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Why Does My Lawn Mower Backfire When I Try to Start It?

Posted by Darrell Ross on
Gardening Tools
Why Does My Lawn Mower Backfire When I Try to Start It?

Ever tried starting your lawn mower only to have it sputter, cough, and backfire in protest? Frustrating, right? You are ready to finish your yard work, and your mower has other plans. No, you are probably stuck in a rut asking: Why does my lawn mower backfire when I try to start it? Don’t worry; we’ve got you covered. To care for your lawn organically without issues, ensure you read this to the end. We’ll walk you through the possible reasons your mower is backfiring and the steps you can take to get it running smoothly again.

Common Causes of Lawn Mower Backfiring on Start-Up

When your lawn mower backfires and sputters when you try to start it, it can be frustrating and mess with your gardening plans. But don’t worry; the problem is usually an easy fix. Here are the most common causes of lawn mower backfiring and what you can do about them:

Dirty or Clogged Air Filter

The air filter cleans the air before it enters the lawn mower engine. Not enough air can get in if it’s clogged with dirt and debris. Replace your air filter once a season or every 25 hours of use.

Old or Bad Gasoline

Gasoline starts to break down after just 30 days, and it releases volatile compounds that ignite too early. So, drain your mower’s fuel tank and refill it with fresh, high-octane gas. You may also need to clean the carburetor.

Spark Plug Issues

Faulty or fouled spark plugs won’t ignite the fuel-air mixture properly. In such a case, remove and inspect your spark plug. Clean or replace it if the electrode is worn down or coated in carbon buildup. Ensure the spark plug gap is set to the recommended distance for your mower’s engine.

Carburetor Problems

If your carburetor is stuck, clogged, or out of adjustment, it won’t supply the right fuel-air mixture to the engine. In fact, it may cause too much air or insufficient air. Consequently, you may need to clean or rebuild the carburetor. Check your mower’s manual for the proper carburetor settings and adjustment procedures.

Troubles With Combustion Chamber

If lawnmower engines backfire when you try to start it, it might indicate a disturbance in the normal operation of the engine. Most times, it’s linked to the combustion chamber. The combustion chamber is where the air-fuel mixture ignites to produce power. The balance of this mixture is critical. Hence, an incorrect ratio causes the engine to behave erratically.

Check Exhaust System

When the combustion process is interrupted or doesn’t complete correctly, unburned fuel can make its way into the exhaust system. Given the high temperatures within this system, this fuel can spontaneously ignite. When this occurs, it results in a loud pop or bang – the characteristic sound of a backfire.

Fuel-Related Issues Leading to Backfiring Lawn Mowers

If your lawn mower is backfiring when you try to start it, it is likely fuel-related. Check these common culprits:

Incorrect Fuel-to-air Ratio

The carburetor mixes fuel and air to power lawn mower engines. If the mixture is off, it leads to backfiring. First, check that your air filter is clean and unobstructed. A clogged filter reduces airflow and leans out the mixture. You may also need to adjust the carburetor settings – consult your owner’s manual for the proper procedure.

Vacuum leak

If there’s an air leak in the fuel system, extra air gets mixed into the fuel-air mixture and causes backfiring. Check that all fuel lines, gaskets, and seals are intact and securely connected. Tighten or replace damaged components as needed.

By draining old fuel, ensuring the proper fuel-to-air ratio, and fixing any vacuum leaks, you should be able to get your lawn mower started without the backfire. If issues continue, it’s best to have your mower serviced by a certified technician to check for additional carburetor or ignition problems before further damage occurs.

Ignition System Problems Causing Backfiring

Weak Magneto or Ignition Coil

The magneto and ignition coil produce the spark plug’s electrical current. If either part fails, it won’t generate a robust enough charge. You can test the magneto and ignition coil to determine if it is a replacement.

Incorrect Spark Plug Gap

The spark plug gap refers to the space between the center and side electrodes on the spark plug. The spark may be too weak to ignite the fuel properly if the gap is too wide or narrow. Check your owner’s manual for your lawn mower model’s correct spark plug gap. Carefully adjust the gap or install a new pre-gapped spark plug.

Faulty Ignition Timing

The ignition timing controls when the spark plug fires to ignite the fuel in the engine. If the lawnmower engine timing is off, the spark may fire at the wrong time, leading to backfiring and trouble starting. Have your lawn mower serviced to check and adjust the ignition timing. The technician can also test other ignition system components to diagnose and repair the issue.

How to Troubleshoot a Backfiring Lawn Mower

A man checking a lawn mower

If your lawn mower is backfiring when you try to start it, it likely means there’s an issue with the carburetor or ignition system. The good news is that basic tools and troubleshooting can fix many of these problems at home.

Check the Air Filter

A clogged air filter prevents enough air from getting into the carburetor, causing the engine to run rich in fuel. Replace or clean your mower’s air filter—this is an easy fix and could solve your backfiring problem.

Clean the Carburetor

Over time, gunk and debris build up in the carburetor, disrupting the air-fuel mixture. Remove the carburetor bowl and main jet, then clean them out with carburetor cleaner or solvent and compressed air. Be sure all holes and passages are clear before reassembling the carburetor. Cleaning the carburetor is one of the most effective ways to remedy a backfiring engine and get your mower running smoothly again.

Inspect the Spark Plug

A faulty spark plug won’t ignite the air-fuel mixture properly, causing the engine to backfire. Remove your mower’s spark plug and check that the electrode gap is set correctly, usually around 0.030 inches. The spark plug should also be clean and undamaged. So, replace the spark plug if needed.

Check for Vacuum Leaks

An air leak in the intake manifold or vacuum hoses upsets the air-fuel ratio, often leading to backfiring. Inspect all hoses and the intake manifold for cracks or holes and replace damaged components. And ensure you seal all connections tightly and properly.

Review the Engine Speed

Also, the rapid deceleration causes backfiring. If your mower is prone to backfiring when shutting down, gradually reduce the engine speed before turning it off.

Consider Engine Tune-Up

If the above steps don’t solve the problem, servicing the mower by a small engine mechanic is a good idea. They can perform an engine tune-up to replace additional parts like the ignition coil, magneto, or engine valves. Plus, they will ensure everything is properly adjusted to prevent backfiring and keep your mower running well.

Final Thoughts

Your lawn mower engine requires maximum care- the same goes for other parts. This is especially crucial when wondering why does my lawn mower backfires when I try to start it. Now, you can troubleshoot the issue, determine the culprit, and get your mower running smoothly again to get back to making your garden look pristine or attractive to hedgehogs.

Remember, it could be something simple like old gas, a clogged air filter or spark plug, or an issue with the carburetor. With some basic mechanical skills and the correct replacement parts, you’ll be back to mowing in no time. And when your mower starts on the first pull, you’ll feel a surge of satisfaction knowing you solved the problem yourself. Happy mowing!