Lawn Mower Engine Troubleshooting

Welcome to our step-by-step lawn mower engine troubleshooting guide. If your lawnmower is acting up, you're in the right place.

Whether you're a seasoned DIY enthusiast or a first-time fixer, this article equips you with the insights needed to diagnose and address common mower engine problems.

Get ready to roll up your sleeves and master the art of lawn mower engine repair.

Kohler, Briggs and Stratton, Kawasaki , Tecumseh and Honda are the five most popular small engine manufacturers, and this mower troubleshooting guide applies to them all.

Lawn Mower Engine Troubleshooting

Perform a Visual Inspection

By visually checking your lawn mower for loose parts, disconnected wires, leaks, or visibly damaged components, you can quickly spot issues that might be causing the trouble.

These can include loose belts, disconnected fuel lines, oil leaks, or damaged spark plug wires.

Often, these visual cues can help you address straightforward problems without the need for advanced troubleshooting or professional assistance.

Lawn mower engine troubleshooting is like letting the mower tell you what's wrong, you just need to look for the signs in a methodical way.

Test Safety Features (If Applicable)

Ensure that mower safety mechanisms, such as the safety key, pressure switch, and blade control, are in proper working condition.

Test the safety key to confirm it engages and disengages as intended. Verify that the blade control stops the blade's rotation promptly when released.

Riding mowers often have a safety pressure switch under the seat. It's purpose is to protect the mower operator from harm by shutting the engine off when it senses nobody is sitting in the seat.

Malfunctioning safety features can prevent your lawnmower from starting or operating.

Inspect The Throttle/Choke

Improperly adjusted throttle and choke components can cause your lawn mower to run poorly, or not at all.

The throttle controls the engine speed, and the choke regulates air and fuel mixture during starting.

Adjust Mower Throttle and Choke

Throttle Inspection

Move the throttle lever to the "Start" or "Choke" position before starting the engine.

After starting, gradually adjust it to the "Run" or "Fast" position for normal operation.

Choke Inspection

Set the choke to the "Choke" or "Closed" position when starting a cold engine. As the engine warms up, gradually open the choke to the "Open" or "Run" position.

Proper throttle and choke settings ensure a smooth start and efficient operation of your lawnmower engine.

Refer to the owner's manual for specific instructions on your model.

Check The Fuel

Maintaining fresh fuel is vital for your lawnmower's peak performance. To ensure your mower runs smoothly, follow these steps:

First, do a smell test. Open the gas tank and, from a few feet away, do a quick sniff. If the fuel smells sour, it's likely bad.

Next, check to make sure the fuel is clear and free from debris. Check the color too; it should be pale yellow, not dark.

Some fuel stations are now selling gasoline with up to 15% ethanol. This E15 product is not recommended or approved for use in small engines.


To keep fuel fresh, use stabilizers for long storage, choose ethanol-free options, run the mower regularly, and buy fuel as needed.

Proper disposal of old fuel is crucial.

Check The Oil

To determine the freshness of your lawnmower's oil, follow these steps. First, check the oil level on a level surface using the dipstick.

Fresh oil is translucent amber. If the oil appears dark or black, it might need changing. Rub a bit of oil between your fingers; fresh oil is smooth, not gritty.

Fresh mower oil shouldn't have a burnt smell. Regular oil changes are crucial for a well-functioning engine.

Check The Air Filter

Check The Air Filter

A clogged air filter can restrict airflow to the engine, leading to poor performance or mower starting issues.

To check, locate the air filter, remove it, and inspect its condition. If it's dirty or clogged, clean or replace it.

This simple step often improves engine performance and helps resolve common issues.

Is The Carburetor Clean?

A dirty carburetor can disrupt the fuel-air mixture and cause starting or running problems. In fact, in my 25+ years of experience, I have found that a dirty carburetor is a leading cause of mower engine problems.

Common symptoms of a dirty carburetor include difficulty starting, rough idling, reduced power, engine surging, or stalling during operation.

In my years of experience, lawn mower engine surging is most often caused by a worn or dirty carburetor diaphram.

To check, locate the carburetor, inspect it for visible dirt or deposits, and, if needed, clean the carburetor using carburetor cleaner.

This simple mower maintenance step can often restore proper fuel flow and resolve issues.

Is The Ignition System Faulty?

If your lawnmower is experiencing starting or running issues, a faulty mower ignition system could be the cause.

Inspect the spark plug for dirt, damage, or wear. Replace it if neccessary and ensure the spark plug wire is securely connected.

Test the ignition coil for damage, examine the wiring for any issues, and consider performing a spark test.

If you're uncertain about the ignition system, it's advisable to consult a professional for diagnosis and potential repair.

Check The Battery (If Applicable)

Check the Mower Battery (If Applicable) - Battery on a Workbench

Riding and zero-turn mowers typically have an electric starter motor and a keyed ignition switch that requires a battery to operate.

To check, locate the battery, inspect its connections for corrosion or looseness, and measure its voltage using a multimeter.

Mower battery voltage rating is marked on the side of the battery and that's how much voltage your multimeter should read when testing it.

Symptoms of a battery issue include slow or no cranking when starting, dim lights, or erratic electrical behavior.

Ensuring a fully charged battery with clean connections can often resolve starting problems and restore normal operation.

Inspect The Wiring

Faulty wiring can lead to starting problems and poor performance. To check, examine visible wires for fraying, damage, or loose connections.

