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Lawn Mower Surging

5 Easy Steps To Fix Lawn Mower Engine Surging

Is your Lawn Mower Surging up and down every time you use it?  If so, you’ve come to the right place! Today, we’ll share 6 simple steps that you can inspect and repair on your own – to prevent lawn mower engine surging.

We probably agree that mowing your lawn is a real chore – and that’s when your lawnmower is running well!

Unfortunately, small engines don’t always perform as well as we hope for; which usually means one of these three (3) engine requirements is not working properly:

Lawn Mower Engine – Performance Requirements”

  1. Fresh Gasoline (fuel)
  2. Fresh Air (oxygen)
  3. Good Spark (spark plug)


To make sure each of the above 3 requirements Is working properly – Look at the Six (6) potential problems below – that can cause Lawn Mower Engine Surges:


Six (6) Problems That Cause Lawn Mower Surging:


1)  Bad Fuel

The number one cause of mower engine surge is using gasoline that is old or contaminated – perhaps by water condensation, usually as a result of leaving gas in the mower over the winter.

Many homeowners forget or choose not to drain the gas from their mower’s tank at the end of each season. At the start of a new season, they either add fresh gas to the old gas or try to start the mower engine using the old gas.

The quality of gasoline deteriorates quickly over time. For this reason, buy only enough gas to be used within 30 days. When your lawnmower sits idle over the winter with a half-empty gas tank – condensation builds up inside the gas tank and mixes that water (condensation) with the gas.

If the lawnmower does happen to start – it will not run properly with old or diluted gas in the tank. If your lawnmower surges up and down – you can repair it as follows:

How To Repair Bad Fuel in Lawnmower

  • Drain old gas from the tank. Use a siphon if you have one – and save spills on the engine and floor.
  • Add fuel stabilizer to the gas tank (manufacturers recommended mix ratio) and top with a full tank of gas.
  • Run the engine for 2 minutes to circulate the fresh gas. Check for mower engine surging.

Tip:  Briggs & Stratton recommends only using gas for small engines, with no more than 10% Ethanol, and that has a minimum 87 Octane rating.

Briggs & Stratton Video Best Fuel For Lawn Mower Engine:



Alternative Fuel:  “Pre-Mixed” Lawnmower Gas:

You can also choose to use “Canned (or Boutique) Fuel” for 2-Cycle small engines, discussed here at Popular Mechanics.

It may sound weird – but canned fuel for mowers does provide 3 important benefits to your lawnmower engine vs. typical gas pump fuel.

  • Canned fuel contains Gas – but No Ethanol (Ethanol is a grain-based by-product) used to make petroleum fuels more affordable, and actually do more damage than good to engines.
  • Canned fuel has a High 94 Octane rating – Provides superior engine performance
  • Canned fuel includes Stabilizer to protect engines

We recommend VP Small Engine Fuels at Amazon – as the best choice of canned fuel available online.



2)  Dirty Lawn Mower Air Filter


Replace Lawnmower Air Filter - Best Home Gear
Replace Lawnmower Air Filter – Besthomegear.com


If you find that your Briggs & Stratton mower engine revs up and down (surges), it may be likely that your lawnmower air filter is dirty.

As noted previously, mower engines require air (oxygen) mixed with fuel, then spark, to ignite the mixture and run the lawnmower engine.

If your air filter is dirty – it’s like forcing your lawnmower engine to “suck air through a straw” – and nothing runs efficiently under those conditions.

How To Repair Dirty Lawnmower Air Filter  

  • Remove the lawnmower air filter
  • If your lawnmower has a “Foam” type reusable air filter, follow instructions to wash, dry, and reuse the lawn mower filter.
  • If your lawnmower has a “Paper” type (disposable) air filter – simply replace the air filter with a new one to match your lawnmower air filter requirements.

If You Need To Replace a disposable mower air filter – You can Use the links below for the most popular brands.  Be sure to verify specifications that the air filter you select matches your existing filter:



3)  “Clogged” Lawnmower Gas Cap


Clogged Mower Gas Cap - Best Home Gear
Fixing Clogged Gas Cap – Besthomegear.com


Most people are unaware that the fuel tank “cap” has a small hole on the top, which allows air to enter the gas tank and create back-pressure. This air pressure helps deliver fuel to the carburetor and run correctly.

Sometimes, when a lawnmower surges up and down it’s because the gas cap hole is clogged with dirt or debris. Not to sound too redundant; lawnmower engines need a steady flow of fuel delivered from the gas tank to the carburetor to perform as designed.  Use 2 steps below to make sure the gas cap is not clogged:

How To Repair Clogged Lawnmower Gas Cap 



4)  Vacuum Line Leak 


Vacuum leak lawnmower - Best Home Gear
Inspecting Vacuum Leak on Lawnmower – Besthomegear.com

The lawnmower carburetor is designed to operate with a controlled amount of air intake. If you have leaks in your air intake – the carburetor and therefore the engine – will not run smoothly.  Try these 5 steps to inspect your lawnmower for vacuum leaks:

How To Repair Lawnmower Vacuum Leak

  • Inspect vacuum hoses leading from the air filter to the carburetor.
  • Check vacuum hose for cracks, holes, or loose hose clamps, and fix or replace as needed.
  • Check the condition of Carburetor Gasket – Replace if needed.
  • Tighten carburetor (4 bolts)
  • Tighten engine manifold bolts to ensure a tight vacuum seal.


