Lawnmower Engine Surging — Causes & How to Fix

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Lawnmowers are very efficient in cutting large grass areas quickly. But if the engine surges, it can lose power in the middle of the job, which is frustrating.

Our lawn care specialists have provided everything you need about lawnmower engine surging to eliminate this problem. 

Why Does a Lawn Mower Surge?

When the ratio of gasoline to air in an engine is off, a phenomenon known as surging occurs. The carburetor is the most likely point of the mower engine surging, with odd fuel-air mixtures running low or rich due to the problem.

lawn mower on the ground

Lawn Mower Surges: 8 Reasons and Solutions

Reason #1: Poor Gas Quality

Lawn mowers surging can occur if you put old, contaminated gas in the tanks. In addition, the engine will have trouble running due to erratic combustion brought on by the poor quality of the gasoline.

If you suspect gas in your mower is contaminated, you can check for it visually. You can tell if the gas in your lawn mower is bad by comparing it to new gas in color and clarity.

Solution: Remove the Bad Gas

Step 1: Get a Gas Can

Get a gas can and fill it with the bad fuel. Put it near the lawnmower’s fuel tank so you can easily supply fuel with it.

adding oil to lawn mower
Step 2: Place the Pump

Take off the gas cap, then connect the pump/siphon to the lawnmower’s gas tank (inlet) and the gas can (outlet).

Step 3: Start Pumping the Gas

Proceed with the gasoline transfer by starting the pumping/siphoning operation. It’s important to be careful not to damage the siphon, as it’s difficult to resume the process with so little fuel left.

Interesting Read: How to Drain Gas from a Lawn Mower Without Siphon 

Step 4: Refill the Gas Tank and Test the Mower

Once the old fuel has been removed, replace it with fresh fuel. Check to determine if the surge is still present by using the mower. The process of flushing out the old gas from the mower’s gas tank and introducing new fuel could take a few minutes.

Extra Tip

For Briggs & Stratton small engines, we suggest using gas with no less than an 87 Octane rating and no over 10% Ethanol.

Reason #2: Clogged Vent of a Fuel Cap

The gasoline filler cap on most lawnmowers has a small vent that keeps the fuel at the right pressure and vacuum. Lawn mower engine surging can come from the wrong back pressure, which could force fuel into or away from the carburetor.

lawn mower fuel cap

Find the little hole on the cap that you use to fill the mower’s gas tank. The dirt within is producing pressure problems, and the consequent swells.

Solution: Clean the Fuel Cap Vent

Step 1: Detach the Cap and Clear the Clogs

Take off the fuel tank’s top and find the release valve. Remove the dust and debris blocking the vent hole with a needle or pin. The greatest tool for the job is a sewing needle.

Step 2: Put the Cap Back and Test the Mower

After cleaning the vent, reattach it to the tank by twisting it back onto the mower.

Reason #3: Fuel Filter is Dirty

The carburetor won’t get enough fuel if the gas filter is clogged, causing an engine surge. A cursory visual examination will do the trick. It’s important to ensure the gasoline filter is free of debris that could clog it. Keep a brand-new replacement filter handy.

lawn mower bad fuel filter

Solution: Replace the Fuel Filter

Step 1: Detach the Ignition Cable

To avoid any potential catastrophes, disconnect the ignition cable and tuck it so it does not get in the way. There will be spillage, and you must keep the area free of sparks.

Step 2: Turn Off the Fuel

Finally, turn off the fuel supply to the lawn mower.

Step 3: Detach the Spring Clips

Pull the line away from the dirty fuel filter and undo the two spring clips holding it in place with a pair of pliers. You can slide the clamps along the pipe to do this.

Step 4: Unlink the Fuel Line

You can detach the gasoline lines from the filter with the clamps out of the way. To remove the gasoline filter,  twist it while pulling. A cut-off valve can prevent the engine oil from leaking from a tank, but you can simply choke the line if you don’t have one.

unlinking fuel line
Step 5: Link the New Filter

Get a new gasoline filter and install it in the system. Since petrol can only move in one way, you’ll want to ensure that your orientation is proper. We recommend a directional arrow on the side of the filter.

Step 6: Put Back the Spring Clips

Use the pliers to replace the spring clips, and I Inspect that they are in their appropriate places and securely fastened. If you notice that they are loose, you should replace them.

Step 7: Turn On and Test the Mower

Turn the gas on and give the mower a spin. Since the system is filled with air when the filter is changed, it may take many pulls to get the gas to the engine.

Reason #4: Faulty Spark Plug

An unreliable spark is produced as a sparkplug deteriorates from soot buildup. When this happens, the dirty spark plug will not provide a steady spark but rather intermittent sparks that will cause short bursts of combustion or you’ll experience your mower will start then dies soon after. 

mower spark plug

Solution: Replace the Spark Plug

Step 1: Take Out the Ignition Cable

Disconnect the bad spark plug cable and push it out of the way first.

