Why is My Lawn Mower Engine Surging?

Why is My Lawn Mower Engine Surging

Imagine you are mowing your lawn on a typical sunny Sunday. You are halfway there and look behind to appreciate the evenly laid out grass. But as you continue to mow the remaining of the lawn, you start noticing your lawn mower engine surging. The lawnmower surge is too little to start with at the beginning. However, as you continue the mow, the surge starts getting stronger. 

The one thing you don’t want your engine to make is noise, in many cases those noises could be due to lawn mower backfiring. Although you have opted for a noise-free lawn-mower in the first place, you notice your lawn mower engine surging. Noising out an engine cause concern and must be addressed immediately. So, if you hear your lawn mower engine surging, then we are here for you. This article will walk you through all the possible causes that contribute to a lawnmower surge. Finally, we will lay down some easy fixes you can implement to tackle this issue. 

Causes Of Lawn Mower Surging

Knowing the probable cause behind your lawn mower surging is key to going ahead. Once you have known all the probable causes behind the issue, it becomes relatively easier to tackle the problem the next time it arises. One of the primary causes behind a lawnmower engine surge is a blockage in the fuel supply. If your mower is subject to regular use, dust or other particles may tend to enter the engine and block the fuel supply. But apart from the fuel supply blockage, here are some other probable causes that can contribute to the lawnmower engine surge

  • Poor Quality Of Fuel 
  • Malfunctioning Spark Plug
  • Faulty Carburetor (Find where it is)
  • Blocked Gas Filter
  • Blocked Fuel Tank
  • Blocked Gas Line
  • Gasket Vaccum Leak
  • Manifold Vaccum Leak
  • Governor Control Fault
  • Dirty Or Blocked Air Filter

Conduct A Surging Test

A surging test is perhaps the best way to determine any issues within your mower that need addressing. Primarily, a surging mower engine results from gas starvation or any other blockages that may have occurred within the fuel tank, air filter, or the fuel line. Meanwhile, some carburetors are prone to vacuum leaks, further intensifying the problem. 

The majority of modern-day mower engines are designed for maximum efficiency. However, this has made the carburetors more prone to clogs and blockages. To quickly conduct a surging test, all you need to do is trace the given steps.

NOTE: You will need to have a helper to help you conduct this step

  • Make the helper hold the bail lever.
  • Ensure both of you are wearing protective gear such as earplugs, eye goggles, safety gloves, and safety boots
  • Identify the type of mower you have to know the type of choke it holds, namely manual, auto, or a primer bulb.

For Manual Choke:

  1. Apply half choke with the engine running
  2. If the engine runs surging, there is a lack of gas present
  3. If there is no noticeable change in the surging, there is a vacuum leak

For Auto Choke:

  1. Remove the air filter and housing 
  2. Place a clean rag over the filter and keep the engine running
  3. If the engine runs surging, there is a lack of gas present
  4. If there is no noticeable change in the surging, there is a vacuum leak

For Primer Bulb:

  1. Add some extra gas to the engine by pressing the bulb
  2. If the engine runs surging, there is a lack of gas present
  3. If there is no noticeable change in the surging, there is a vacuum leak

How To Fix A Surging Lawn Mower?

Once you become familiar with the problem, it becomes relatively easier to tackle them. Similarly, you can always conduct a surging test to ensure everything’s in a good working condition and check whether all the fixes you have applied are implemented successfully. So, if you find your lawn mower engine surging, you can implement the following fixes.

NOTE: Before implementing either of the fixes,

  • Turn off your machine and allow it to cool off for at least an hour
  • Wear protective gear such as earplugs, eye goggles, and safety gloves
  • Do NOT touch anything with your bare hands
  • Do NOT touch the mower blades 
  • Ensure that the machine is turned off at all times during the repair

1. Bad Fuel

This is one of the primary causes behind a lawnmower engine surging. If you live in an area where there isn’t a lot of rain, your mower is bound to be off at all times during the off-season. With the lawnmower, the engine fuel remains dormant during the period. This largely enhances the chances of the fuel getting contaminated. We cannot stress the importance of good fuel enough. You must fill the machine with the right kind of fuel. 

The right fuel will only enhance your machine’s overall durability and longevity. Whereas subjecting your machine to bad or poor fuel quality will automatically take a tool on the mower’s engine. Know that gasoline is extremely important for appliances that run on fuel-injection technology, such as cars and lawnmowers. Gasoline is used to power up the engine by causing the combustion required for sparking. Good quality fuels will ensure a good fuel economy, smoother engine performance, higher engine power, and lower carbon emission. 

