Lawn Mower is Surging (Causes + How to Fix)

Surging is a common problem that almost every lawn mower owner faces. Unfortunately, no matter how much I look after my equipment and stay ahead of any issues, I still hear my lawn mower surge from time to time. So let’s take a look at several potential causes and fixes for your surging engine.

Why is My Lawn Mower Surging? (The Short Answer)

Surging results from the engine having an unbalanced fuel-to-air mixture ratio. The most likely fault you’ll run into will be a problem with the carburetor resulting in the fuel-air mixture erratically running lean or rich.

Potential Reasons for a Lawn Mower That is Surging

Before reaching for your tools to pull off the carburetor, it’s best to rule out any other potential reasons that may explain why your lawn mower is surging. For example, problems can stem anywhere from the fuel to the fuel system. Because of this, you will want to check and cross off the other potential causes before you tackle the carburetor. So, let’s go over those items you’ll want to check off.

Quality of Gasoline

Bad or old, unprotected gasoline in your lawn mower will cause the engine to surge. In addition, the degraded quality of the fuel will combust irregularly and cause various poor engine performance-related issues.

You can do a visual inspection if you’re suspicious that you have bad gas. By comparing the color and clarity of gas from your lawn mower tank and fresh gas, you’ll be able to tell if you have bad gas in your mower.

Blocked Fuel Cap Vent

Most lawn mower fuel filler caps have a tiny vent that enables the fuel within the tank to maintain the correct vacuum and pressure. Incorrect back pressure could either push or pull fuel to the carburetor and result in surging. Look at your fuel tank filler cap and locate the tiny hole in the top. You could find that it’s filled with dirt and causing pressure issues and therefore surging.

Dirty Fuel Filter

Having a dirty fuel filter will affect how the fuel can flow to the carburetor. Reduced or increased flow and fuel pressure will cause fluctuating revving of the engine. A simple visual inspection here will do just fine. You’ll want to check that the filter looks clean and doesn’t have dirt inside that could affect the fuel flow. I like to have a spare new fuel filter on hand to make a comparison.

Dirty Air Filter

A dirty air filter will reduce the flow of air into the carburetor. This will alter the fuel-air mixture the same as the fuel filter and potentially result in surging. So again, you’ll want to inspect the air filter visually.

Fouling Spark plug

As a spark plug goes bad or is sooted up, it creates an irregular spark. Instead of the spark plug giving a consistent spark, it will work on and off, resulting in bursts of combustion.

For me, the easiest way to test a sparkplug is to use an inline spark plug tester. I’ve never found watching a plug spark when you pull the starter cord a very accurate method. It’s difficult to tell if it’s the plug or how you pull the starter.

You can remove the plug and inspect the electrode if you don’t have an inline tester.

Clogged Carburetor Jet

There are small jets inside the carburetor. The fuel passes through the jets under pressure and turns into a mist/vapor that is then mixed with air and delivered to the combustion chamber. As a result, they can block easily if any debris and fuel dirt get in them.

Whenever I hear surging from my lawn mower engine, I’m always confident that this is the root of the problem. The tiniest of blockages can cause some very noticeable issues. To test for blocked jets, you’ll need to remove the carburetor and conduct an inspection.

Leaking Carburetor Gasket

Carburetors are sealed to the engine using gaskets. Once these fail, the combustion chamber can draw more air. I’m sure you’ve already guessed that this changes the mix ratio and results in a surging lawn mower engine.

Firstly, check to see that the carburetor is securely attached to the engine. Secondly, with a running engine, spray engine starter onto the gaskets and listen for a change in the surging. The engine starter spray will immediately have an effect, replacing the air being sucked in with a combustible substance.

Solutions If Your Lawn Mower is Surging

Let’s run over what tools, parts, and steps you’ll need to tackle each of the potential causes for a surging lawn mower.

Removing Bad Gasoline

If you have tested your gasoline and realized that you have bad gas, then you’ll need to know the steps to remove it effectively. So let’s take a look.

Position Gas Can

Grab yourself a gas can that you can use to fill with bad fuel. Position it within reach of the lawn mower so you can transfer the gas from its fuel tank.

Position Pump/Siphon

Remove the gas cap and put the inflow end of the pump/siphon into the lawn mower’s gas tank and the outflow end in the separate gas can.

Begin Pumping/Siphoning

Start the pumping/siphoning process and transfer the fuel. Just watch out that you don’t break the siphon since it’s hard to restart the process with just a little fuel left in the tank.

Refill Tank & Test

After you have removed the bad fuel, refill the tank with fresh fuel. Give the mower a test to see if the surging has stopped. It may take a few minutes to get the new fuel through the system and flush out any old gas.

Cleaning the Fuel Cap Vent

Here are the steps you’ll need to follow if you have discovered that you have a block or dirty fuel cap vent.

