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Problem with Lawn Mower Engine Running Rough


My lawn mower was running rough, so I changed the spark plug. It started easily and ran fine; but the next time I used the mower, it was hard to start and ran poorly again. I replaced the spark plug several more times, but it keeps happening. What could be the problem? -Judy

Hi Judy,

The odds that you would purchase several bad spark plugs are pretty low, but you might want to buy a different brand of spark plug and try it just to be sure. Here are some other common problems to check:

  • Spark Plug Gap: Check the spark plug with a spark plug gap gauge to make sure the gap is set to the distance specified in the manual for your lawn mower. A spark plug with the wrong gap setting can quickly become fouled with carbon from not firing properly.
  • Air Filter: Check to be sure the air filter on your lawn mower is clean, and replace or clean it (depending on the type filter) if needed. A dirty air filter is one of the most common causes of a poorly running lawn mower, since it starves the engine of the oxygen needed for combustion.
  • Fuel Filter: Check the fuel filter on the mower to make sure it’s clean, and replace it if needed. A clogged fuel filter restricts the flow of gasoline to the engine, starving it of fuel.
  • Gasoline: Using gas with 10% ethanol is believed to cause problems in small engines like lawn mowers by gumming up the carburetor. This is particularly true in two-cycle engines such as string trimmers, blowers, and chainsaws. Also, the shelf life of ethanol blended gas is considered to be only a few months. Your best bet is to use ethanol-free gasoline in lawn mowers and other small engines, add fuel stabilizer to gas to extend the shelf life, and drain your gas tank and run the mower until it stops before putting it up for the winter. To find a list of stations that carry ethanol-free gas in your area, go to pure-gas.org.

If you’re still having problems with your lawn mower not running smoothly after trying these fixes, take it to a small engine repair shop to see if there’s a more serious engine problem.

Good luck with your project,


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5 Comments on “Problem with Lawn Mower Engine Running Rough”

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  1. John Says:
    May 26th, 2012 at 12:37 pm

    I have this same issue. But all I do is clean the spark plug and put it back in. Is there a maintance issue where the spark plug is covered in soot/ carbon do much to where it stops performance?

  2. David Says:
    June 17th, 2012 at 11:30 am

    I have a Husquvarna lawn tractor with Briggs and Stratton 24hp. motor. When I tried to start it 3 weeks ago, the engine made a loud backfire and began to run very poorly. I replaced the air filter, spark plugs and fuel filter with no change. I inspected the new spark plugs and found them to be coated with black soot. Any suggestions on how t correct these problems?

  3. Michael Lynch Says:
    June 15th, 2014 at 2:03 pm

    This seems to be a very common problem. I have a Toro mower which just completely won’t run. Tried fixing everything. Shop takes it, says its the gasoline, fixes it, but it only runs once, then stalls. I bought a new Troy-bilt with Honda motor, run fine once, then very rough. Drained gasoline, put new in, with additive to stabilize it, better but still rough. Brand new oil, carburetor, filters, plugs.

  4. Eric Blaise Says:
    January 26th, 2015 at 2:06 pm

    My lawnmower was running rough, and like most people I thought I might’ve been the air filter. Turns out, the spark plug was going foul and the engine was running rich as a result. Changing the spark plug and properly gaping it did the trick, just remember that it will take a while for the mower to run smoothly as the mower is adjusting to the new spark plug if this is the case for you.

  5. Viet Thang Says:
    March 24th, 2015 at 2:24 am

    I have the similar problem as you got. But i think E10, which contain 10% of Ethanol is fine. Could you please give me the most suitable fuel type?

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