Mower Engine Surging at Idle and How to Fix

Engines for lawnmowers are designed to be dependable and effective at cutting grass and keeping a tidy lawn. Even the greatest lawnmowers, nevertheless, are susceptible to engine surging at idle, which can cause the engine speed to constantly swing up and down.

Multiple factors could be the root of this issue, including:

  • Clogged air filter
  • Old or defective carburetor
  • Faulty spark plug
  • Loose or damaged fuel lines

After viewing this article, mower owners will quickly understand what causes Mower Engine Surging at Idle and how to resolve the problem.

Mower Engine Surging at Idle
Mower Engine Surging

4 Reasons Why Mower Engine Surging at Idle (Solutions Added)

This section will examine common causes of engine surging:

1. Clogged Air Filter

Clogged Air Filter

A lawnmower engine can surge when the air filter is blocked because it restricts the airflow to the engine. The engine is unable to receive the air it requires to burn gasoline effectively when the air filter is blocked with debris, dirt, or grass clippings.

This may lead to an excessively high fuel mixture, which makes the engine run rich or lean and results in surging.

It is to note that the engine uses less fuel when it is idling since it is operating at a reduced pace. The engine won’t receive enough air to burn fuel effectively at idle if the air filter is clogged.

As a result, the engine may struggle to maintain a constant idle speed and experience ups and downs in speed. A clogged air filter can also make the engine stall or run erratically, especially while accelerating or under load.

Fix: Examine the air filter

It’s critical to routinely check the air filter and clean it or replace it to avoid surges brought on by a clogged air filter. By performing this, you can be convinced that the engine is getting enough air to idle smoothly and burn fuel effectively.

Read Also: Lawn Mower Leaking Oil From Air Filter: Here’s How to Fix It!

2. Old or Defective Carburetor


An out-of-date or damaged carburetor may not be providing the right amount of air and fuel to the engine, which might result in a lawnmower motor surging at idle.

Regulating the flow of fuel and air into the engine is the carburetor’s responsibility to ensure effective combustion. Surging could occur if the carburetor can’t carry out this task adequately due to wear or corrosion.

Moreover, a precise amount of fuel and air at idle to keep the speed constant is important for the engine to run. The engine may receive too much or too little fuel if the carburetor isn’t working properly, which might cause the engine to surge.

The engine may run excessively richly or too leanly as a result, which can result in additional problems including stalling, harsh running, or trouble starting.

Fix: Check the carburetor

Checking the carburetor is important when the engine is surging. It can be necessary to clean, repair, or replace the carburetor to address surging which occurs because of an outdated or malfunctioning carburetor.

A trained mechanic should perform this technique because it can be difficult. You can easily avoid carburetor faults with regular carburetor maintenance, which includes cleaning and tuning.

3. Faulty Spark Plug

Spark Plug
Spark Plug

A lawnmower engine might surge at idle due to a malfunctioning spark plug because it can disrupt the process of ignition.

For the engine cylinder’s air-fuel mixture to ignite and generate power, a spark plug must be present. Surging may occur if the spark plug is defective because it isn’t regularly igniting the fuel mixture with a strong enough spark.

A steady spark at idle plays a great role in the engine to keep its speed stable. The engine may malfunction or run erratically if the spark plug fails, which might cause surging.

A faulty spark plug might also result in additional problems like difficult starting, harsh running, and reduced engine output.

Fix: Replace the spark plug

You may learn the importance of using the proper spark plug for the mower and proper gapping by checking the spark.

Simply execute regular checks on the spark plug, which includes cleaning and replacement at the proper intervals, to avoid the problem of surging and other challenges caused by spark plug concerns.

4. Loose or Damaged Fuel Lines

Damaged Fuel Lines

Due to their ability to alter the flow of fuel to the engine, loose or damaged fuel lines may result in a lawnmower engine that surges when it is idling.

The fuel lines are in charge of transporting fuel from the fuel tank to the carburetor, and then to the engine. Fuel pressure may drop as a result of broken or faulty fuel lines, which may lead to surging.

Not to forget that the engine needs a specific amount of gasoline at idle to keep its speed consistent. Fuel pressure may drop and surging may occur if the fuel lines are broken, cracked, or loose and are admitting air into the fuel system.

Fix: Verify the fuel lines

It may be necessary to inspect and replace the fuel lines if necessary to correct surges caused because of loose or damaged fuel lines.

It’s crucial to utilize the proper gasoline line type for the mower and to make sure the fuel lines are attached correctly and are not broken or kinked.

5. Finally, Adjust the idle Speed

One way to resolve a lawnmower engine surging problem is to change the idle speed. The pace at which the engine runs while it is in neutral and not actively mowing is referred to as the idle speed.

If a lawnmower engine surges or stalls at idle, it may be running too quickly or too slowly.

You must locate the carburetor’s idle speed adjustment screw to change the idle speed. You might need to refer to your lawnmower’s handbook to properly identify this screw, which is typically found on the side of the carburetor.

Related Post: How To Fix Lawn Mower Engine Dies When Clutch Released?

Cost to Fix Mower Engine Surging at Idle Issue

The price of a new air filter can vary from $5 to $25, depending on the retailer where you purchase the replacement filter, and the brand and model of the mower also play a role here.

Depending on the brand and model of your lawnmower and the labor rates charged by the repair shop, the cost of cleaning the carburetor might range from $50 to $150.

In general, a new spark plug costs $5 to $15.  There won’t be any additional labor charges if you feel confident changing the spark plug yourself.

If you intend to repair or replace a damaged fuel line, the cost will be determined by the extent of the damage and the cost of new parts. Replacement fuel lines usually cost between $10 and $50.

Remember that these are merely estimates and that final costs may differ based on a variety of elements, including your location, the scope of the required repairs, and the repair facility you select.


Can an engine that surges harm a lawnmower?

If a surging engine is not fixed, it may result in excessive wear and tear on the lawnmower’s engine and later, more serious problems.

Is it okay to keep mowing with a surging engine?

It is not advised to keep cutting with a surging engine because it could harm your lawnmower much more and pose a safety risk.

How often should I change my lawnmower’s air filter?

Depending on how often you use a lawnmower and how it operates, you may need to replace the air filter more frequently or less frequently. Check the air filter normally on a regular basis, and if it looks dirty or clogged, replace it.

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