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What Causes A Lawnmower Engine To Surge?

Engine surging is a common problem that lawn mower machines have been experiencing over the years. The most common reasons for engine surges are bad spark plugs, faulty carburetor, dirty air filter, gasket vacuum leak and governor control fault. 

Surging can cause poor cutting performance, increased fuel consumption, and even damage to your engine, so it needs to fixed asap.

Engine surges can be easily rectified by following the steps mentioned below. Today we’ll be discussing some of these causes in detail.

What is lawn mower surging?

Lawn mower surging is a condition where the engine speed fluctuates up and down, creating a jerky motion and an uneven cut. Surging can occur at any time, but it is more noticeable when the mower is idling or running at a low speed.

The most common cause of lawn mower surging is a dirty or clogged carburetor. The carburetor is the part of the engine that mixes air and fuel to create combustion.

If the carburetor is dirty or clogged, it can prevent the proper amount of fuel from reaching the engine, resulting in an inconsistent fuel-air ratio.

This can make the engine run too lean (not enough fuel) or too rich (too much fuel), causing it to surge.

A lawn mower that is surging
A surging lawn mower

Why Does My Lawn Mower Keep Surging?

Here are the most common causes we discussed – have a look.

1. Bad gas in the tank

A lawnmower engine could surge due to bad gas, because the gas is too old or because the tank is not filled up. Gasoline can be left in the tank for about a week, but it should never be drained completely. 

A half-empty gas tank is not a problem and should not cause the lawnmower engine to surge. If your gas tank is not very full when you store it for winter, there is a risk that the gas will absorb water and become diluted. 

This might cause it to leave water droplets in the tank or cause an engine surge when you start it up at the beginning of spring. The best way to avoid this is to always keep your gas tank almost full during winter storage. 

2. Bad spark plug fails to burn fuel efficiently

A lawnmower engine is designed with a spark plug to provide the right amount of energy for an engine to start and run smoothly. A lawnmower engine will not function properly without a spark plug. 

The spark plug ignites the fuel and air mixture in the cylinder chamber. A bad spark plug is the cause of a lawnmower engine surging. 

This will lead to high fuel consumption as the engine will not be able to accelerate properly because it’s not getting enough power from the spark plugs. 

A bad spark plug can be caused by an improper electrode gap or worn-out electrodes that no longer conduct electricity properly. Without a proper spark plug, the lawnmower engine will misfire as it tries to turn on. 

The fuel will not be burned as efficiently as it should be, and all this can lead to a surge in engine performance. If your lawnmower engine is surging or misfiring, you might want to check the state of your spark plugs first before replacing them with new ones. 

A lot of people are reluctant to work on their own mower because they think that they might break something or make it worse. However, replacing spark plugs is an easy process that doesn’t require any special training or skillset. 

Even if you break something, the worst-case scenario would be to replace the part with a new one for a few bucks.

3. Faulty lawn mower carburetor can’t mix the fuel correctly

A carburetor is a device that mixes the perfect ratio of gas and air for an internal combustion engine. The purpose of the carburetor is to mix the perfect ratio of gas and air. It is designed to get the correct air/gas ratio.

If the carburetor is out of adjustment, the engine will not be able to use all its fuel mixture for combustion. If the carburetor is set up wrong, there won’t be enough fuel getting to the engine. 

Faulty lawn mower carburetor
Faulty lawn mower carburetor

This will cause the engine to surge because it has too much fuel in it. A dirty or faulty carburetor can’t mix the fuel mixture correctly. If the bowl vent valve is stuck open, it will allow fuel to flow freely through the carburetor. 

This will cause the engine to surge. If this happens, you must replace the bowl vent valve. A bad fuel filter can’t keep debris out of your engine and carburetor, which can lead to surging or lack of power.

You’ll also notice your lawn mower is running slow due to a faulty carburetor. To fix this problem, you should inspect the carburetor for any torn gaskets or dirt. You should also remove and repair any damage that was done to it if there is anything. 

Finally, you should securely tighten the carburetor after cleaning it off to avoid further issues with your car engine in the future. This process might require more expertise than replacing the gasket or tightening bolts, so be sure to consult an expert before starting this step.

4. Dirty air filter causes engine to run with lower output

If you have a dirty air filter, it can cause your mower to surge on startup. This is because the engine is receiving less air than it needs and starts sputtering and misfiring. 

The engine will also run with lower power output and the gas mileage will likely be reduced. When you’re cutting thick weeds or high grass, the engine may sound like it’s struggling with these tasks. 