Remove Cap & Clear Blockage

Remove the cap from the gas tank and locate the vent hole. Using a needle or pin, poke out any dirt to clear the vent. I find a sewing needle works best for this.

Replace Cap & Test the Lawn Mower

Once the vent has been cleaned, you can twist it back onto the tank and test the lawn mower.

Swapping Out a Fuel Filter

If you have found that the fuel filter is no longer usable, you’ll need to grab a few tools and a new filter, then complete the following steps.

Remove Ignition Cable

Start by removing the ignition cable and tuck it out of the way so no mishaps occur. There will be some spilled fuel, so you want to avoid any sparks.

Switch Off the Fuel

Next, if your lawn mower has a fuel cut-off valve, switch it to the OFF position.

Remove the Spring Clips

Using a pair of pliers, remove both spring clips gripping the fuel line to the fuel filter. This can be done by sliding the clips along the pipe.

Disconnect the Fuel Line

You can remove the fuel lines from the filter with the clips out of the way. I find twisting the fuel filter at the same time as pulling helps. If you don’t have a fuel cut-off valve, you’ll need to pinch the fuel line so the fuel in the tank doesn’t spill out.

Connect the New Fuel Filter

Grab your replacement filter and attach it to the fuel lines. Make sure your orientation is correct, since the fuel needs to flow in one direction. The filter should have an arrow on the side to show the flow direction.

Install the Spring Clips

Next, use your pliers to reinstall the spring clips. Make sure that the clips are positioned correctly and that they are nice and tight. You’ll need to replace them if you see that they are loose.

Switch ON the Fuel and Test the Lawn Mower

Finally, switch the gas back on and test the lawn mower. It could take a few pulls to fill the fuel system with gasoline since replacing the filter fills it with air.

Cleaning/Replacing the Air Filter

If your air filter is dirty, it might simply need a good cleaning. However, if it’s got oil and fuel from incorrectly tilting over the lawn mower, you should replace it with a new filter. Here are the steps you need to follow.

Remove the Air Filter Cover

Start by locating the air filter and removing the cover. You may need to use a screwdriver if your cover doesn’t use clips to keep it in place.

Remove the Filter

Remove the filter from the housing and set it to one side.

Clean the Housing

Take a cleaning rag and give the inside of the housing a good clean. You’ll want to remove any oil, fuel residue, and dirt so you don’t get your new/cleaned filter dirty.

Clean the Filter

If your filter isn’t in bad shape, use an air-line from your compressor and blow out the dirt. If you don’t have a compressor, I recommend using a new filter.

Install the Filter

Place the filter back into the housing, making sure that the orientation is correct.

Replace the Cover

Place the air filter cover back onto the lawn mower and secure it with either the screw fixing or the clips.

Test the Lawn Mower

Finally, you can test your mower’s performance with the new/clean filter installed. You will notice an immediate improvement if the filter was causing the surging.

Replacing the Spark Plug

If you have tested the spark plug and it’s bad, or you are unsure if it needs to be swapped out, then here’s what you’ll need to do.

Remove the Ignition Cable

First, remove the cable connected to the spark plug and tuck it out of the way.

Remove the Spark Plug

Next, remove the installed spark plug using a plug wrench or a plug socket.

Install the New Plug

Insert your new spark plug into the engine head and tighten it with your fingers. Once the plug is finger-tight, you can use your tool of choice. When tightening your spark plug, be careful not to over-tighten. You just want a nice, snug fit.

Attach the Ignition Cable

Finally, reattach the ignition cable and test the lawn mower. You should see an immediate improvement.

Un-Clogging Carburetor Jet

To service the jet, you will need to remove the carburetor and clean the internal parts. If this is beyond what you are comfortable doing, then replacing the carburetor is an option. But, first, let’s look at how to clean a carburetor jet.

Remove the Spark Plug

Like any engine repair, you’ll want to disconnect the spark plug. So, remove the ignition cable and tuck it out of the way.

Shut Off the Fuel Flow

Follow this by cutting off the gas using the fuel cut-off valve if you have this option.

Remove the Air Filter Cover and Air Filter

Next, you’ll need to remove the fixings to the air filter cover if your lawn mower has them so that you can remove the cover to the air filter housing. Using a screwdriver should work just fine.

Remove the Air Filter Backplate Mounting Bolt

Next, use your socket wrench to remove the bolts holding the air filter back plate to the lawn mower. Once these are removed, both the backplate and the carburetor will disconnect from the lawn mower.

Remove Crankcase Breather Pipe

To remove the air filter back plate from the mower altogether, you’ll need to remove the breather pipe from the crank. Grab a set of pliers to remove the spring clip that holds the pipe in place. With the clips released, you can slide off the pipe.

Remove Gaskets

You’ll find a gasket between the carburetor and the air filter housing. This can be lifted off and set to one side.