Step 2: Take Off the Spark Plug

Then, use a spark plug wrench or socket to remove the existing plug.

Step 3: Install the Brand New Spark Plug

Put the replacement spark plug in the engine head, then screw it on as tightly as possible with your fingers. The plug should be finger-tight before any additional torque is applied, but be cautious not to overtighten the spark plug in your engine. 

Toro Lawn Mower spark plug
Step 4: Fasten the Ignition Cable

Reconnect the spark plug cable and test the mower. An instant uptick in quality is to be expected.

Reason #5: Carburetor Jet is Clogged/Dirty

The carburetor houses several tiny jets, some of the essential parts of a mower. In case of troubleshooting, you need to know where the carburetor is on your lawn mower

Under high pressure, petrol is forced through the jets and atomized into a mist or vapor before being combined with air and sent to the chamber. So, if dirt and debris get into the gas, it can cause a blockage.

Solution: Unclog/Clean the Carburetor Jet

Step 1: Unfasten the Spark Plug

You should remove the spark plug before working on an engine. Take the ignition cable out of the cylinder and tuck it away.

detaching spark plug
Step 2: Switch Off the Fuel System

The next step, if available, is to turn off the gasoline supply.

Step 3: Take Off the Air Filter Cover and the Filter

If your lawn mower has a removable air filter,  the next step is to unscrew the cover’s mounting bolts. Simply using a screwdriver ought to do the trick.

Step 4: Take Out the Backplate Mounting Bolt of the Air Filter

The bolts attaching the air filter back plate to the mower must be removed using the socket wrench. When you take these off, the lawnmower will no longer be attached to the backplate or the carburetor.

Step 5: Detach the Breather Pipe from the Crank Case

You need to disconnect the breather line from the mower’s crank to take out the air filter rear plate. Pry the pipe out from its spring clamp with a pair of pliers. When you unhook yourself from the pipe, you can slide down the pipe.

person holding a screw driver
Step 6: Take Out the Gaskets

A gasket separates the air filter and the carburetor housing. You can remove this and put it somewhere else.

Step 7: Unlink the Fuel Line

After you remove the air filter, you can connect to the carburetor line. It, too, will be attached with a spring clip that can be easily detached, just like the breathing pipe.

Step 8: Unlink the Spring and Throttle

The governor spring and full throttle linkage are the final pieces of the puzzle that secure the carburetor to the mower. These have to be removed from their carburetor mounts manually. Nothing more than some time and calmness is required.

Step 9: Clean the External Part of the Carburetor

A can of WD-40 and a used toothbrush are my go-to tools. After removing the filth, use an old rag to clean the carburetor.

lawn mower carburetor
Step 10: Detach the Fuel Cap

You can loosen the bolt at the base of the cup with a socket wrench. Removing the bolt makes it possible to separate the cup from the carburetor.

Step 11: Pull Out the Float & Pin

As you can see, a single side of the float serves as a hinge. The float can be detached by first pulling the float pin out of the float with a pair of needle nose pliers and setting it aside.

Step 12: Unfasten the Jet

Remove the jet from the carburetor next. You’ll find a tiny screw easily removed with a screwdriver. The jet can be tipped out after this screw is removed.

Step 13: Clean the Carburetor Jet

To clean the jet, insert one of the tiny files from a carburetor cleaning wire set into the jet. You may need to use force and a can of carburetor cleaner to remove the grime from your carburetor and needle valves if it’s particularly filthy.

cleaning lawn mower carburetor
Step 14: Clean the Internal Part of the Carburetor

Spray the inside of the dirty carburetor thoroughly with the carb cleaner. Wait a minute for the cleaner to work, and then clean the carburetor bowl hold-down bolt.

Step 15: Put Back the Float, Jet, and Fuel Cap

To reassemble, you must do the opposite of what you did to get here. Putting in the cap comes after fitting the jet and screw.

Step 16: Install the Lawn Mower Carburetor

Once you’ve finished, you can reattach the two gaskets, gas line, spring, and throttle linkage, to the carburetors. To finish, remove the air filter backplate and reattach the vacuum hoses.

Step 17: Test the Mower

Now that you’ve finished cleaning and reassembling the lawnmower, you can put it through its paces. After a few pulls of the cable, the engine should sound better because the fresh gasoline flows through it.

Toro lawn mower

Reason #6: Leaks on the Carburetor Gasket

Gaskets are used to ensure that carburetors do not leak into the engine. Additional air can be drawn into the combustion chamber when these stop working. Your hypothesis that this would alter the mixture and cause the lawnmower’s motor to rev faster would be spot on.

