Lawn Mower Surging

6 Easy Steps To Fix Lawn Mower Engine Surging

Is your Lawn Mower Surging up and down every time you use it?  A lawnmower surge is defined as an engine that revs up and down while running, and fortunately, it is something you can fix yourself. 

If your lawnmower falls into this category, this guide will walk you through the six steps you use to stop lawnmowers from surging. 

After all, Lawn mowers surging while you’re using a mower is very frustrating and dramatically affects the performance of your mower’s ability to cut your lawn effectively.

Why does a Lawn Mower engine Surge?  Any of these (3) conditions will cause mower engine surging problems and poor performance:

Lawn Mower Engines Require These 3 Things to Function Properly:

  1. Fresh Gasoline (Good Fuel)
  2. Fresh Air (Oxygen)
  3. Good Spark (Spark plug)

To ensure each of the above three requirements Is working properly – Let’s look at the Six (6) potential problems below related to these functions – and determine which requirement is not working correctly on Your Lawnmower.


How To Fix Lawn Mower Surge: Six-Step Checklist

1)   Bad Fuel

The number one cause of mower engine surge is using old or contaminated gasoline – perhaps by water condensation, usually due to 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 you can use 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 starts, it will not run correctly 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 gas tank.
  • 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, which has a minimum 87 Octane rating.  This is one way to keep your Briggs & Stratton engine surging issue from happening.

Briggs & Stratton Video Best Fuel For Lawn Mower Engine:

Alternative Fuel:  “Pre-Mixed” Lawnmower Gas:

You can also 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 three essential benefits to your lawnmower engine vs. typical gas pump fuel.

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

We recommend VP Small Engine Fuels at Amazon – the best choice of canned fuel 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, mower engines require air (oxygen) mixed with fuel, then a 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” – 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 the instructions to wash, dry, and reuse the lawnmower filter.
  • If your lawnmower has a “Paper” type (disposable) air filter, replace the air filter with a new one to match your lawnmower’s air filter requirements.

If You Need To Replace a lawn mower air filter – You can use the links below for the most popular brands.  Be sure to verify the 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.

The gas cap hole is sometimes clogged with dirt or debris when a lawnmower surges up and down. Not to sound too redundant, lawnmower engines need a steady fuel flow delivered from the gas tank to the carburetor to perform as designed. 

Use these steps below to make sure the gas cap is not clogged:

How To Repair Clogged Lawnmower Gas Cap 


 

4)    How to Inspect Vacuum Leak 

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

The lawnmower carburetor is designed to operate with controlled Air Intake. If you have leaks in your air intake – the carburetor and, therefore, the engine – will not run smoothly. 

Some lawnmowers have a gas tank located “Above” the carburetor. If this is your case, you most likely have an engine that does not use a “vacuum” to deliver fuel to the carburetor vs. a fuel pump and instead relies on Float & Gravity to deliver fuel to the engine.

However, most popular lawnmower engines use a Fuel Pump that delivers gas to your engine. The motion of the piston creates a vacuum, allowing gas to be pumped into the carburetor.

How “Vacuum” is created to draw fuel from the gas tank in small engines.

  1. Pulse-type fuel pumps such as Briggs & Stratton fuel pumps develop pressure by a diaphragm using a vacuum from the crankcase, which is created by the motion of the piston. A fitting on the crankcase cover or the dipstick tube draws the pressure to pump fuel. You can see the two types of methods used to create a vacuum in the diagram below:
Fuel Pump Vacuum line Variations - Besthomegear.com
Small Engine Fuel Pumps: Dipstick or Crankcase Mounted Vacuum – besthomegear.com

In both cases, you will find a Vacuum line, or “Center Port Line,” which should be inspected seasonally for cracks or holes.  Replace the vacuum line if you find either condition.

Pulse Fuel Pump – The most popular type, and usually found on newer small engines, Pulse-type fuel pumps use crankcase pressure pulses to actuate a diaphragm to move fuel. 

