8 Reasons Why Your Lawn Mower Is Surging

When your lawn mower is surging it affects the outcome. You will end up taking more time mowing your lawn and it’s not as smooth looking. Plus you run the risk of damaging your lawn mower more.  

Your lawn mower could be surging for a multitude of reasons, the most common ones are wrong fuel, bad spark plug, faulty or dirty carburetor, clogged fuel lines, dirty air filter, blocked fuel cap vent, notch/throttle faults, vacuum leaks. 

Reasons Why Your Lawn Mower Is Surging 

There are several reasons why your mower could be surging, but the most common reason is that there may be a blockage in the fuel supply. But other possibilities could be:

  1. Wrong Fuel
  2. Bad Spark Plug
  3. Faulty or Dirty Carburetor
  4. Clogged Fuel Lines
  5. Dirty Air Filter
  6. Blocked Fuel Cap Vent
  7. Notch/Throttle Faults     
  8. Vacuum Leaks 

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1) Wrong Fuel 

Not a lot of people know that fuel is not immune to going bad! It has an expiration date mostly due to its composition being compromised.  

For example, your fuel tank could have water seep into the fuel tank in rainy seasons. Or, the fuel may have been mixed and of poor quality when it was filled into the tank. If left without use, fuel can become ‘gummy’. 


Using the wrong fuel is detrimental to your lawn mower’s engine. We strongly advise you to stop using your mower temporarily if you suspect the fuel is wrong. Hold off until you can test the fuel to confirm or dismiss your doubts. 

Test the old fuel against the new fuel to see if it matches in terms of color and clarity. If it does not match, change out the old fuel for new fuel. 

Changing out the wrong fuel should stop the lawn mower from surging and protect the engine from damage. Remove all the old fuel by doing this:

  1. Strategically park your lawn mower over old cardboard to avoid leaving stains
  2. Place a fuel container and take it near the lawn mower (not too close either)
  3. Unscrew the fuel tank cap and put the pump/siphon into the lawn mower’s fuel tank and start the fuel transfer
  4. Ensure you get all the fuel to siphon out in a single go since starting the process again for a small amount of remaining fuel can be next to impossible to restart
  5. Once sure that all of the old fuel is out, you can begin to refill the tank with the new fuel. After a bit of time, test the mower to check if the surging is still persisting. 

2) Bad Spark Plug

Ever wondered how your lawn mower receives the signal to start the engine? A small part called the spark plug provides the spark so to speak. When this small gadget is faulty or burnt out, you are very likely to experience a surging lawn mower. 

As soon as you start to experience such surging problems when adjusting the power settings, you should start to question the spark plug. 


You can solve the spark plug problem by testing it and immediately changing it to see results! Here’s how to replace the spark plug. 

  1. Disconnect the ignition cable from the spark plug
  2. Make sure the ignition cable is out of the way before progressing 
  3. Using a plug wrench, remove the old spark plug
  4. With the wrench and a plug socket, fit the new spark plug into the engine head
  5. Be sure to avoid over-tightening the spark plug
  6. Reconnect the ignition cable 

Try out the lawn mower on several different power settings to make sure the spark plug is working properly. 

3) Faulty or Dirty Carburetor

A lawn mower has a part called the carburetor where air and fuel are mixed to create the spark ignition whereas the spark plug is useful in creating this spark.

The function of this specific part is to ensure there is a steady flow of both air and fuel. Do you know what happens if there isn’t, it could lead to your lawn mower surging? 

The carburetor inlets from where fuel and air respectively come from can create a difference in ratios of the combustion reaction. Causing a few hiccups in your lawn mower’s power.


To get a clearer idea if your carburetor is the problem or not, you will have to remove the whole part from the lawn mower. This is how you can remove it, clean it, and replace it:

  1. Start by enduring the machine cannot start- remove the spark plug and use the fuel-cutoff valve option if it is present 
  2. To remove the carburetor, you will have to remove several other parts such as:
  3. Air filters cover and air filter (unbolt the bolts to remove it completely)
  4. Crankcase breather pipe (removing this helps to remove the air filter)
  5. Gaskets- these sit between the carburetor and air filter as a sort of barrier
  6. Cut-off fuel leading into the carburetor by disconnecting the fuel line 
  7. Unlink the throttle and spring as these are the final items attaching the carburetor to your lawn mower
  8. Clean the outside of the carburetor with a ready-made carburetor cleaner from a can. Additionally, you must also remove other parts such as the fuel cap, float and pin, and jet to properly clean the interior of the carburetor.

Spray the cleaner into the carburetor and leave it for 5 mins. Return with a few specialized brushes that can fit inside the inlets. A rag will also help in cleaning up the excess spray. Once it is dry, you can start reassembling the carburetor from the last item removed and so on. 

4) Clogged Fuel Lines

Sometimes your lawn mower surging can actually be the cause of petrol starvation! Fuel level is usually the last thing on your mind when your lawn mower starts shaking or surging. So, check the fuel level when surging begins to make a fast diagnosis if it is the issue! 

Also, be aware that there is another way that the engine can be starved of fuel. If the fuel lines are clogged, enough fuel won’t be getting through to the carburetor and imminently the engine as well. This will result in jerks and surges.


Clogged fuel lines or the filter may need replacing to avoid jerks and surges while mowing your lawn.