Disconnect the Fuel Line

Now that the air filter is out of the way, you’ll need to remove the fuel line from the carburetor. Again, this will be held on with a spring clip that can be removed like the breather pipe.

Disconnect the Throttle Linkage and Spring

The last parts holding the carburetor to the lawn mower are the throttle linkage and spring from the governor. These must be lifted out of their mounts on the carburetor by hand. You will not need any tools, just a little patience.

Clean the Outside of the Carburetor

Now that you have the carburetor in hand, you can give it a good cleaning. I like to use a can of WD-40 and an old toothbrush. Once the grime has been cleaned, I finish by wiping down the carburetor with an old rag.

Remove the Fuel Cup

Now it’s on to taking the carb apart. On the bottom of the carburetor is the fuel cup. Use your socket wrench to remove the bolt in the bottom of the cup. With the bolt removed, the cup will detach from the carb.

Remove the Float and Pin

With the carburetor’s fuel cup removed, you’ll be able to see the carb’s float. This will usually be a white plastic part that sits inside the cup. You’ll see that the float is hinged on one side. To detach the float, you’ll need to remove the float pin using a pair of needle nose pliers and set it to one side.

Remove the Jet

Next, remove the jet from the carb. Depending on the design of your carb, you may need to remove the screw holding it into place. Looking down the carb from where you removed the float, you’ll see a hole centered on the carb. This is where the jet sits. In this location is a small screw you can remove using a screwdriver. Once this screw is removed, you’ll be able to tip out the jet.

Clean the Jet

To clean the jet, I use a carburetor cleaning wire set. This is a set of tiny brushes and files that are small enough to fit through the jet hole. Poke one of the small files through the jet. If your carb is very dirty, you’ll need to be a little forceful to clean the dirt.

Move the file back and forth to clear the dirt. Having a can of carburetor cleaner is really going to help break down the dirt, so having one handy will help.

Clean the Inside of the Carburetor

It’s a good idea to clean the rest of the inside before putting the carburetor back together. Give the inside of the carb a good spray with your carburetor cleaning spray. Allow the cleaner to sit for a minute, and then tackle the carb with your cleaning tools. I like to use a rag, a toothbrush, and my air-line.

Reinstall Jets, Float & Fuel Cup

With the carb all cleaned up, it’s time to put it back together. Here you’ll want to reverse the disassembly process. Start by installing the jet and screw, followed by the float and pin and then the fuel cup.

Install the Carburetor

Finally, reconnect the carbs fuel line, two gaskets, throttle linkage, and spring. Once this is done, take the air filter backplate and reconnect the breather pipe.

Next, take the two bolts you removed that were holding the carburetor to the engine and slide them through the backplate, gasket, carburetor, and second gasket, then into the lawn mower. This is where you’ll wish you had an extra set of hands.

With all of the parts lined up and in position, you can go ahead and tighten up the bolts nice and snug. You could crack the head or snap the bolts if you make it too tight.

The last part is installing the air filter and cover and switching the fuel back on. Last but not least, attach the spark plug ignition cable.

Test the Lawn Mower

With everything cleaned and installed, you can test the lawn mower. It’s going to take a few pulls of the cord to get gas through the engine, but once it does, you should hear an improvement.

Changing a Carburetor Gasket & Securing the Carburetor

You’ll notice that during the carburetor’s removal steps, you removed the gaskets and resecured the carburetor once the cleaning was complete. Let’s just run through the steps briefly again so you know which steps to skip.

• Remove the Sparkplug
• Shut Off the Fuel Flow
• Remove the Air Filter Cover and Air filter
• Remove the Air Filter Backplate Mounting Bolt
• Remove Gaskets
• Replace Gaskets
• Install the Carburetor

Riding Mower Surging? Causes + Solutions Specific to Ride-Ons

The only real difference is the size when it comes to gas-powered ride-on and walk-behind lawn mower engines. Some manufacturers have extra parts and more sophisticated designs, but the principles of combustion engines remain the same.

Fuel Cut-Off Solenoid

One part on a ride-on mower to check is the fuel cut-off solenoid connected to the bottom of the carburetor. This opens and closes the path of fuel flow through the carburetor. You will find these on walk-behinds, but usually, they are on much bigger models. So if your riding mower is surging and it’s not because of one of the causes I have already covered, it could be the carburetor solenoid.

Like any solenoid, they can go wrong, and they can go wrong in different ways. The solenoid is activated when the ignition is switched on and off. You should feel it kick on by placing your hand on the solenoid and turning the key.

To inspect the inner workings, you can disconnect the power and unscrew the unit. Once you remove it, you can check to see if it can operate smoothly. Any dirt or gum in the mechanism can cause the solenoid to function erratically and cause the engine to surge since it controls the fuel flow. Cleaning or replacing the solenoid may fix your problem with your surging engine.