Lawnmower Won’t Stay Running – 4 Common Causes & Fixes

Lawnmower Won’t Stay Running

4 Common Causes & Fixes

lawnmower wont stay running

This problem bothered me in the past and I’m sure this also happens to you – my lawnmower won’t stay running even if it starts fine! Just when I’m in the mood to stretch up my legs and ready to turn my lawn into a beautiful masterpiece, my lawn mower won’t stay on which ruined my day. But hey, I did not give up and did some research and found out there are 4 common causes of engine stalling. Now, I know how to troubleshoot my lawn mower and don’t get bothered by the same problem anymore. Do you want to know the solutions? Keep reading. 

From what I have learned, a lawnmower won’t stay running because of 4 common issues – bad old gas that’s been in the tank for months, flooded carburetor and engine, dirty air filter, and a problem with the spark plug. However, aside from these, there could also be some issues that can cause the engine stalling in the middle of the operation. But mainly these are the four most common causes. 

In this blog, we will discuss why these issues commonly happen and we will also tell you the troubleshooting tips you can do to solve them. 

Lawn mowers only have basic components , unlike cars. Therefore, you can easily find what’s wrong with them through observation, doing research and practicing what you have learned and fixing the machine yourself. 

Here are the four major reasons that cause why your lawn mower doesn’t stay running and what you should do about each. 

Old Gas in the Tank - Lawn Mower Bad Gas Fix

lawnmower wont stay runningAlthough gasoline is kept securely in the lawn mower’s gas tank, if it stays there for a long time, some of it can evaporate slowly due to the high volatility of the gas component. The result would leave gunk-like sediments that could result in the clogging of the outlet and inlet ports that lead to the carburetor and can make the carburetor and the fuel filter malfunction.  

Another reason why a lawn mower won’t stay running is this – 

During the cold season, unfortunately, condensation can occur inside the gas tank and moisture is produced. This moisture will eventually fall into the gas and because water and gasoline won’t mix together, there will be incomplete combustion of the air-fuel mixture in the combustion chamber and your lawn mower won’t stay running even if it starts nicely. This can also happen if the gas gets left in your lawn mower’s gas tank for three seasons or more and you did not use your lawnmower during these times.  

Moreover, if you think adding more fuel to the old gas can solve the problem, this is not the right lawnmower bad gas fix because the sludge will still be there to continue blocking the tubes and the moisture will still be affecting the process of combustion. 


lawnmower wont stay runningThe best solution to this problem is to completely drain the old fuel out from the tank and not to recycle it but dispose of it properly. This is to dispose of all the sludge that remained in the tank. Then you have to follow this up by cleaning the carburetor using a carb cleaner before refilling the gas tank. 

Just a reminder, don’t fill up the tank with fuel yet when refilling but only enough to let the engine run for a few minutes and observe if the engine will stay running. If the lawnmower won’t stay running still while there is still gas, drain out the gas again and better disengage the carburetor and clean it thoroughly. Let the remaining gas flow out from the tubes to ensure the clogs will also flow out. 

Lawn Mower is Flooded - How to Clean a Lawnmower Carburetor

lawnmower wont stay runningOne of the typical causes, why a lawnmower won’t stay running, is because your lawnmower is flooded. What do we mean by flooded? Flooding is the presence of an excessive amount of fuel in the carburetor that hinders the normal release of gas infusion to mix with the air. Thus, when an engine gets flooded, this starts in the carburetor and the proper air-fuel ratio does not happen in the combustion chamber and the lawn mower won’t stay on even for a few minutes.  

Why does flooding in the carburetor happen? It could be that the needle valve that should stop the gas from flowing in is not completely blocking the entry hole where the gas comes in even if the float valve moves. The reason for this could be the dirt or sludge blocking the inlet hole or the needle gets stuck due to dirt. It could also be that the float valve is stuck due to dirt. So when there is a continuous inflow of fuel into the carburetor, not only that the carburetor gets flooded with too much fuel but also the engine, this explains how a lawnmower engine gets flooded.  


The best solution when a lawnmower is flooded is to check for signs for a flooded carburetor first. If there is a strong smell of gasoline coming from the carburetor as well as seeing some drips on the connecting tubes, this is a sure sign of a flooded lawnmower. Other signs could be hard starting, reduced acceleration, rough idling, and misfiring of the engine.

How to clean a lawnmower carburetor easy. 

✅ Uncap the spark plug wire from the spark plug’s tip to avoid accidental starting. 

✅ Close the gas valve then unscrew the nut found under the carburetor with an adjustable wrench to drain the gas. Clean out the hole you see at the bottom of the nut using a small pin. This hole maintains the air pressure inside the carburetor. 

