My Lawn Mower Starts Then Dies: What’s Wrong?

Lawnmowers are mechanical things and as such can encounter a wide range of problems – the most common being lawnmowers starting and dying.

If you’re facing lawnmower startup issues, this guide should help diagnose and fix the problems. However, if your lawnmower is still under warranty, I recommend having it looked at by an authorized technician.

My Lawn Mower Starts Then Dies What’s Wrong

Why Is My Lawn Mower Starting and Dying?

When your lawn mower starts up but cuts out before you have a chance to get anything done, the usual reasons are…

  • A dirty carburetor or an obstruction in the carburetor bowl
  • Stagnant gasoline
  • Damaged or dirty spark plug(s)
  • Excessive oil levels

That may seem like one hell of a line-up, but I’m happy to report that they’re all incredibly easy to fix, and I’m going to show you how.

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Carburetor Chaos

Besides being incredibly difficult to spell, the carburetor is your lawn mower’s most important component, so when your grass-eating beast starts to misbehave, all eyes should turn to the carb.


I Thought Only Old Cars Had Carburetors. What Are They Doing in Our LawnMowers?

The carburetor in your lawn mower does the same thing it does in your car: mixes precisely the right amount of gas with precisely the right amount of oxygen to facilitate combustion.

These tiny controlled explosions spark a chain reaction that gets the crankshaft spinning and the engine fired up, but if the fuel: oxygen ratio is off due to a buildup of dirt on the carburetor or a clogged bowl, it can throw the whole system out of whack.

Combustion will still be taking place, which is why your lawn mower fires up when you pull the cord, but it’s not a sustainable power source, which is why your mower chokes moments later. Learn more about the carburetor air-to-fuel ratio.

Okay, So How Do I Fix It?

In this scenario, all you need to do to revive your lawn mower is give the carburetor a thorough clean, and what better to do than an aerosol can of specialized carburetor solvent?

I can personally recommend WD-40 Specialist Carb/Throttle Body & Parts Cleaner. It’s super affordable and takes care of both carbon contaminants and gumming. I’ve been using it for years, and it’s never let me down.

That said, it does have one shortcoming… it lacks a straw applicator for precision use, but that’s okay, as many alternatives do, such as Gumout Carb and Choke Cleaner. I’d advise you to pick up a can of both, use the WD-40 for the heavy lifting, and then switch over to the Gumout for precision spot cleaning.

How To Clean Your Carburetor

Step 1. Open The Windows

Carb cleaner may be great for your carburetor, but it’s hell on the lungs, so make sure you’re working in a well-ventilated area.

Step 2. Finding The Carb

Your carburetor will either be bolted to the top or side of the motor. Found it? If not, check behind or below the air filter.

See also my detailed article on finding the carburetor in your lawn mower.

Step 3. Remove The Fuel Line

Detach the fuel tank from your mower (they usually slide out of place), then use a pair of needle nose pliers to shimmy the fuel line loose.

Step 4. Remove And Clean The Jet Screw

The jet that feeds fuel into the carb doubles up as a bolt that holds the bowl in place. Unscrew the jet from beneath your carburetor, and give it a generous spritzing with your carb cleaner.

I find it’s best to use the precision applicator for cleaning the jet channels, as the concentrated stream can help dislodge debris.

For the best results, you should consider using a carb cleaner kit, as they arrive with all the special brushes you need to rejuvenate your jet.

Step 5. Poke The Jet Channels

Take a piece of wire, or even just an unfolded paper clip, and poke it through the tiny holes in the jet screw, just in case the carb cleaner missed anything.

Step 6. Remove And Clean The Bowl

With the jet removed, the carb bowl should pull free. Spray it with plenty of carb cleaner, and, if you have one, give it a good scrub with the brushes from your carb cleaning kit.

Step 7. Check The Gaskets

The gaskets are the black, rubber O-rings that seal the discrete sections of your carburetor. Check for any crumbling or blemishes. If you find any, replace them with fresh gaskets.

Step 8. Push The Float

When you removed the bowl, you revealed the carb float, a little disk attached to the rest of the motor. Gently push it up and listen closely. If you hear a quiet clicking sound, all is well. If not, you may need a specialist to look into the matter.

Step 9. Put It All Back Together

Okay, so now your carb is looking slick and shiny, it’s time to re-assemble. Be sure not to overtighten the jet screw, as you might strip the threads and disfigure the seal.

Step 10. Spray Some Carb Cleaner In The Intake Hole Of The Engine

The intake hole is usually behind the air filter. Give it a light spritzing, and when you start your mower up, it will be pulled through the system, giving everything one last clean.

Step 11. Take Your Mower For A Spin

With any luck, that quick maintenance session solved your mower mishap, and now you’re free to cut all the grass you want. If it didn’t work, don’t sweat it. We’ve got three more fixes to try.