Solution: Change the Gasket and Secure the Mower Carburetor

After you clean the carburetor, reinstall it and ensure the gaskets are removed during the removal process. Simply reviewing the procedure will remind you which steps you can safely omit.

removing carburetor gasket

Reason #7: Leaks on the Manifold Tube

The carburetor mixes the petrol and air and then sends it through an air intake manifold tube to the engine’s air intake valve.

Most surging engine problems can be traced back to a faulty air intake manifold gasket.

surging lawnmower engine intake manifold leak

As a result, the vacuum leaks, and the engine is not getting as much air as it needs.

Solution: Fix Tube Leaks

Reason #8: Air Filter is Dirty

If the air filter is clogged, less air will reach the carburetor. This can affect the air-gas ratio like a gasoline filter, causing the lawn mower engine surging. Therefore, a visual examination of the air filter is required.

dirty lawn mower air filter

Solution: Clean or Replace the Air Filter

Step 1: Detach Your Air Filter Cover

Locate the dirty air filter (foam or paper filters) and take the cover off of it as a first step. If your cover doesn’t secure with clips, we suggest using a screwdriver to remove it.

Step 2: Detach the Filter

Take the filter out from the housing, then put it on one side.

Step 3: Clean the Casing

Grab a rag and clean the housing interior. Remove debris, residue, and oil not to get the filter dirty. For foam filters, you can use soapy water to clean them.

Step 4: Clean the Air Filter

If your filter isn’t in horrible shape, utilize an airline from the compressor and blast away the dirt. Getting a new filter is preferable to utilizing an old compressor.

putting back cleaned air filter
Step 5: Install the Air Filter

Return the filter to its housing in the proper alignment.

Step 6: Replace the Air Filter Cover

Replace the lawnmower’s air filter cover and fasten it with the screw or clips.

Step 7: Test the Mower

Now that you have a new, clean filter in your mower, you can see how it performs. If the filter were to blame for your surging lawn mower, you’d see an immediate change for the better.

Other Possible Reasons for Lawn Mower Surging

Governor System Malfunction

Worn-out governor springs are a common cause of the lawnmower’s governor not functioning properly, which in turn causes the engine to surge.

Fuel System Issue

A tiny hole in the top prevents a vacuum leak from building in the gas tank. The gas is pumped into the engine in waves rather than a steady stream preventing the vacuum from forming if the system becomes obstructed.

Troy Bilt Lawn Mower fuel line

Dirty Gas Line/Filter

If insufficient gas is not delivered, it can cause the engine to surge. Blocked fuel lines, contaminated gasoline, and a filthy fuel filter are possible causes of insufficient gas in the engine.

Vacuum/Air Leaks

If the carburetor is not properly sealed to the engine block, air might get sucked into the intake, attract dirt and force the engine to surge.

This causes vacuum leaks, limiting the gas supply to the piston chamber.

How to Fix Lawn Mower That's Pulsing

Take the lawn mowers to a spot with plenty of airflows and follow these instructions to fix the motor.

Cub Cadet lawn mower

Why Does a Lawn Mower Engine Rebound?

An engine rebound may sound dreadful, but it usually does not cause any significant harm to a tiny engine. However, it may indicate the following problems:

fix mower engine

Estimated Cost of Fixing a Surging Lawn Mower Engine

The cost of fixing a lawn mower is usually a little more than the expense of replenishing the gas, provided that you can fix the surging engine and no parts need to be purchased.

lawn mower surging

Carburetors may be replaced on several popular lawn mower engines for around $50.

For a lawn mower surging that needs to be sent to a repair shop, expect to pay between $50 and $100, even for small engines. 

FAQ

How do I know if my mower’s spark plug is bad?

Your lawn mower engine will fail if the sparkplug is defective. Besides, a faulty spark plug may cause your mower to release white smoke randomly. The good news is that it’s not hard to tell whether your lawnmower spark plug is bad and must be replaced. 

What’s the best way to store your mower during winter?

Add a quality fuel stabilizer and a full fuel tank of fresh gas in your lawnmower, then let it run for 2 minutes before putting it away for the winter. It is possible to store gas for up to 24 months without deterioration with the help of a fuel stabilizer.

How can you change the lawn mower oil?

To change the oil in a lawnmower, ensure the engine and oil are cool and follow Briggs & Stratton’s easy 4-step oil-changing guide. Also, make sure you know how much oil can you put on your lawn mower

Conclusion

To prevent lawnmower engine surging, you need to perform regular maintenance. Still, it is reliable outdoor machinery, especially considering the tasks and the environment in which it operates. 

The repairs listed above are not difficult and can usually be done by someone with only basic mechanical skills. 

Robert Johnson is a woodworker who takes joy in sharing his passion for creating to the rest of the world. His brainchild, Sawinery, allowed him to do so as well as connect with other craftsmen. He has since built an enviable workshop for himself and an equally impressive online accomplishment: an extensive resource site serving old timers and novices alike.
Robert Johnson
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