Dirty air filter on a lawn mower
Dirty air filter on a lawn mower

The exhaust system may give off a strong gasoline smell that fills the air when you’re mowing in hot weather. A dirty air filter will make the lawn mower inefficient. 

When the air filter is dirty, it can cause internal damage to the engine. You should fix the air filter to overcome the issue.  Removing the air filter from a lawn mower is one of the easiest tasks to do. 

Start by removing the spark plug wire from the spark plug. This will prevent you from accidentally starting your lawn mower and getting injured. 

Pull out your lawn mower’s air filter and wash it with soap and water, then dry it before putting it back in. If you would like to reuse your old filter, rub some mineral spirits on it before putting it back in.

If you find that your old filter is too worn out or damaged, replace your old filter with a new one so that you can always have a clean machine for any job!

5. Carburetor vacuum leak symptoms

The lawnmower carburetor is designed to work with a controlled amount of air intake. A gasket vacuum leak causes the engine to surge because when not enough air enters the combustion chamber.

This throws the gasoline mixture out of balance. This unbalances the gas-to-air ratio and releases unburned hydrocarbons. This can cause irregular engine performance. 

The unbalance in the gas to air ratio will make it difficult for the engine to run smoothly.

Fixing a vacuum leak in a lawn mower engine is the best way to prevent surges. Properly routing hoses can help prevent surge due to vacuum leaks. 

When you are repairing vacuum connections always make sure that they are tight and secure. Find the location of your vacuum leak. 

When you have found it, take off the hose and inspect it for signs of wear or damage. If there is no visible damage, try fixing the connection by either tightening it or replacing the hose entirely.

6. Faulty governor controller

Governor control fault is one of the most common causes for this problem. The governor is a system that regulates the mean speed and controls the fuel to the engine cylinders. 

When an idle speed control motor becomes fouled by carbon buildup from gasoline residue, it will cause surges in engine power and leads to high fuel consumption. 

This will make it difficult for the governor to maintain a constant mean operating speed and may lead to shutdowns or malfunctions in other parts of the fuel system. 

The best way to fix this problem is to clean the component and repair or replace it if needed.

Some other possible reasons might be –

  • A loose or damaged throttle cable, which can interfere with the throttle control and cause the engine speed to vary.
  • A worn or damaged governor spring, which can affect the engine speed regulation and cause the engine to overcompensate.
  • A low or uneven oil level, which can affect the lubrication and cooling of the engine and cause it to overheat.

How to fix lawn mower surging?

The first step to fix lawn mower surging is to identify the source of the problem. You can do this by inspecting and testing different parts of your engine and checking for signs of wear, damage, or dirt. Here are some steps you can follow to troubleshoot and fix lawn mower surging:

1. Check the carburetor

The carburetor is usually located on the side of the engine and has a bowl-shaped part underneath it. Remove the bowl and check for any dirt, debris, or water inside it. If you find any, clean it with a rag or a brush and some carburetor cleaner.

You can also use a thin wire or a needle to clear any clogs in the small holes or jets of the carburetor. Reassemble the bowl and make sure it is tight and secure.

2. Check the air filter

The air filter is usually located on top of the carburetor and has a cover that can be removed by unscrewing a knob or a clip.

Remove the cover and check for any dirt, dust, or grass clippings on the filter. If you find any, clean it with a rag or a brush and some compressed air.

You can also wash it with some warm water and mild soap if it is very dirty. Let it dry completely before reinstalling it.

3. Check the spark plug

The spark plug is usually located on top of the engine and has a wire attached to it. Remove the wire and use a spark plug wrench to unscrew the spark plug from the engine.

Check for any signs of corrosion, carbon buildup, or cracks on the electrode or ceramic part of the spark plug. If you find any, replace it with a new one that matches your engine specifications.

You can also use a gap tool to measure and adjust the gap between the electrodes if needed.

4. Check the throttle cable

The throttle cable is usually located on top of the engine and connects to a lever on your handlebar that controls the engine speed.

Check for any signs of looseness, fraying, or damage on the cable or its connections. If you find any, tighten or replace them as needed. You can also use some lubricant to smooth out any kinks or bends in the cable.


For any engine to run smoothly, it needs to be properly maintained. If you neglect these routine maintenance tasks, your lawn mower engine could experience problems like surging or backfiring as the Kholar engine do.

Some of the vital way to avoid these problems is by performing the necessary fixes and following certain guidelines like ensuring clean gas, good air flow and proper spark. 

We’ve discussed some of the major problems associated with surging lawn mower engines. If you still have problems performing these fixes, contact a professional repair service.

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