Lawnmower Running Rough? (Possible Causes & Fixes)

Lawnmower Running Rough

In a perfect world, your lawnmower would run perfectly every time you started it. Unfortunately, because these machines are complex and rely on many different components, they will wear down eventually. Usually, the most common cause of mower problems is a lack of care and maintenance. When this happens, one side effect can be that your lawnmower starts riding rough.

Typically, the reason for rough mowing is either a dirty carburetor or a faulty spark plug. Since both of these elements help control the amount of air and gasoline inside the engine, a problem with either one can create a rough experience.

Once you start to notice a change in your lawnmower, we recommend inspecting and cleaning (or replacing) these parts as soon as possible.

Because there are multiple causes for why your mower might be running rough, we’ll dive deep into each one. We’ll also show you how to fix these problems, step by step.

Top Reasons for a Lawnmower Running Rough

Problem 1: Clogged or Dirty Carburetor

Although modern cars don’t use carburetors anymore, most small-engine machines like lawnmowers do. The carburetor’s job is to mix gasoline and air for combustion. This mixture has to be precise for the engine to run smoothly. Unfortunately, carbs are not precise components (which is why they are obsolete for larger motors). As they get dirty and worn, they will stop working as effectively, causing a rough ride.

The Solution: Clean or Replace Your Carburetor

If your mower isn’t too old, you should be able to clean the carb instead of replacing it. You can find out more about how to do that here. However, if you discover that the carburetor is beyond saving, it’s almost as easy to replace it as it is to clean it. Follow these steps:

  • Step One: Disconnect the Spark Plug Wire
  • Step Two: Remove the Air Filter – Usually, there is a plastic covering over it that just pops off.
  • Step Three: Remove the Air Filter Base – The carburetor is behind this. There is a tube that connects to the carb, so you will have to disconnect that as well.
  • Step Four: Drain the Fuel Tank – Since the carb stores gas, you want to drain the tank before pulling this piece off. We recommend putting a rag down to catch spills. Also, use a gasoline-safe container so that you can reuse the fuel afterward.
  • Step Five: Disconnect the Pull Cord – Usually, you just have to loosen the top bracket, and it will retract to the housing unit.
  • Step Six: Remove the Blower Housing Cover – This is fastened by screws, so you will need a screwdriver.
  • Step Seven: Remove the Old Carburetor – Two brackets are holding it into place, so you will have to push these to the side.
  • Step Eight: Disconnect the Rods – There are two rods connected to the carb – one for the governor and one for the choke. They should just pull out.
  • Step Nine: Remove the Fuel Line – You will have to loosen a clamp to get this piece off the carburetor.
  • Step Ten: Connect the Fuel Line to the New Carb – You will need to place an O-ring inside first to form a tight seal.
  • Step Eleven: Reconnect the Rods – Make sure that they are oriented correctly. Then push the carburetor into place.
  • Step Twelve: Replace the Blower Housing – Be sure that the starter pull cord is accessible so that it doesn’t get stuck inside the engine.
  • Step Thirteen: Reconnect the Pull Cord to the Mower Handle
  • Step Fourteen: Replace the Air Filter Housing – Reconnect the hose to the filter base and place the filter inside.
  • Step Fifteen: Secure the Fuel Line – Reattach the line to the gas tank and secure it with the spring clamp.
  • Step Sixteen: Refill the Gas Tank and Test the Mower – Don’t forget to reconnect the spark plug wire first.

Problem 2: Dirty Air Filter

Sometimes, the simplest problem can cause the biggest issues. While the carburetor is the likely culprit, the reason for a rough ride could be a dirty or clogged air filter. If you notice that the filter is filthy when looking at it, now is the perfect time to replace it.

The Solution: Clean or Replace Your Air Filter

Usually, it’s better to put a new air filter onto your lawnmower than try to clean the old one. This piece is relatively cheap, and it will do wonders for your mower’s performance. The only reason to clean it is if there are big pieces preventing air from getting into the engine. Otherwise, don’t bother.