Symptoms of wiring problems include intermittent starting, lack of power to electrical components, or flickering lights.

Look for melted insulation, exposed wires, or disconnected plugs as visual signs of wiring issues.

Check The Engine Exhaust System

A blocked or damaged exhaust can cause poor running conditions.

To check lawn mower exhaust functionality, inspect the exhaust for debris, carbon buildup, or damage.

Symptoms of mower exhaust problems include reduced power, excessive smoke, or unusually loud noise.

Visual signs of issues include a clogged muffler outlet or visible damage to the exhaust components.

Clearing obstructions or addressing damage in the exhaust system can often restore efficient engine operation and resolve related issues.

Note: If your mower backfires it is not likely to be an exhaust problem, it's an engine timing or worn valve issue, most of the time.

Test Engine Compression

Low compression can lead to poor performance and starting difficulties. To check, use a compression tester on the spark plug hole.

Symptoms of low compression include hard starting, reduced power, and excessive smoke.

Checking engine compression helps diagnose internal issues, allowing you to address potential problems and restore proper engine function.

Checking Compression Without a Tester

Checking engine compression without a compression tester can provide a basic indication of engine health.

On a mower with a starter and battery, remove the spark plug with a spark plug socket of the right size, turn the engine over, and listen for consistent airflow.

On a mower with a pull cord, gently pull the cord and feel for cord resistance. You should feel resistance with each cycle of the engine.

Low mower engine compression can lead to starting problems, reduced power, or smoke.

While not as precise as using a tester, this method can help identify major problems and guide further actions, such as seeking professional help or evaluating engine condition.

2-stroke vs 4-stroke Mower Engines

2-stroke engines do not have a separate oil reservoir. Instead, oil is mixed with fuel.

This is a critical step because the oil in the fuel protects 2-stroke engines from overheating and prevents piston damage.

If you suspect your mower is running on un-mixed gasoline you should immediately drain the gas from your mower.

Billowing Blue Smoke from the Exhaust

Lawn Mower Billowing Blue Smoke

It's hard not to notice when a mower engine billows a lot of bluish smoke from the exhaust. There are three most likely reasons for this.

  1. The pistons are worn and the piston rings are no longer preventing oil from entering the combustion chamber.
  2. The is an excessive amount of oil in the fuel and it is burning during the combustion process.
  3. The mower has been improperly tipped on its side.

Yes, the blue smoke is almost always a sign of oil burning. In fact, in 25 years of repairing small engines, I've only had the blue smoke be caused by dirty water once, and that mower had been sitting outdoors for two years, uncovered.

If the problem is worn pistons(#1), you can buy a little time before needing an engine rebuild by using a thicker oil.

If you've been using 5W-30, for example,, switch to straight SAE30 oil.

This will make the oil flow more slowly while the mower is not in use and less will leak past the cylinders at startup.

If the engine is burning oil consistently through an entire mowing session, it's time to inspect the fuel for an excessive amount of oil. If the gas is oil-free, engine wear or damage is likely, bring the mower to an expert.

If the mower burns a LOT of oil suddenly after you first start it, but stops after a few minutes and doesn't return, it's likely that the mower was tipped over improperly.

In ALL cases, you'll need to remove and clean the spark plug after you diagnose the problem as the oil will have left a buildup of gunk and residue on the piston.

When to Call a Professional

Here are some signs that indicate it's time to bring your mower to a professional.

Safety Concerns

If you suspect electrical issues, fuel leaks, or other safety-related problems, it's best to leave the repairs to experts who can handle these potentially hazardous situations.

Unresolved Issues

If you've attempted basic troubleshooting steps and the problem persists or worsens, it's a clear indication that a deeper problem exists.

A professional can diagnose the root cause more accurately.

Related: In extreme cases you'll need to fix a seized lawn mower engine, or find someone who can.

Specialized Tools

Specialised Diagnostic Tools on a Bench

Some repairs require specialized tools that you might not have on hand. Professionals are equipped with the right tools to diagnose and fix the issue efficiently.

Warranty Coverage

If your lawnmower is under warranty, attempting repairs yourself could void the warranty. A professional can address the issue while preserving the warranty.

Time Constraints

If you need your lawnmower back in operation quickly, a professional can diagnose and repair the issue more swiftly than if you're learning on the go.

Major Engine Issues

If the problem involves major engine components like the crankshaft, piston rings, or camshaft, it's best to have a professional handle the repairs due to the complexity involved.

Modern Technology

Newer lawnmowers often feature electronic components and computerized systems.

If you're not familiar with these technologies, a professional can effectively diagnose and repair them.

Prioritize Safety

Wait for your mower to cool down before attempting to troubleshoot any issues.

Remember to disconnect the spark plug wire to prevent accidental starts.

Also, always work in a well-ventilated area to avoid inhaling exhaust fumes.

Safety first!


Lawnmower engine troubleshooting involves a systematic approach to identifying and resolving issues that affect performance.

From checking fuel and oil levels to inspecting components like the spark plug, carburetor, and wiring, each step targets potential problems.

Addressing safety features, exhaust, and engine compression, while using visual and auditory cues, can help pinpoint issues.

This methodical process ensures a well-maintained lawnmower that operates efficiently and safely. I use it myself when I troubleshoot mowers and other outdoor power equipment.