Why My Lawn Mower Revs Up and Down

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There’s nothing more frustrating than a lawn mower that revs up and down. It’s like a car that can’t decide which gear to be in, and it can turn a relaxing afternoon of yard work into a mechanical nightmare.

But why is this happening? And more importantly, how can you fix it? In this article, we’ll explore the common causes of a lawnmower that revs up and down.

We’ll also provide effective solutions to rectify this issue for good, turning your mechanical nightmare back into a relaxing afternoon of yard work. Stay tuned!

Lawn Mower Revs Up and Down

Why My Lawn Mower Revs Up and Down (Short Guide)

Lawn mower revs up and down because it has an issue with airflow, fuel delivery, ignition, or compression. A clogged air filter restricts airflow into the engine; debris clogs passages in the carburetor, leading to an inconsistent fuel mixture. Fixing surge issues involves cleaning or replacing the air filter, rebuilding the carburetor, installing new properly gapped spark plugs, and engine repairs to restore lost compression.

Causes of Lawn Mower Revving Up and Down 

Let’s delve deeper into each cause and its corresponding solution:

Dirty air filter

The air filter in a lawnmower allows the engine to breathe by trapping dust and debris from entering the engine. Over time, the filter element can become clogged with dirt, dust, and grass clippings. This restriction starves the engine of air, causing it to run lean.

The lack of airflow forces the engine to compensate by revving up and down as it struggles to draw in more air, similar to how a person might gasp for breath when their airways are blocked.

Solution

To address this, you need to check the air filter. If it’s a paper filter and is excessively dirty, it’s best to replace it. If it’s a reusable foam filter, you can clean it by washing it in warm, soapy water and letting it air dry completely.

If the foam filter is too dirty to be cleaned, it should be replaced. By ensuring a clean air filter, you allow proper airflow into the engine

Bad fuel

Just as a car won’t run well on bad gasoline, a lawnmower can also have performance issues with old or contaminated gasoline. Gasoline evaporates and breaks down over time, leaving stickier residues that clog carburetor components.

Water or dirt in the gas tank also affects performance. This causes the engine to surge up and down when it gets a burst of fuel, then drops as it sputters through dirty or watery spots.

Solution

Drain the old gas from the tank and carburetor and refill with fresh 87-octane gasoline, preferably from a top-name brand like Toro or Honda

If you’re storing gas for over 2–4 weeks, use a fuel stabilizer because using quality fuel ensures smooth combustion.

Also check out:

Faulty carburetor

A carburetor is responsible for mixing air and fuel in the proper proportions for efficient combustion. Tiny jets, ports, and passages inside the carb can become blocked by varnish, dirt, and debris over time.

This affects the fuel-air ratio delivered to the engine, often making it too lean. The engine revs up and races as it tries to compensate for the lack of fuel. It drops back down when it gets a burst of fuel.

Also, many carburetors have external adjustment screws that alter the air and fuel flow to fine-tune performance. Over time, improperly setting vibrates out of position.

Solution

Cleaning the carburetor of debris and varnish restores proper fuel flow. Severely damaged carbs may need rebuilding or replacement. Follow your mower manual for carburetor service procedures.

Lawn Mower Revs Up and Down
Faulty Carburetor

Proper cleaning and rebuilding ensure the carburetor provides a balanced fuel mixture, preventing revving issues.

Click here for a complete guide on how to clean lawn mower carburetor

Spark plug issues

Spark plug is the main component in the ignition system, it provides the electric spark to ignite the compressed fuel-air mixture inside the cylinder. Problems with spark plugs can lead to uneven burning and misfiring.

Bad or worn-out spark plugs cause uneven revving; it struggles to fire properly and compensate through changing rpm. It’s like having an irregular heartbeat of a mower

Solution

Check the spark plugs regularly. If they’re not in good shape, removing the spark plugs one by one and inspecting their condition identifies issues. Replace plugs that are excessively worn or fouled. 

Use a spark plug gap tool to ensure the gap meets manufacturer specifications, usually around 0.030″.

Installing new or cleaned spark plugs with the proper preset gap will restore strong, consistent sparking and eliminate revving behaviors related to ignition misfiring.

Engine compression issues

Proper compression in each cylinder is absolutely essential for complete combustion and generating usable power. Compression squeezes the fuel-air mixture to create sufficient heat and pressure to ignite the mixture and produce controlled expansion.

Over time, normal engine wear leads to loss of compression from worn piston rings, leaking head gaskets, and damaged valves.

To compensate for this power loss, the engine attempts to rev up higher in a futile effort to generate more combustion force. Which causes Surgung and inconsistent rpm.

Solution

Test the compression in each cylinder using a compression gauge to determine if pressures are within specification or low. Typical small engine compression should be around 90–110 PSI.

If one or more cylinders are low by more than 10–15 PSI, internal repairs will be required to bring compression back to optimal levels and rectify surging issues. This may involve a piston ring, head gasket, valve, or cylinder head replacement.

If you’ve checked all these things and your lawnmower is still revving up and down, it might be best to take it to a professional for a more thorough diagnosis and repair.

Preventative Maintenance to Avoid Lawn Mower Revving Issues

While repairs may be needed to stop current revving issues, prevention is key to avoiding problems down the road.

Follow these tips for smooth mower operation:

  • Change oil regularly – Helps prevent engine wear to maintain compression.
  • Inspect belts – Replace cracked or slipping belts that affect governor operation.
  • Sharpen blades yearly – Keeps engine from working too hard and surging.
  • Clean mower after use – Prevents debris buildup around engine and carburetor.
  • Drain fuel for storage – Eliminates gum and varnish from stale gas.
  • Replace air filter yearly – Stops dirt from entering the engine.
  • Follow maintenance schedule – Do tune-ups, and adjustments to minimize rev issues.

With proper care and servicing, your lawn mower can deliver years of reliable service. Stop problems before they start with preventative maintenance. Give your mower the TLC it deserves so you can worry less and mow more.

Conclusion

In summary, maintaining a consistent and even engine speed is crucial for efficient mowing. It ensures that your lawnmower cuts the grass evenly, providing a neat finish to your lawn.

By understanding the causes of a lawn mower revs up and down and how to fix them, you can save both time and money. Instead of spending on professional repairs or troubleshooting services, you can handle most of these issues yourself with a little bit of knowledge and effort.

FAQs

Why does my Briggs and Stratton lawnmower rev up and down?

Just like any other lawnmower, a Briggs and Stratton lawnmower can rev up and down due to issues like a dirty air filter, a faulty carburetor, bad fuel, spark plug issues, governor problems, or engine compression issues.

How often should I check the air filter on my lawnmower?

It’s recommended to check the air filter after every 25 hours of operation. If it’s dirty, clean it, and if it’s damaged, replace it.

What type of fuel should I use for my lawnmower?

Always use fresh, clean, unleaded gasoline. If you’re storing gas for over 2–4 weeks, use a fuel stabilizer to keep the fuel fresh.

How often should I replace the spark plugs on my lawnmower?

It’s recommended to replace the spark plugs after every 100 hours of operation or every season, whichever comes first.

What is the number one cause of mower engine surge?

The number one cause of mower engine surge is using old or contaminated gasoline

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