What To Know About A Lawn Mower Running Slow And Rough

Maintaining your lawn can sometimes be a war of attrition, especially during peak growth seasons like summer or spring. Your trusty lawnmower is a noble battle companion, but all those duels with feisty runner grasses may eventually…creep up on you and your mower.

A lawn mower running slow is usually due to obstructions or cleaning issues with decks and filters. Mowers running rough normally points to cold engines, low fuel to air mixtures, or bad engine parts. Both symptoms, rough and slow, can also lead to the other.

Today we’re looking at the problem of a mower running slower, and rougher, than normal. What are the causes and, more importantly, how can they be fixed? Let’s take a look.


Why Is My Lawn Mower Running Slow?

Over time, or even right out the box (sometimes), your lawnmower may begin running slower than usual, which may result in longer mowing times. This is a common problem and can be caused by various issues. Luckily most of these root reasons can be repaired or even avoided with routine maintenance.

Lawn mowers can run slowly due to faulty or damaged parts like spark plugs or carburetors. There are also causes that mess up the firing of the engine or air to fuel mixture like starting without the choke or dirty air filters. Then there are simply blade obstructions like tall wet grass.

First we will look at two of the main causes for slow running mowers. Then we will touch on some of the other causes that could be making your mower run slowly in your situation.

Faulty Or Old Sparkplugs Can Make Mowers Run Slowly

The first area you want to explore, if you are experiencing this problem, is the sparkplug. The sparkplug’s role is to provide…er, spark to the fuel/air mix in the combustion chamber. Essentially, the sparkplug initiates the combustion process during every cycle.

With time, the spark plug may become worn out or covered in dirt, which can lead to delayed sparks. For the engine and, ultimately, the mower to operate at full power, the plug must fire during peak compression. If this doesn’t happen, the engine will lag noticeably.

Dirty Carburetors Can Slow Mowers

The carburetor’s job is to feed fuel into the engine cylinder during each cycle. Like the spark plug, the carburetor can also get bogged down with dirt and debris, as well as the sedimentation found in fuel. If this happens, the combustion chamber will not receive adequate volumes of fuel, which leads to underpowered performance.

Other Reasons For A Slow Mower

Sparkplugs and carburetors are the most common problem points here, but you also have to watch out for…

Starting A Cold Mower With The Choke Off

It is important to have the proper fuel-air mixture in your mower’s engine for it to run smoothly and at a normal pace. Newer mowers can sometimes run even with a cold, chokeless start, but older mowers and even some newer ones will run rough and slow until the engine heats up.

The engine needs to warm up for most of the parts to work in unison and as intended. To help the engine run smoother while it heats up the choke is employed to give that fuel rich air mixture in the engine to spur it on to the right temperature.

Trying To Cut Excessively Tall Or Dense Grass

The unofficial term that lawn crews and landscapers use for what happens when trying to mow overly tall or thick grass is ‘bogged down’. The resistance against the spin of the mowers blades sometimes can’t be overcome by the horsepower of the engine.

This is doubly hard in tall as well as wet grass. Many of the grass clippings will adhere to the sides of the deck and really slow down the blades and thus the engine.

This leads us to our next point.

A Dirty Mowing Deck

This can be wet grass that collects along the inside of the deck, mulched leaves, dirt, or I have even seen small tree branches or brush get lodged and slow the mower blades. When the mower blades are slowed in this way, you will hear the result in a slower firing engine as it attempts to push the blades through the obstructions.

For mowing decks that get clogged by wet leaves or grass clippings, leaving the problem can lead to buildup and slower turning blades. I recommend a good Wire Brush Set like this one from Amazon to help clean out the debris or the build up if left over time.

Clogged Air Filter

Here again we are talking about the engine firing rhythm itself. Clogged air filters effect the fuel-air mixture in the engine. Without proper ratios the firing may be slower and irregular.

Air filters should be regularly cleaned and then replaced once worn or if permanently damaged. But more on this in a bit.

If you would like to see more articles like this one, read my others here…

Why Is My Lawn Mower Running Rough?

Sometimes, the mower may not run as smoothly as it once did, as it produces a labored gurgling or sputtering sound.

Like with slow running engines, lawn mowers running rough is usually due to one of several factors. There could be component issues like with sparkplugs or carburetors. The oil may need to be changed or the air filter cleaned. It could also be a problem with the blend or type of fuel used.

Luckily, this problem is a close cousin of the one we’ve just discussed above.

Bad Spark Plugs Can Make Mowers Run Rough

If the mower is running rough, you should check the sparkplug first. The plug must be clean and at a specific distance from the combustion chamber. If these conditions are not met, the combustion process won’t occur as efficiently as it normally would. The mower will sputter inconsistently and even produce a little to a lot of smoke from the exhaust.