Having your mower sit idle for long is likely to contaminate the fuel or the oil present. Hence, if you think the fuel is at fault, we suggest you empty the tank and refill it at once. To do so:

  1. Tip-over your mower and drain away from the fuel and oil.
  2. Ensure that NO COMBUSTIBLE OBJECT is present in the vicinity.
  3. Once you have drained the fuel, fill the mower with fresh fuel.

2. Clogged Air Filter

Apart from the fuel tank, the air filter is the first thing you will need to check in case there is a surging lawnmower. The air filter is responsible for the regular supply of fresh oxygen to the mower engine. However, if the air filter is clogged, it starves the mower engine of fresh oxygen required for combustion. To check if the air filter is clogged, trace the given steps.

  1. Disconnect the spark plug wire
  2. Remove the air filter
  3. Remove the air filter housing
  4. Using soapy water, clean the foam filter
  5. Once the cleaning is done, keep the filter to dry
  6. Once the filter is dried up, smear an oil coating over the surface of the air filter.
  7. If the filter is still dirty despite the cleaning, replace it with another filter that suits your model.
  8. Wipe the dirt out of the filter before reinstalling it

3. Fuel System Issues

Just like bad fuel, fuel system issues are quite common when it comes to working with a lawnmower. Every fuel tank comes with a small cap that allows air to flow into the tank and mix with the fuel to ignite a spark and cause combustion. But just like air, dust, and dirt can sometimes flow through the cap and clog the fuel system preventing enough fuel from reaching the carburetor. To tackle this, implement the given steps.

  1. Inspect the vent on your gas cap
  2. If the vent is dirty, clean it 
  3. Drain the gas tank 
  4. Clean the gas bowl
  5. Add a fresh batch of fuel to the tank
  6. Alternatively, you can inject a fuel cleaner to clean the fuel line
  7. Once all this is taken care of, run the mower to check if the issue is taken care of. 

4. Dirty Carburetor

The carburetor is an integral part of every machine that runs on fuel-injection technology. The carburetor that allows the right amount of air-to-fuel rato into the engine is required to cause combustion. However, blockages within the carburetor are probably why you have a surging lawnmower. To eliminate this possibility, trace the given steps.

  1. Disconnect the air filter
  2. Disconnect the fuel tank
  3. Disconnect the governor control link
  4. Disconnect the breather pipe
  5. Disconnect the manifold seal and the keeper ring
  6. Finally, disassemble and disconnect the carburetor.
  7. Use a carburetor cleaner to clean the carburetor thoroughly
  8. If the carburetor is damaged, replace it at once
  9. After cleaning the carburetor, reassemble the carburetor and reinstall it onto your mower.

5. Vacuum Leaks

A loose carburetor tends to suck in air through the crevices and traps it between the engine block. This will unbalance the air-to-gas ratio, compromising the vacuum required to transport fuel and air into the mower engine. All this collectively causes an irregular engine performance.

This is because a vacuum leak affects air and fuel rate flow through the carburetor at the recommended rate. A possible solution to fix a vacuum leak is to check the bolts on the carburetor and tighten them if necessary.

6. Spark Plug Issue

Lastly, the issue with the spark plug is probably your mower revs up and down. If you have an old or worn-out lawn mower, there are high chances of the spark plug being worn out. A worn-out spark plug may not have the strength to cause combustion and ignite the flame required for the mower to run. If you have a worn-out spark plug, you will find that your mower dies every other minute. A solution to this involves

  1. Disconnect the machine
  2. Remove the spark plug
  3. Check the spark plug for any wear and tear
  4. Check for the gap between the metal end and the protruding end of the plug
  5. If the gap is too big or too small, adjust it accordingly to allow sparking to occur
  6. Check to see if the mower is back to normal functioning

Any issue with the spark plug generally calls for a replacement. So, if you have little luck implementing this fix, we suggest you replace the spark plug with a new one designed for your mower.


Despite its impeccable architecture, a lawnmower is subject to various issues throughout its lifetime. One commonly reported issue is a surging mower engine, and the last thing you want to do is disturb your neighbors with a broken mower. Therefore, we suggest you look at our troubleshooting guide to help tackle the problem. However, if there is little to no luck in the fixes you have implemented, call in an expert to have a look at the machine at once. 

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