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How to Fix a Backfiring Gas Lawnmower

Updated on August 12, 2013

The Problem

This is based on a true story.

One day, I was happily mowing my lawn, and with about one third of the lawn left to mow, my mower backfires with flames spewing out of the exhaust, then abruptly stops. I checked the oil and gas levels and they were all good. Yet, after pulling the start cord multiple times, nothing worked. I could not get the engine back to life again. How can I complete mowing my front lawn now?


After some deep thinking, I realized that the problem could be the spark plug. Fortunately, I have some basic tools my garage (see Figure 1) to get me through this. I found my special set of spark plug sockets which I was able to use with the ratchet to unscrew the spark plug from the engine head.

Figure 1.  My 3 Spark Plug Socket Set
Figure 1. My 3 Spark Plug Socket Set | Source

After removing the spark plug, I examined it. It looks as though the conductors are burned out or completely covered with carbon (see Figure 2).

Figure 2.  Spark Plug:  Burned Out or Covered in Carbon
Figure 2. Spark Plug: Burned Out or Covered in Carbon | Source

Fixing it

This problem has two possible paths to resolution--plan A (use a wire brush to clean the spark plug head) and plan B (replace the old spark plug with a new one).

I took the first path, plan A. I wire brushed the spark plug until the conductors, especially at the spark plug gap, was clear.

I then replaced the spark plug and attempted to start the lawnmower. After multiple attempts, I gave up. The attempts to start the engine didn't even result in the engine igniting the fuel in its cylinder. We'll that didn't work.

The second path--plan B--took some additional effort. I had to figure out the spark plug type and purchase one to replace the existing burned out plug. Using the spark plug socket, I ratcheted the new plug back into the engine, then reconnected the spark plug cable. See Figure 3.

Figure 3.  New Spark Plug Installed
Figure 3. New Spark Plug Installed | Source

With the new spark plug in place, I pulled the starter cord; and to my surprise the engine started right away.

The mower was fixed, and I was able to happily resume my mowing chore to completion.


If you find your lawnmower backfiring and spewing out flames, it is time to stop and fix it. Note that the spark plug could just be clogged up and thus preventing a nice clean spark between the metal conductors on the igniting tip of a spark plug. If that is the case, then you are done.

However, if the spark plug needs replacement, make sure to get the same or equivalent plug. Install the new plug into the lawnmower engine head, then give it a test start. At this point, the lawnmower should be functional again.

Hey! If all else fail, you can always purchase a new lawnmower. Maybe this time get one of those green lawnmowers that doesn't require gasoline--just human push/pull power.


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