Problem 3: Faulty Spark Plug

As with the carburetor, your spark plug is a crucial component. Any issues with this single piece can cause various problems, from trouble starting the machine to a rough ride. Most experts recommend changing the spark plug at the beginning of each spring season, so if you haven’t done that in a few years, the plug is likely worn and dirty.

Solution 4: Replace Your Spark Plug

In some cases, you might be able to clean this piece instead of replacing it. However, since spark plugs are inexpensive, it’s almost always better to get a new one instead. Follow these steps to replace the plug.

  • Step One: Disconnect the Ignition Wire
  • Step Two: Remove the Spark Plug – You will need a specialized socket to do this. While a regular socket wrench can work, it might crack the plug and cause further problems. Although any damage won’t matter for the old unit, it can ruin your new plug.
  • Step Three: Inspect the Housing – Look for any signs of dirt or corrosion. Clean everything thoroughly as necessary.
  • Step Four: Use a Feeler Gauge on the New Plug – There is a gap between the two electrodes. This gap is critical for a smooth performance. You can buy a feeler gauge with your new unit to ensure that the space is precise. Fortunately, if you buy the right plug for your lawnmower make and model, you shouldn’t have to adjust the gap much, if at all.
  • Step Five: Insert the New Plug – Again, use a spark plug socket to tighten it. One-half turn to the right should be sufficient.
  • Step Six: Reconnect the Ignition Wire

Problem 5: Bad Gasoline

If you know that the spark plug and carburetor are not creating any problems, the cause could be your gas. Typically, ethanol fuel (like the kind at most gas stations) is not ideal for small engines because it can leave a residue. Over time, this residue will build up and cause clogging within the carburetor. Also, gasoline can go bad after a few months, so you will often notice rough mowing during the first session of the new season.

The Solution: Improve Your Gas

There are several ways to ensure that your fuel is optimized for a lawnmower, including:

  • Use a Stabilizer – Put these additives into your gas every time you fill the tank.
  • Use Non-Ethanol Fuel – You will have to do some research to find a station that offers 100% gasoline.
  • Avoid Leaving Gas Inside for Too Long – Empty the tank if you know that you’re not going to mow for a few weeks or months.

Problem 6: A Dented Flywheel Key

The flywheel and crankshaft are integral elements of your lawnmower’s engine. If the crankshaft gets bent or damaged, you will likely have to replace the whole machine. To help mitigate this problem, there is a flywheel key that acts as a buffer. If this component gets dented (i.e., when you run over a rock or obstacle), it can affect the mower’s performance.

The Solution: Replace Your Flywheel Key

This job is relatively complex, so we usually recommend taking your mower to a repair shop to get it done correctly. However, here are the broad steps to give you an idea of what it will take to replace the key. Only attempt this fix if you know that you’ve run over something heavy. If you haven’t, the problem is likely a different piece.

  • Step One: Remove the Starter Rope
  • Step Two: Remove the Engine Cover
  • Step Three: Remove the Fuel Tank
  • Step Four: Remove the Oil Dipstick
  • Step Five: Remove the Blower Housing
  • Step Six: Tilt the Mower and Immobilize the Flywheel
  • Step Seven: Remove the Flywheel
  • Step Eight: Remove the Damaged Flywheel Key

Once you have finished all of these steps, you can replace each component in the reverse order. You also need some specialized tools, such as a two-jaw puller, to remove the flywheel itself. Overall, not a job for casual lawnmower owners.

Related Questions

What if my mower shuts off and I can’t get it started again?

If that happens, the likely culprit is an overheated engine. You can find out more about that here.

What if my mower won’t turn on?

The most common cause is a dead battery, but the problem could be the solenoid or the spark plug as well.

Upgraded Home Team

We are a team of homeowners and home improvement enthusiasts who enjoy sharing decorating, gardening, home improvement, and housekeeping tips with other homeowners! Whether you're looking for advice on furnishing your living room or the next outdoor DIY project, we've got you covered.

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