, it only takes the timing to be in just the wrong place and the lawnmower will get spark from the coil but spark at the wrong time. You’ll have fuel and spark, and compression but it won’t run because the spark was at the incorrect time.

Here’s a video that shows the easiest way to do it

How To Remove A Lawnmower Flywheel

NOTE:  You really do need to check the flywheel key properly. The best way to do it is to take the flywheel off itself. This can be a bit of a fiddly job.

If you have a flywheel puller, you can put a flywheel puller on and basically, just pull the flywheel up, and off. What a lot of people do, is to just tap the flywheel from underneath with a rubber mallet and just knock around it until it comes off.

If you choose to tap the flywheel off from underneath be very careful because if you hit the top of the crankshaft in an effort to remove it and ruin the threads on the crankshaft, you might as well say bye to the lawnmower. You will never refit the flywheel nut!

The best way to do this, of course, is with the right tool which would be with a flywheel puller. So, once you’ve got this flywheel removed look closely at the flywheel key. If it looks like it’s been knocked across so it’s not a perfect shape in any way, you want to replace this. Don’t be fooled just because this looks like a small part, this is crucial to the timing of a petrol lawnmower.

This massively affects the timing of the petrol lawnmower and as I’ve said, if you don’t get it timed properly and it’s not sparking properly, it won’t start.

Why Does A Gas Lawnmower Have A Flywheel Key?

The idea of having a flywheel key is to stop the crankshaft bending. It is designed to shear to prevent any damage to the crankshaft.

Check For Stuck Valves On a Lawnmower That Won’t Start

If you’ve been really unlucky you may even have a stuck valve. This has only happened to me once or twice because the valve has actually come out of the valve seat.  To get to the valves, what you need to do is just take the head off the mower or remove the overhead valve cover.

Occasionally, I’ve had lawnmowers where the actual valve guide popped out and the valves actually don’t go in and out fully. They get stuck so either the exhaust or the inlet valve just stays open no matter how much you’re turning the flywheel.

 If you remove the head of the lawnmower and turn the flywheel you can see if the valves are opening and closing properly. If a stuck valve  happens, you could have fuel going through correctly, you could have spark, and you could have compression, but if the valve guides that sit around the edge of the valve has popped out and the valves aren’t moving in and out freely, you won’t get the correct compression and your lawnmower won’t start.

Here’s a video on how to set the valves on a petrol lawnmower.

Use Quality Spark Plugs!

One important thing to mention that’s really simple is this, buy decent spark plugs!! I can’t emphasise this point enough so please don’t dismiss it! – I buy  NGK B2LMs for most of my mowers. I bought B&Q’s own brand when I first started doing this and they were brand new, and those would not work at all. 

NGK are the best ones. I’ve had no problems with them whatsoever, I certainly won’t scrimp on buying  NGK spark plugs, and I must have had about fifty lawnmowers over the last seven or eight years that wouldn’t start simply because they needed a new plug.

You can find them here

Check out my other article on why a lawnmower won’t start after winter here.

A tip is to keep one separate that you know works fine and use it to test all the lawnmowers you repair for profit! I would highly recommend doing this with a petrol tank/ carburettor set up also. It will save you a lot of time and effort.

So, I hope these few tips have helped you out if you have a lawnmower that won’t start at all. Now, if you’ve got a lawnmower that starts and dies, or revs up and down, or just generally runs badly – puffs out black or white smoke, that can be a whole separate issue altogether.

Take a look at my article on hunting and surging for that issue

You may also be interested in my beginner’s complete guide to repairing lawnmowers for profit

Happy Mowing


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