Geriatric Gasoline

You may not realize it, but just like those plums shriveling up in your fruit bowl, gasoline has a shelf life, which depends on the fuel type. Over time, the volatile compounds essential to combustion thin out, which, understandably, can affect engine efficiency.

pouring gasoline into mower

When the quality drops below a certain threshold, it starts to damage the internal components of your motor. What’s more, it leaves gummy deposits on surfaces, which, if left unaddressed, can impede gas flow, leading to, you’ve guessed it… sudden mechanical failures.

Got it! But How Do I Fix It?

If you’ve got, say, less than half a tank left, you may be able to get away with diluting it with fresh gas, thereby strengthening the combustive properties. But if you’ve got more than half a tank left, I’m afraid you’re going to have to dispose of the contents (responsibly, of course).

See also my guide on the type of gas to use for your lawn mower

No matter which route you end up taking, my advice is to throw some sort of fuel stabilizer into the mix, and if you’re looking for a recommendation, in the past, I used STA-BIL 360 Protection Ethanol Treatment and Fuel Stabilizer.

This stuff prevents corrosion and halts the buildup you’ll see using traditional fuel, ensuring your engine always runs buttery smooth! Wondering if you need an ethanol fuel treatment?

Alternatively, you could do what I do now and use TruFuel-4-Cycle Ethanol-Free Fuel. Ethanol is the stuff that causes all the problems when fuel stagnates, so removing it from the equation is kind of a no-brainer.

A tank full of TruFuel will survive a long, cold winter in the shed, so when the warmer weather arrives, and your grass gets a growth spurt, you can break out the mower sans hiccups!

Cards on the table, TriFuel is a little more expensive than the standard gas/stabilizer combo, but in my opinion, the reliability and peace of mind it brings are well worth the premium. It gets my 140cc Black+Decker mower fired up with a single pull every single time — a real shoulder saver!

Spark Plug Panic

Once the carburetor has mixed the fuel and air, it’s the spark plug’s time to shine.

These incendiary little components provide the spark that ignites the fuel/air cocktail, thereby powering your mower’s engine

spark plug

As a pivotal part of the combustion process, should they fall into disrepair or perhaps get a little dirty, it can cause the irritating start-stop mower behavior that brought you to this article.

And What About the Fix?

As long as your spark plug isn’t completely coated with a stubborn carbon build-up, you can simply remove it, give it a thorough clean, and then replace it. Hopefully, that will smooth the combustion process out and return your mower to its former glory.

If you do find a lot of soiling on your spark plug, it’s best to call it quits and order a new one.

How to Remove and Clean A Spark Plug

Step 1. Locating Your Sparky

Spark plugs are typically front and center on your mower, so they’re not difficult to find. Look for a component covered with black cabling at the front end of the engine.

Step 2. Removing Your Spark Plug

Removing your spark plug requires a socket wrench of a certain size, so have a quick thumb through your user manual, and see what info you can dig up.

Don’t worry if you’ve misplaced your manual, as you can purchase a universal spark plug wrench here.

Once you’ve got the necessary tool, pull the black cable free from the plug, and use the wrench to unscrew it from the engine.

Step 3. Cleaning Your Spark Plug

I use standard multi-use WD-40 and a small wire brush for cleaning my spark plugs.

Start by spraying the plug down with your cleaning solution, then leave the chemicals to work their magic for a few minutes. Next, take your small wire brush, and give your sparky a good scrub to dislodge any dirt or carbon residue.

Once the debris has been agitated, wipe it away with a soft rag to wrap this step up.

Step 4. Replacing your spark plug

If you’re refitting your old spark plug, simply insert it into the end of your wrench and twist it back into place (not too tight).

Things are a little more complicated if you’re fitting a replacement plug, but stick with me, and we’ll have it sorted in no time!

  • First, you have to “gap” the plug, which essentially means that you are thinning or widening the gap between the two electrodes at the end of the plug to suit the requirements of your mower. To do so, you’ll need one of these handy CTA Tools spark plug gappers.
  • Check the recommended plug gap in your user manual, then run your spark plug gapper in the space between the electrodes. Turn it until you feel resistance, then check the measurement on the face of the tool.
  • If the gap isn’t quite large enough, you can use the hole in the gapper to pry the top electrode upwards, as if you’re cracking the cap on that cold one you were dreaming of earlier (and still are). To make the gap smaller, all you have to do is use the gapper tool to push down on the top electrode.
  • Once gapping is complete, you can set the plug in place as normal. Take your time, and don’t force it. You need to get the threads in just the right place to secure it properly, and remember, it shouldn’t be too snug, as overtightening can damage the plug seal.

Spark Plug Advice For The Future

A good rule of thumb is to switch up your spark plug at least once every two years, but I like to bump it up to once annually as part of my standard maintenance schedule.

Oil Overload

If you’re anything like me, you felt the urge to fill your oil reservoir to the brim. “Gosh”, you think… “Aren’t I a responsible machine owner!”, but surprisingly, there is such a thing as too much oil.

You may have noticed that before your mower decided to take a siesta, it was spewing some white smoke, which is a dead giveaway that you did get a little overzealous with your oil.

