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,

Further Information

• Lawn Mower Maintenance (article/video)
• How to Keep Your Lawn Mower Running Efficiently (video)
• How to Change a Lawn Mower Spark Plug (article)
• How to Change a Lawn Mower Air Filter (article)
• Gas Ingredient Causing Lawn Mower Problems (jennstrathman.com)

Tags: , gas, lawn mower, lawn tractor, repair