Video:  Differences Between Gravity-fed and Pulse Fuel Pump Small Engines:

 

How To Repair Lawnmower Vacuum Leaks

  • Inspect the vacuum hose leading to the fuel pump (typically the center port hose)
  • Check the vacuum hose for cracks, holes, or loose hose clamps, and fix or replace them.
  • Check the condition of the Carburetor Gasket – Replace it 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

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

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

How To Replace 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.
  • We recommend you replace the spark plug with a new one – use the same 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, debris, or corrosion, which can also be caused by using old gas.

Therefore, a surging lawnmower engine can be fixed by 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 clogged, the engine stalls, as no fuel is 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, causing the engine to die and then surge. 

Try these five steps to clean a dirty lawn mower carburetor:

How To Repair a Dirty Lawnmower Carburetor

  • If you’re handy with tools, remove the carburetor and clean it with carb cleaner
  • Use Stabil Small Engine Treatment
  • Inspect and replace any torn gaskets
  • Securely tighten the carburetor after cleaning.
  • Alternate method: If you’re uncomfortable removing and repairing the carburetor – try using a carburetor cleaner spray. For under $4.00, you can spray the carb cleaner directly into the carburetor 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

Q:  Why does my Lawn Mower Rev Up then Down then Quit?

A: The 3 main reasons a lawn mower surges up and down and eventually quits include: 1) Bad or Old Fuel, 2) Bad Sparkplug, or 3) Lack of Air/Oxygen. Any of these shortcomings will cause a lawn mower to rev up and down and not run smoothly.

Q:  Why Does a Lawnmower Engine Backfire?

A:  Even though it sounds terrible, an engine backfire won’t 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). Get rid of old gas and buy fresh gas.
  • 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)

Q:  How Do You Know If Your Spark Plug Is Bad?

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

Q:  How Should You Store Lawn Mower for Winter?

A:  Before storing a lawnmower for the winter, you should always add a fuel stabilizer to the gas tank (such as Sta-Bil) – then fill the tank thoroughly with fresh gas.

Next, run the mower engine for 2 minutes to circulate mixed gas through the engine. The fuel stabilizer will fix your problem by keeping gas stable for up to 24 months.

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

A:  Bare patches in your grass lawn can result from many causes. The most prevalent are pet urine and bug infestation, which can be fixed in a jiffy.  Check out our article and learn how to plant grass seeds in bare spots.

Q:  Can You Clean A Lawn Mower Carburetor Without Removing It?

A: You can clean most lawnmower carburetor issues easily and without removing the carburetor. Buy a good brand of carb and choke spray, such as Gumout. Follow the instructions, and the result should be a clean carburetor.

Q:  How Do You Change the Oil On a  Lawnmower?

A:  Changing the oil on a lawnmower is relatively easy. First, make sure the engine oil requires a change, then before you start, you can read this quick 4-step oil-changing guide from Briggs & Stratton.

Conclusion:

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

As with owning equipment with a small engine, lawnmowers also have three 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 these five solutions does the trick!

Thanks for Visiting Besthomegear – And please don’t hesitate to leave us a Comment or Suggestion Below!

References and Additional Reading:

Kevin Carroll
Kevin Carrollhttps://www.besthomegear.com
Kevin is the author and editor for Best Home Gear, and uses his 25+ years experience in Commercial and Residential Construction Management to author and publish the work for this website. In addition to publishing Best Home Gear; Kevin enjoys the outdoors in Michigan and Arizona, hiking, cycling, fishing, golf, and completing DIY projects at his Home and Garden.

2 COMMENTS

    • Nicole, thanks for the question (or statement), regarding vacuum line on mower engines. You may have, or be referring to a Float/Gravity fuel delivery system, where the gas tank is located “above” the carburetor, in this case there is No vacuum line because there is no fuel pump.
      In the case of most mower small engines, they will incorporate a “Pulse Fuel Pump” which does rely on Vacuum. I have updated this article “how to fix lawn mower surging” to provide more description as well as a video on the two types of fuel delivery. Hope this helps resolve any questions,
      Kevin

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Author and Publisher for Best Home Gear

Kevin is the Owner, Author, and Publisher @ Best Home Gear, which he began In 2018.

As a Professional in Construction, Real Estate and Property Management, Kevin uses that experience to publish Useful Articles, and help homeowners improve (DIY) their own homes, lawns, and outdoor spaces.

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