Problem with Lawn Mower Engine Running Rough

Cutting grass with Toro lawn mower.

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

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


  1. 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. 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. 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. 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. 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?

  6. I have a mower that always runs rough when I fill the tank. I changed the plug, cleaned the filter and put in some dry gas. When the gas goes to near empty, the thing runs fine. But when I stop to fill it up, it runs real rough. Ever hear that one before?

  7. I just had the same problem engine running rough. took off the spring for the throttle control put a piece of wire in place of spring made the wire tight. Engine ran perfectly temp fix only

  8. I have same problem. Changed for new plug and cleaned the carb, but when it runs it’s like the whole thing shaking.
    Please advise. David

  9. My neighbor has a Murray lawn mower, 4.5 horse, starts fine and runs fine without the breather on, once it is on, it runs rough and eventually dies. I put new air filter on, new spark plug and new gas. Still happens.

  10. My lawn mower does not run at all. I put more gas in it to make sure that it wasn’t a fuel level problem, but it won’t turn on. Can I use these same steps to try to fix the problem?

  11. Check the gap of your coil between the flywheel and the coil. Three pieces of copy paper is about right. Mine was backfiring and running rough, the coil now called (electronic ignition) was touching the flywheel and causing a misfire. Just loosen the two screws and put 3 pieces of paper in between and re tighten. The gap is like .125 or .0125 what ever 3 pieces of paper are

  12. Ethanol gasoline is the ruination of these small engines, buy gasoline without ethanol and your problems will be over. Same for weedeaters!

  13. The Briggs and Strutton 17.5hp. Runs uneven on full revs. I cleaned the carburetor and adjust the spark plug gap and spark gap on the fly wheel. It is still running uneven – what else can be the problem?

  14. my sons toro self propelled lawn m stopped running. put in new air filter was filthy. new spark plug pulled off gas line fuel line was running very slow. put on new fuel line from tank to carb. when I drained fuel tank and put in new found a small plastic tube coming out of the outlet fuel tank going to carb. it fit inside the gas line removed it and mower started but runs alittle rough. cant find a replacement tube or is it even necessary. runs but drives me crazy what is going on?

  15. Craftsman/ Husqvarna 19.5 BS Motor – Ran fine. Only used 4 or 5 times, mainly pushing snow. Battery went dead last year, and sat nearly year without use or longer. Bought new battery. Started today and it popped and cracked unless I left throttle in choke position. Mowed for a few passes and pulled back into garage before it died. Pulled plug, not burnt, not fouled, but dry. Neighbor came over thought was dirty carb. I drained fuel, checked fuel filter and it was clear and flowing freely. Checked valve/ jet on bottom of bowl on carb. (electronic btw– stupid in my opinion–old fart standing here saying, ” Its a lawnmower not a friggin Porsche!”) The jet/valve was sticky. Actuated and freed it up. Started up with everything unhooked just to get rid of residue and crap. Ran smooth as silk for a couple seconds and died. Put everything back together, all I had was old gas, and it started its popping and cracking again. I shutdown. Going to drain gas, and go get fresh gas later. “It is a lawnmower! Not a race car! WTH” It sounds like a burnt valve until I choke it and it runs smooth. To be continued……

  16. My craftsman lawn mower with a B&S engine, after a pull start, runs rough for five to ten seconds as if having no power (bogging down), but smooths out. Not sure if this is a clogged fuel line, carburetor or something else going on. Usually I’ve read that it would be the other way around where it would start and then run rough or stall out.

  17. lawnmower starts up easy and runs good for about 1 minute then starts to run rough cleaned air filter replaced sparkplug cleaned gastank same thing need help

  18. Thank you SO much for the tip about the gap setting for the spark plug. Had NO idea that could cause the engine to not run properly.
    Yesterday, I was talking to my brother, and he mentioned that with it getting warmer out he wants to make sure all of his tools still work. It sounds like his mower won’t start. My brother is no mechanic, so I don’t think he knows what is wrong. But I’ve emailed him the article. Maybe there’s hope for him after all!

  19. My grandfather is experiencing similar problems with his lawnmower as it has been running poorly. It’s interesting that you mention that using gas with 10% ethanol could be the problem as it messes up the carburetor. He uses gas with ethanol for his lawnmower which may be causing problems. I will suggest that he look for ethanol-free gas for his lawnmower and take the proper steps in ensuring his mower is kept for winter.

    • Glad to hear you enjoyed this content, Hazel! Please share it with friends — that’s how we’re able to create similar content.


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