My Lawn Mower Made A Loud Pop And Won’t Start – Now What?

Lawn mower engines, while not entirely complex, have a certain number of issues that can arise. One is the engine backfires, or you hear a loud pop, and the machine no longer starts. Several different things can cause a lawn mower engine to pop, dealing with the spark plug, engine timing, and more. 

You were excited to mow your lawn. Well, maybe not, but you had plans to do it anyway. However, the mower made a pop, you jumped, and now it won’t even start. We know what you’re thinking, but no, this is not an excuse to forfeit mowing for a cool glass of lemonade. What it is time to do, is release that inner mechanic (we know you have it in you). Roll up your sleeves, and let’s take a look at why that engine won’t start. 

What causes a lawnmower engine to pop?

Your lawn mower may have worked perfectly fine until the engine pops, but why? Ok, maybe you don’t have an inner mechanic in you, or if you do, they aren’t all that knowledgeable on lawnmower engines. Thank goodness for google, hey? Let’s dive in.

The popping noise you hear is commonly known as the lawnmower’s engine backfiring. This happens when gas ignites outside the combustion chamber of the engine. It is a rather loud noise and can come across as very startling. To further understand how to fix it, let’s take a look at the fine print. 

Inside of the mower is an engine (obviously). It is a combustion engine, producing power through an air-fuel mixture. The air-fuel mixture combusts to start the engine, creating a spark. This ignites the moisture, causing an explosion and your engine to fire up. All of this takes place in the combustion chamber, where it is specifically designed to take place. We begin to have issues when the combustion occurs outside the combustion chamber. 

Perhaps it’s all a bit confusing, and that’s ok. Now that we have a better understanding of the inner workings of a lawnmower, the solutions are a bit easier to find.

Why Does My Lawn Mower Backfire And Not Start?

Lawn Mower Flywheel

When it comes to your lawn mower backfiring and then not starting, it most likely has to do with the engine’s timing. When it comes to poor engine timing, most of the time, the flywheel is at fault.

Flywheels get damaged as time passes from regular use. However, if you tangled with a sizeable rock the last time you were mowing, there is a very high possibility something is wrong with the flywheel. 

To check this, you must disassemble a portion of the mower. Keep in mind that all lawn mowers are slightly different when it comes to taking parts off, so be sure to check the manual before disassembling the mower; anything from Craftsman to John Deere mowers has slightly different builds. 

Fixing A Flywheel 

  • To begin, disconnect the spark plug connector.
  • Remove any screws holding the housing in place.
  • Remove the flywheel nut. A socket wrench will effectively remove the flywheel nut in most cases.
  • Remove anything preventing a clear view of the flywheel key. 
  • The key should be in line with the keyway hole. If it is not, the flywheel is damaged, and the spark plug is not in line with engine valves, which causes the engine to backfire
  • Replace the flywheel key.

Because the flywheel is connected with the timing of the lawn mower, once replaced, the timing of the mower will also improve, causing the engine to start correctly. 

Why Does My Lawn Mower Backfire When Starting?

Now, if your mower will start but still backfires, check the flywheel first. If you ensure the flywheel is working properly, it could be the carburetor. 

The carburetor provides the air-fuel charge the engine needs to run correctly. As a rule of thumb, routine maintenance on a carburetor is essential to prevent backfires and keep the engine running smoothly. 

Backfiring usually occurs when the carburetor is not properly adjusted. To fix this, use a carburetor cleaner. Another option is to disassemble the carburetor, clean it, and then reassemble it.

Carburetor problems occur on many mowers and are common on pull start engines and weedeater engines. 

Other Common Issues That Cause An Engine To Backfire

Repairing a Lawn Mower

Many things can cause a lawn mower engine to work improperly. Here are a few other things to check if your engine continues to backfire. 

  • High Engine Temperature- Engine temperatures are essential to keep an eye on. Most lawn mower engines are air-cooled. If you are dealing with a continuously heated engine, this is a sign of a clogged air-port. To fix this issue, clean and check the air filter and air intake. 
  • Spark Plug- Over time, spark plugs become worn and do not produce an adequate spark. This is a simple fix; replace the spark plug.