5)  “Bad” Lawnmower Spark Plug


inspect lawnmower spark plug - best home gear
inspect lawnmower spark plug – Besthomegear.com

Without a proper spark delivered from the lawnmower engine spark plug, the engine will not run correctly.  If you’re not sure How to tell if a lawn mower spark plug is bad, check out this article from Best Home Gear.

We recommend the following 4 steps to inspect and repair a bad spark plug:

How To Repair a Bad Lawnmower Spark Plug

  • Disconnect the spark plug wire
  • Carefully remove the spark plug
  • Inspect the spark plug for any cracks or a worn-out electrode.
  • If worn – replace the spark plug with a new spark plug – use the size recommended by the manufacturer.


6)  Dirty Carburetor


clean lawn mower carburetor -
How to clean lawnmower carburetor – Besthomegear.com

The role of the carburetor is to mix the perfect ratio of gas and air. It does so, with tiny jets and passages designed to get the correct air/gas ratio to the spark plug. Sometimes the jets can get clogged with dirt or debris or corrosion which can also be caused by using old gas.

Therefore, fixing a surging lawnmower engine can be just a matter of cleaning the carburetor.

In short: The main carburetor jet supplies gas to the engine during the engine’s full throttle. When that main jet gets blocked the engine stalls, as no fuel is being delivered.

As soon as the engine stalls, a governor on the engine automatically closes the engine’s throttle. This situation causes the carburetor idle circuit to supply fuel and the engine – which will fire again, returning the throttle to full.

If the carburetor is not cleaned, this stall and firing cycle will repeat itself, again and again, causing the engine to die then surge.  You can try these 5 steps to clean a dirty lawn mower carburetor:

How To Repair Dirty Lawnmower Carburetor

  • If you’re handy with tools, remove the carburetor and clean it with carb cleaner
  • Inspect and replace any torn gaskets
  • Securely tighten the carburetor after cleaning.
  • Alternate method: If you’re not comfortable removing and repairing the carburetor – try using a carburetor cleaning spray. This allows you to spray cleaner directly into the carb without removing it.
  • If this method doesn’t effectively clean the carburetor – you may require the service of a small engine repair technician.



Frequently Asked Questions (FAQ) Lawn Mower Surging


Why Does a Lawnmower Engine Backfire?

Even though it sounds bad – an engine backfire does not typically damage a small engine.  It can, however, be a sign that one of the following issues is occurring:

  • Using gasoline with too much alcohol – (Use maximum recommended 10% Ethanol)
  • Turning the engine speed down too quickly (lower engine throttle slower)
  • High engine temperatures (mow during cooler temperatures – or mow in two shifts – if possible)
  • Lack of adequate air (dirty filter)
  • The carburetor is running lean (adjust carburetor for proper air/gas mix)

How Do You Know If Your Spark Plug is Bad?

A bad spark plug will certainly cause your lawnmower to run poorly or not at all. Fortunately, it is fairly simple to determine if your Spark plug is bad, and needs to be replaced. If you’re not sure what to look for, check out this quick article How To Read a Spark Plug

How Should You Store Lawn Mower for Winter?

Before lawnmower storage – you should always add a good fuel stabilizer to the tank (such as Sta-Bil) – then fill the tank completely with fresh gas. Next, run the mower engine for 2 minutes to circulate mixed gas through the engine. The fuel stabilizer will keep gas stable for up to 24 months.

What Is The Best Way to Fix Bare Patches on Lawn?

Bare patches in your grass lawn can be the result of many causes. The most prevalent being pet urine and bug infestation, both of which can be fixed in a jiffy.  Check out our article and learn how to plant grass seed in bare spots.

How Do You Change Oil On Lawn Mower?

Changing the oil on a lawnmower is actually quite easy. Make sure the engine and engine oil are cool – and before you start, read this quick 4-step oil changing guide from Briggs & Stratton.



While lawnmower engines require some routine maintenance, they are usually a very dependable piece of outdoor equipment – considering the work and conditions that they perform in.

As with owning any equipment with a small engine, lawnmowers also have 3 basic requirements to ensure dependable performance.  Clean gas, Good Air Flow, and Proper Spark.

We hope this Mower Guide for solving lawn mower surging has been beneficial – and that one of our five solutions did the trick!

Thanks for Visiting Besthomegear.com – Please don’t hesitate to leave us a Comment or Suggestion Below!

References and Additional Reading:


  1. I have a surging problem but I did all of the steps and the mower still surges after start-up and warming up the engine. The mower just keeps surging. The engine has a new spark plug, new air filter, fresh fuel. Valve adjusted to manufactory specs and still surges . Also, the gas tank cap is also clean and has good flow of fuel. But the engine still surging, what else can it be?

    1. Jossy, thanks for the inquiry and for referring to our post. The only items left to check from your list, which we refer to in our lawn mower surging repair article, would be to check the Vacuum lines from the engine to the carburetor. if there is a vacuum hose leak that could be the problem. Good luck with your repair!

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