Why Your Lawn Mower Is Backfiring And How To Fix It

There are few things more frustrating than trying to mow the grass with a backfiring lawn mower. It always seems to happen just as you're settling into a rhythm. The noise is annoying — or even startling when it comes out of nowhere — and a sign that something isn't working the way it should be with your mower's engine. Backfiring happens for various reasons, including old fuel, incorrect carburetor settings, issues with the spark plug, or problems with airflow inside the engine. As with most machinery, prevention is the best cure. Many of these issues can be avoided by doing lawnmower maintenance after every mow, like checking the air filter for clogs and replacing old spark plugs and gas. However, too many homeowners simply put the mower back in the shed once the chore is done.

Checking the air filter is one of the lawn mower maintenance tasks that everyone forgets — and to their detriment. A dirty or clogged filter is a common culprit causing a lawn mower to backfire. Over time, dirt and debris can build up in the filter, reducing the amount of clean air getting to the engine. When the air supply is restricted or dust-filled, the fuel mixture becomes unbalanced, causing the engine to misfire or backfire when you push the mower around a lawn. Fortunately, this is a simple issue to diagnose and fix. Remove the filter and clean it or, if it's not reusable or beyond saving, replace it with a new one. Your air filter should be cleaned after every 25 hours of use and replaced after 150 hours to keep the air flowing.

While a faulty spark plug can cause backfiring, it's not as common a problem as a clogged air filter, but it can still happen. If the spark plug is damaged, worn out, or the gap is incorrect, these can all cause issues. A mower with a poorly performing spark plug uses more gas than usual, leaving deposits of carbon on the plug and the cylinder head. They ignite with the heat of the engine, causing backfiring. This is another easy, cheap DIY lawn mower repair. Before replacing the plug, use a wire brush or spark plug spray cleaner to clean the existing plug. Put it back in and see if that fixes the problem. If it doesn't, you'll need to buy a new spark plug.

Old gasoline — defined as gas that has been sitting in the fuel tank of your lawn mower for several months — is another issue that can result in backfiring. Gasoline starts to degrade after about three to six months without use. If you didn't empty your gas tank when you put the mower away last winter, you're going to have a tank full of old gasoline come spring. The fix is simple: Replace the old fuel with new, then start the engine and leave it running to clear out anything left in the system. Always winterize your lawn mower and protect it until spring by emptying the tank of gas or adding fuel stabilizer before storing the machine in your shed or garage.