Running Rough Can Be A Sign Of Clogs

There are three things to check for clogs that have in the past given me or the crews I worked with trouble.

  1. The carburetor is something to check for obstructions big and small.
  2. Also, check the fuel filter. The filter is especially prone to clogging over time if you regularly use mid to low-octane gasoline.
  3. Inspect fuel lines to ensure efficient delivery of fuel to the engine.

Dirty Air Filters Can Be The Cause Of Rough Running Mowers

You should also inspect the air filter to ensure it is clean. A build-up of dust and debris can block the engine’s intake of air, which in turn limits the amount of oxygen available in the combustion chamber. Oxygen, as you know, is an essential part of the combustion equation, so inconsistent air intake equals inconsistent combustion.

Fuel Type Can Determine Rough Or Smooth Mower Operation

Speaking of fuel, the type of fuel you use can also impact the smooth running of your mower. 

Ethanol blends and low-octane gasoline have bits of sedimentation that can build up over time and clog up the fuel filter and carburetors. Ethanol blends, in particular, also tend to be corrosive, which can lead to internal parts being chipped away to add to the debris.

Oil Viscosity Affects Engine Performance In Mowers

Do not neglect to check your oil too. Oil lubricates vital engine components, which helps with engine consistency, smoothness, as well as longevity. Clean thinner oil means a smoother running engine.

Oil changes are usually scheduled for every 50 hours or so, but it doesn’t hurt to keep an eye on your levels. Also keep a check on the quality of the oil in the engine. If used too long it can pick up particles and break down, losing some of its lubricating power.

Note: If you find yourself having to add more oil frequently, you might have a leak on your hands.

How To Fix A Lawn Mower Running Slow

The best way to restore your mower’s engine to its regular speed is to identify the root problem. This can be done by inspecting all the problem areas mentioned above. 

There are several fixes for slow running lawn mowers. You can replace old or worn parts like sparkplugs and carburetors. There is also cleaning like air filters, fuel filters, fuel lines, and mower decks. Other fixes include using a choke when starting and not cutting high or wet grass.

Usually the fixes for these involve cleaning or replacing. If there are other more serious issues that require actual repair, qualified professionals will usually find the offending part and simply replace it.

Replace The Mower’s Sparkplug For Faster Running

The sparkplug must be inspected for dirt and debris. You must also ensure that the gap between the plug and the chamber is adequate, as per your instruction manual.

You might want to just get a new plug altogether, especially if you’ve been using the same one for a while. Most manufacturers recommend a sparkplug change after about 100 hours of use.

Cleaning Filters Can Speed Up Mower Engines

Next, inspect the air filter. Ensure that it is clean and free from debris. If it’s not, clean it or replace it with a fresh one.

You should also take a look at your fuel filters and carburetor. Clean out any built-up blockages to ensure efficient fuel delivery.

Use The Choke To Help The Engine Of Your Mower Warm Up

In addition, if your mower has a choke, you may have to engage it to warm up the engine if it is cold. The choke limits the volume of fuel delivered to the combustion chamber to make the process easier on the cool engine.

Clean Out The Mower Decks For Faster Blade Spin

You also want to inspect your mower deck and clear the dirt and any accumulated grass clippings that may affect the movement of the blades. Cutting wet grass, in particular, can lead to clumps of grass collecting in the deck.

Regular Grass Cutting Helps Mowers Cut Faster

Lastly, make sure to avoid cutting grass that is too tall, dense, or wet. Adjust the mower deck height, if your mower allows it, to reduce strain on the blade spin and thus the mower.

How To Fix A Lawn Mower Running Rough

So, how do you stop your mower from sputtering?

The solutions to a rough mower engine are largely the same as those for a slow-running engine. Check and correct problems with the sparkplug, carburetor, fuel filter, and air filter. You must keep the mower deck clean and also avoid cutting grass that is too thick.

If your oil levels are low, or if you haven’t changed the oil in a while (or ever), make sure to add more. This should increase the engine’s performance levels and smoothen things out.

We also recommend that you switch to high-octane gasoline. Lower-grade and ethanol-blended gas may be cheaper, but these fuels increase the likelihood of you encountering these problems.

The Final Touches On Lawn Mowers Running Slow And Rough…

To fix the problem of rough or slow running mower engines, you first have to diagnose the problem. Check out all of the points above and you will more than likely find the cause of your specific issue.

Once you have found the offending part it is usually just an issue of cleaning or replacing. Regular maintenance can even stave off these problems before they become an issue. There are always going to be parts that fail, but taking care of your equipment can make those events fewer and farther between.

Take a look at some of my other articles…




Mathew Booe

Mathew has worked in landscaping professionally for over 10 years. He is a grandpa and frequently interviews other experienced landscapers and lawn care experts who are also grandpas for these articles.

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