The reason your mower seems to be working fine at first is that the excess oil is yet to drown out the engine, I know “drown” is a strong word, but this isn’t as grave a situation as it implies. Let’s take a look at how you can make things right again.

Fixing Your Oil Surplus

The easiest way to solve this issue with a walk-behind mower is to simply pop the top of your oil reservoir, tip your mower on its side, and drain some of the contents; however, this can get pretty messy.

A much cleaner way to reduce your oil level is to use an oil extractor pump. These things allow you to siphon the oil away without spilling it, meaning (as long as it’s clean) you can use it again at a later date.

Be careful not to extract too much oil, though, as that can be even worse for your lawn mower than overfilling it.

Once you’ve sorted your oil level out, fire your mower up and keep an eye out for the dreaded white smoke. If it seems to be running clear, then you’ll know the problem has been rectified, and you can even out your wonky lawn post-haste.

How To Prevent This Oily Issue Repeating Itself

Now that your motor is feeling its old self again, take a break, check the oil with your dipstick, and note down the level. You can use this for future reference.

When the time comes to replenish your oil reserve, the trick is to take it nice and easy. Instead of pouring your oil into the reservoir willy-nilly, pour it in a little at a time, pausing every so often to check the level with your dipstick.

None of These Fixes Worked. What Do I Do?

Lawn mower still giving you grief after trying all four of these fixes? Dang!

Starting and dying shortly thereafter can be symptomatic of more serious mower issues. They’re incredibly tricky to diagnose, and even more difficult to fix.

So, I’m afraid it might be time to bite the bullet, contact a specialist, and have them take a look at your mischievous machinery. The problem could be…

A Defective Choke

The choke restricts the airflow in the carburetor to optimize the fuel-air ratio.

If it’s allowing the passage of too much or too little air, the engine won’t be working efficiently, and, in some cases, combustion may not occur at all, causing the sudden mower death we’ve been trying to resolve today.

A Spent Carburetor

Sometimes, no matter how much you scrub your carb, the damage is already done, and a replacement is the only way to get your grass gobbler purring again.

If you’re particularly handy, you may be able to make the switch yourself, but, in my opinion, it’s best left to a professional.

A Blocked Gas Line Or Tank

Engines require consistent fuel flow to run efficiently. Even minor disruptions in the gas feed due to internal blockages can transform your lawn mulching monster into a wheezing mess.

It takes some pretty invasive snooping to identify a blocked gas tank or line, so I’d recommend consulting a pro and paying them to do it for you.

Wait, Wait, Wait… Professionals? This Is Starting to Sound Expensive

I know you’re probably bracing your bank account for a severe hit, but if your mower is covered by a warranty, you may be able to get the manufacturer to fix it for you for free — hooray!

Some companies will even send an engineer to your door to either fix the mower right then and there, assess the issue, and order parts, or collect it for repairs at a company service depot. You won’t have to lift a finger!

If your warranty has elapsed, or you never had one to begin with, it’s best to look for a small, reputable repair shop in your area. These modest outfits are often staffed with highly experienced workers, and the prices are far less scary than you might imagine.

Lawn Mower Maintenance: Prevention Is Better Than the Cure

Before we go our separate ways, I thought it’d be a good idea to discuss some pro lawn mower maintenance tips, as the best way to ensure you’re never stuck with this issue again is to take good care of your machine.

Preventative Lawn Mower Maintenance
  1. Clean your air filter — It’s common practice to clean a mower’s air filter once every 25 operating hours, so if it takes you 2 hours to mow your yards, it should be given a good once over every 12 ½ mows.
  2. Replace your air filter — I replace my air filter once annually, but if you have quite a lot of ground to cover, and you hit 300 operating hours before the new year, don’t hesitate to switch it out earlier than scheduled.
  3. Clean your carburetor — It’s generally recommended that you clean your carburetor around three times a year. I give mine some TLC once a season, excluding winter.
  4. Stabilize your gas — Unless you’re taking the TriFuel route (a wise choice), be sure to mix your gas with a fuel stabilizer, especially before putting your lawn mower into storage for the winter.
  5. Degrease and clean your engine — If you’re often up against long or wet grass, you should degrease and clean your lawn mower motor once a season, but, in most cases, twice a year is plenty.
  6. Try not to overfill your oil reservoir — Remember, your dipstick is your best friend!
  7. Replace your spark plugs — As long as you keep them clean, your spark plugs should last for 2 years.
  8. Find suitable storage — Your mower needs to be kept in a dry location, protected from the weather, but if you have no other option but to keep it outside, be sure to elevate it and cover it with a durable tarp.

Summing Up

And that brings us to the end of my mower guide. If you’re still here reading this, thank you for your time, but a part of me hopes you never even made it this far.

With any luck, one of the fixes mentioned earlier will have resolved your issues, and you’re now busy tidying up your yard before your fussy neighbor, Val, catches a glimpse and crosses you off her Christmas card list.

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