  1. Disconnect the ignition cable and turn the fuel valve to OFF (if it has the switch)
  2. Pry away the spring clips that attach the fuel line to the fuel filter
  3. Removing the fuel filter is tricky. It is well wedged in and you should try pulling and twisting simultaneously to dislodge it. NOTE- if no fuel valve is present, pinch the fuel line to avoid spillage. 
  4. Assess the fuel filter by comparing it to a fresh filter
  5. As for the fuel line, try running fuel through it to see if it passes through unhindered and also how fast it passes through 
  6. To replace the filter, fit in the new one ensuring its position is correct and fitted securely. An arrow on the filter should help you know what direction is correct
  7. Unclip the fuel line and connect it back to the filter
  8. Place the spring clips. Loose clips call for a new set to be placed on the filter
  9. Turn the fuel valve back to ON
  10. Test the mower to know if the issue is resolved or not 

5) Dirty Air Filter

As you have found out, the engine makes use of both air and fuel to combust and create the energy needed to run the lawn mower. Both are needed in constant supply in the right amounts. So, when air is being restricted from entering the carburetor, surging is expected. 

What’s to blame for a lack of air in the air carburetor? It’s the air filter to blame, it is probably too clogged up with dirt. 

Dirty Air Filter


You will have to clean out the air filter to ensure the air supply is guaranteed to be constant. Sometimes you will have to replace it completely, here’s how you can do that. 

  1. Start by removing the air filter cover
  2. Take out the air filter 
  3. Clean the housing of the air filter
  4. Take the new filter and place it in the housing and make sure it is facing the right place 
  5. Put the cover back on and close it by screwing the bolts back on 
  6. Check to see if the surging has stopped

NOTE- If the air filter appears to be in good shape, a cleaning should do it instead of a replacement. 

6) Blocked Fuel Cap Vent

Did you know that your fuel cap has a minute hole in it? Most people don’t know about this hole and don’t even know that it can be the simple reason behind a surging lawn mower.

This tiny hole is responsible for ensuring the correct vacuum pressure to store and transfer fuel to the carburetor. 

If the fuel cap vent is blocked shut it can cause higher pressures which affect its properties and the next step’s reaction. Since the hole is so small you will have to make frequent efforts to keep the hole unblocked as it is easily filled up.


Luckily the process to keep this tiny vent clear and functioning is relatively simple and not time-consuming at all. 

  1. Simply take a needle or a pin and poke the hole. You will immediately see how much of it is dirt/dust. Therefore, you can clean it away easily. 
  2. You are better off removing the whole cap while doing this. But place another cap over the opening to avoid evaporation or the introduction of dirt into the fuel tank. 

7) Notch/Throttle Faults

Have you ever stopped to think that maybe lawn mower surging issues could be the direct result of something related to speed notches? 

Most lawnmowers feature a notch or throttle as it is also called which can be moved clockwise or anti-clockwise to increase or decrease speed. This dial is connected to the throttle cable. 

The throttle cable controls the flow of fuel to the carburetor, either increasing or decreasing it. 


The speed notch is attached to the carburetor and has a direct effect on engine speed. Sometimes the throttle is faulty or defective and this affects the ability to influence the carburetor. 

  • Unbolt the one screw holding the throttle/lever to the lawn mower handle
  • Run the wire down to the engine.
  • Loosen the bolt at the base which holds the cable close to the engine 
  • Then unhook the cable completely from the carburetor, this portion will be completely uncased and just bare wire 
  • Take your new throttle and cable (it comes combined and is ready to install) and fix it at the handle with a screw and bolt. Do not overtighten the cable
  • Next measure out the cable, try not to undercut it
  • NOTE- while measuring the cable, ensure the throttle is open or set to high power to expose the bare wire
  • Put the cable against the mower and fasten it using the already existing bolt and screw 
  • You will have to bend the exposed bare wire to fold it into place to attach to the carburetor 
  • Now you can start the mower and test the speed 

8) Vacuum Leaks

The carburetor has specific inlets for fuel and air. However, air may seep in from loose areas that you may not be tight enough. Hence, this excess air will throw off the desired ratio of air to fuel that powers the engine. 


  1. Turn off the machine and turn off the fuel 
  2. Just as before, you will have to remove several parts to get to the carburetor. This includes the air cover, air filter, crankcase breather pipe, and gaskets 
  3. Once the carburetor is visible check each of the bolts
  4. Maybe the part itself is not fixed well 

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Is Your Lawn Mower Surging Still?

If you have tried out all of the above remedies and are still experiencing a surging lawn mower, you may have to visit a specialist to solve the problem. You should not continue with the surging and get expert advice right away. 

It could be a defective piece in the machinery or something a bit more technical. Don’t rule out the fact that your lawn mower is just old now and needs replacing! 

Why is my lawn mower smoking?

Spilling excess oil on the engine may be burning off causing your lawn mower to smoke while using it. This is harmless and you can let it burn off. If it persists for a long time, it could be that the air filter needs cleaning. 


Upon finding that your lawn mower is surging, you will want to fix it as fast as possible to ensure your lawn mowing is not interrupted. 

Lawn mower surging is usually due to:

  • Poor fuel quality
  • Bad spark plug
  • Dirty carburetor/air filter/fuel lines
  • Blocked fuel cap vent
  • Vacuum leaks
  • Faulty throttle 

Usually replacing the faulty part or cleaning clogged pipes and parts can rectify the problem and stop your lawn mower from surging. But if it persists, a professional is necessary!