✅ Spray the inside of the carburetor with a carb cleaner thoroughly. 

✅ As you wait for the carb cleaner to dry, take out the spark plug and wipe the excess gas and oil from its terminals. Clean also its seat by inserting a clean rag inside to absorb the gas and oil. 

✅ After reinstalling everything, open the gas valve, connect the spark plug’s wire and start the engine. 

✅ If the lawnmower won’t stay running still, disengage the carburetor and check its movable parts. 

✅ You may also want to replace your spark plug if it is too dirty. Also check  the air filter and clean or replace it as necessary.  

Lawn Mower Spark Plug Problem,
Lawn Mower Spark Plug Replacement

flooded lawn mowerlawn mower spark plug replacementAnother cause for a lawn mower won’t stay running can be caused by a bad spark plug. The spark plug, as you know, is the part that’s responsible for producing spark or electricity to ignite the air-fuel mixture and make the pistons move, and forces the engine to run. So without the spark plug, no combustion can happen. And with a faltering spark plug, the engine will run but may die in the middle of operation. A spark plug may fail because of oil or carbon deposits or dirt that get clogged up on its electrode. Also, when a spark plug gets old, this will normally fail and will call for a lawn mower spark plug replacement.  


The best solution we can suggest to solve the problem of your  lawnmower won’t stay running due to the spark plug problems  is essentially by replacing it. For this, you will need a socket wrench. 

  • Detach the spark plug wire cap from the spark plug’s electrode. 
  • Insert the socket wrench into the nut part of the spark plug and turn counterclockwise to release the hold. 
  • Once the spark plug has been removed, clean inside the wire cap for any carbon buildup. Clean also the spark plug seat by inserting a clean rag and wriggling it inside to absorb engine oil and fuel mixed with carbon.  
  • Check the new spark plug’s electrode using a gap tool. The gap between the ground electrode and the center electrode should be from .030 cm to 0.060 inches. But if the spark plug is brand new, it is already gapped. 
  • Screw in the new spark plug with your finger until you feel that it’s getting tight. Then use the socket wrench to tighten it up but don’t over tighten it. Once you feel that the spark plug is secured, that’s good enough.   

The recommended replacement for spark plugs in lawnmowers should be at least for every 25 hours of use especially if the machine is new. However, if your spark plug is just working fine, you can leave it longer until your machine experiences hard starting.  

Dirty Air Filter

how to clean a lawn mower carburetorAnother cause your  lawnmower won’t stay running is a dirty air filter.  The function of the air filter is to trap debris and dust and stop it from getting these into the engine. If the air filter is blocked with dirt, this can prevent the air from getting through. Air is needed for the gas to burn with the right air-fuel mixture so if the mixture is not right, this will cause the engine to stall even if it starts and runs. 

Engine oil can also render the air filter dirty and sticky.  With regular use of your lawnmower, oil can splash out into the air filter and can clog it up. This is like choking the engine air supply  and the engine can overheat and even stop working altogether.  


✅ Check the air filter regularly for dirt or oil or carbon deposits. If the air filter only has accumulated dust and debris, this can be cleaned out. But if oil and carbon have found their way into the air filter, then it’s time for air filter replacement. Otherwise, more dirt will get trapped and will harden which will block the engine’s air. Air filters are recommended to be replaced every after 25 hours of use if it’s made of paper material. But if it’s made of foam, you can still try cleaning it with detergent soap and water. But if oil has fouled up the foam, replace it altogether. 


lawnmower wont stay runningWe would also like to add that when a lawnmower won’t stay running, this can also be caused by over-choking. Over-choking happens when you leave the choke on even when the engine is warmed up. The choke feature of lawnmowers is necessary to flood the carburetor with more fuel and to have smooth engine starting. But if you leave the choke on for a long time after the engine has started and the engine is warm, this may cause over flooding of the carburetor with gas which can affect the burning of the air-fuel mixture in the combustion chamber. The result would be engine stalling or the lawnmower starts but dies in the middle of operation.  

How to start a flooded lawnmower may only require a quick fixing. First, make sure that the choke is turned off. Then pull the rope repeatedly and let the engine run while stationary to let the excess fuel in the carburetor and in the combustion chamber burn. Then refill the tank with gas, do the choke and once the engine starts, switch off the choke. 


If your lawnmower won’t stay running this can be difficult for some people to deal with and solve. But if you know what causes the stalling, using the troubleshooting we gave you here can easily solve the problem. 

So, for other problems with your lawnmowers that start then stops or What Causes Flooded Lawnmower and How To Fix It?How To Clean Carburetor On Lawn Mower Save Money on Repairs Checkout our other lawn mower tips to save you money.