Why Does My Lawn Mower Sputter When Blades Are Engaged?

Sputtering noises from your lawnmower can be very bothersome, but it can be fixed. The sputtering sound indicates a problem which can be from the engine, faulty switches, or bad fuel, among others. Proper diagnosis and checks can help you figure out what the exact problem is, and you can have your lawnmower working fine again after fixing the problem. 

The sputtering sound from the lawnmower on engaging the blades is caused by one or more of these problems- dirty pulleys, faulty safety switches, bad drive belts, clogged air, fuel filters, carburetor problems, or bad fuel. In this article, you will have a simple guide on detecting and dealing with the situation. 

Why Does My Lawn Mower Sputter When Blades Are Engaged?

With many possible reasons for the sputtering sounds, you must check the necessary components patiently. Below are the steps to take to diagnose and fix the problem:

  1. Check the fuel 
  2. Check the filters (air, oil, and fuel filters)
  3. Fix the fuel cap
  4. Check and clean the carburetor
  5. Check for water in the fuel line
  6. Replace faulty spark plugs

Checking the fuel

The quality of fuel used can affect the proper functioning of your lawnmower. Some fuels are bad fuel, dirty, or mixed with impurities. Low-quality fuel can damage the plastic parts of the lawnmower, producing a soft explosive sound when it is in use. Avoid using low-quality fuel for your mower, even if they are cheaper. Go for ethanol-free gas, and you can add fuel stabilizers to improve its quality and help the fuel last longer. 

Check the filters

A lawnmower has three filters, namely the fuel filter, air filter, and oil filter. The fuel filter prevents dirt particles from mixing with the fuel; it sieves out these foreign particles allowing clean fuel to pass through. The air filter does a similar job; the only difference is that it filters dirt from the air instead. It lets clean, debris-free air into the carburetor. The oil filter also removes debris from the engine oil and keeps it clean. 

Accumulating dirt and particles will make the filter clog over time, and a clogged filter can cause the engine to make noises, stall or even suddenly stop while you engage the lawnmower. 

How to fix a clogged fuel filter

The fuel filter cleans and supplies debris-free fuel to the engine. A faulty fuel filter leads to a shortage of fuel supply. The engine tries to draw in more fuel to meet demand, but oxygen is drawn up instead. This leads to sputtering and hiccups within the machine. 

To clean a fuel filter, empty the fuel tank and remove the fuel line from the carburetor. Use a cleaning fluid, such as carburetor fluid, to clean the filter and fuel line. Allow it to dry and put it back in place. If nothing changes, you may have to replace the fuel filter. Note that filters made of paper and nylon are better replaced, and metal filters can be washed and reused. 

How to fix a clogged air filter

Air filters come in paper or foam material. Paper air filters need to be replaced when clogged, but foam filters can be cleaned and reused. Use dish soap or detergent to wash the foam filter, rinse, and dry for about two hours. After that, apply a thin coat of oil to it. The oil aids the filter in trapping particles. 

Check the fuel cap 

An improperly sealed gas cap can cause vapor to get locked in. Remove the gas cap and try blowing some air through it. If it seems blocked, clean it, but it is best to get a replacement if you are unable to. 

Clean the carburetor

If the sputtering noise persists after all the previous checks, then the culprit is most likely a dirty carburetor. It could also be that the carburetor is not rightly placed. 

To clean the carburetor, you need to use cleaning spray and a piece of cloth to clean the bowl, surfaces, the jet, and the central gasket.  

Inspect the fuel line for water

Water in the fuel can be caused by condensation in the fuel tank. The water interferes with ignition and causes the engine to cough, making sputtering noises. Water is heavier than fuel; it will sit at the bottom of the fuel tank and go into the fuel lines. Drain out the water and fill the engine with new gas to fix this. 

Damaged spark plugs

Routine inspection of spark plugs should be done from time to time. A bad spark plug is a culprit for many engine problems. 

Remove the spark plug, making sure the engine is off and cooled. Use a spark plug tester to diagnose, and clean any dirt around it with a brush. If it is damaged, replace it with a new one. 

What Causes My Lawn Mower To Sputter 

  • Low gas level
  • Faulty belts
  • Broken safety switch
  • Worn out pulleys 
  • Overuse or careless use
  • Clogged cutting deck

Low gas levels

The fuel level might be what the motor responds to with the noises. Check the fuel levels and refuel if it is low. Also, check for leakages that might be causing fuel to waste. If refueling does not work, then the problem might be something else that has to do with the lawn mower’s parts. 

Damaged belts 

Adjust the mower’s deck to the lowest setting and remove the belt guard to locate the belt. Inspect the belt for wear and tear, slacking, or bad routing. If it is not routed correctly or stuck, you can consult your manufacturer’s manual to reroute it yourself. If the belt is worn out, broken, or damaged, it needs to be replaced. 

Broken safety switch

Safety switches stop the blades from rotating to prevent accidents. Contact with sharp lawn mower blades can lead to severe injury, hence the purpose of the safety switch. The motor stops when the safety switch is triggered. To check the switch, open the mower and inspect it. If it is worn out, replacing it is the next step. 

Worn out pulleys 

Faulty or badly positioned pulleys can be the problem. The lawnmower pulley is located near the belt. If it is damaged, replace it with new ones. If the placement is wrong, remove it and place it properly. The manufacturer’s manual will contain a guide on setting it. 

Overuse or careless use

It is likely that the lawnmower is being handled in a way that affects engine health—for example, using the wrong type of mower for a particular kind of grass, overworking the lawnmower, hitting it, not cleaning components regularly, and storing it wrongly. 

Clogged cutting deck

Mowing wet or tall grass with a small lawnmower can clog up the cutting deck, making the blades sputter when engaged—cleaning out the cutting deck after each mowing session is advised. 

Safety Precautions 

Work on your lawnmower in a well-ventilated space: 

It is likely to be working with fuel, which is highly flammable. Fuel has a choking smell that is toxic to the body. Working in a well-aerated space, preferably an open one, reduces the chances of air pollution. 

Keep a fire extinguisher nearby:

Always have a fire extinguisher within reach in case of an emergency. Fires are not rare occurrences when working with gas, so it is only wise to be ready to control them should they occur. 

Wear safety gear: 

Use protective gears like goggles and hand gloves when working on your lawnmower, especially in a case of an overhaul such as this. 

Disconnect the spark plugs:

One of the first precautions to take is to remove the spark plug to avoid unintended starting. This also stops a fire hazard from happening.  

Be careful with the blades: 

Exercise extra caution when handling the blades to avoid severe injury. You can use a piece of wood to block the blade’s sharp edge when taking it off or rotating it. 


What does vapor lock mean?

Vapor lock happens when the hole on top of the gas cap is open. It causes fuel to evaporate faster, making gas bubbles accumulate. This tampers the amount of fuel that gets into the engine, causing problems for the carburetor and combustion chamber. 

How long do lawnmower blades last?

Lawnmower blades can last up to 300 hours if adequately managed. It is recommended that you replace them after three to four sharpenings. Sharpening should be done after an average of 22 hours of run time. When the blades look worn out and rusty, it is probably time to replace them. 

Can I reuse a paper air filter?

Paper air filters are not designed for reuse. Once it is clogged up, discard it and replace it with a new one. Foam air filters can be washed, dried, and reused up to four times before replacement.  

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