Simple Lawnmower Troubleshooting Guide

You are ready to mow your lawn, you start your lawnmower, and it stops a few seconds later, or it works for a couple of minutes and dies. You restart it and it stops again, and you sigh… it’s one of those days.

If your lawnmower keeps stopping it might be as simples as checking your fuel cap is not clogged, changing your sparkplug, cleaning your airfilter or unblocking your muffler. But it can sometimes be something more complex like a damaged carburettor and you need a professional to service it.

Many of these lawnmower problems are easy to troubleshoot, diagnose and fix. If you have time, patience, and some mechanical skills, you can do it yourself. Don’t worry! Below you will find a list of the most common problems to help you with your lawn mower troubleshooting.

Why does my lawn mower keep stopping?

Your lawn mower might keep shutting off because as something simple as something is clogged up, you need a new spark plug, or you need to add new fuel.

Of course, the first thing you should look at is your owners manual. As it contains information unique to your lawnmower. But this guide goes through some of the most common basic troubleshooting steps from the easiest to the worst problems. The idea is for you to use it as a guide to check your lawnmower problems and at the same time as a checklist to troubleshoot your lawnmower.

1. Check if your lawnmower has a clogged fuel cap vent

If your lawn mower keeps cutting out, check you fuel cap. Fuel caps have one or more small vents to let the air in as the fuel level is going down inside the fuel tank. This air compensates the inner pressure with the outside pressure and lets the fuel flow.

If the fuel cap is clogged, fuel will be unable to reach the engine, making it stop. One quick way to test your lawnmower’s fuel cap is to loosen it, making sure that air is allowed into the tank. Now turn on your lawnmower.

If it keeps running, the problem is the fuel cap. You can try to clean it or just replace it.

2. Grass blockage can cause your mower to stop – Check the lower end of your mower

If your lawnmower is not very powerful, a build-up of grass clippings underneath the mower can make it difficult to mow and ultimately cause your mower to stop.

Carefully inspect the underside of your lawnmower and check for clipping build-ups. Try to remove all lumps of grass, clean the area and try again. You can also try to adjust the cutting height to a higher setting to reduce problems with grass build up.

3. Blunt blades and long grass can make your lawn mower stop

Blunt, unsharpened or damaged blades could also cause the lawn mower shut off while mowing. This is because blunt blades will need more power to cut through the same amount of grass as sharp blades. If the blades are not sharp enough, they will tear the grass instead of cutting it. This grass could stick to the blades, preventing the correct functioning of the machine.

blocked mower blades

4. An exhaust blockage can cause lawn mower problems

This is a long shot but it’s very easy to check. Inspect your mower’s exhaust pipe and muffler. If it’s clogged with dirt or debris, the exhaust fumes won’t be able to leave the engine freely and will stop the engine. Wasp nests are more common than you think!

5. Your lawnmower problem could be due to leaking oil

If your lawnmower’s oil level is low, it’s dangerous to your lawnmower’s engine. It could also be the reason why your lawnmower keeps stalling out. Before further testing, top your mower’s engine oil according to the user’s manual, don’t overfill it and try again. Your lawnmower should always run with the right amount of oil.

Once you know there is the right amount in there, you need to look for what part of the engine is leaking. Unfortunately, there are a few places where the leak could be coming from, and it could be coming from more than one spot! To start to work out where the leak is coming from, you need to wipe the whole engine over with a rag and a brush to get rid of any oil and dirt that has built up on the outside. Once it is clean, it make it easier for you to see what is going on. Then, run the engine for a few minutes as often seals will only leak during use or immediately after. If there are leaks, hopefully this will help you identify them.

The most obvious thing to check with a lawnmower leak is whether the seals, o-rings and gaskets are worn. The first one to check is the lower bearing ring seal (aka oil seal). This is underneath the engine and the crankshaft that spines the mower blade goes through it. If the lower bearing ring seal is leaking, then you will find an oil leak directly underneath your engine.

If your valve gasket needs replacing you might see a build up of oil around the valve chamber where the cover attaches. To work out if that is the source of your leak, clean it up, and run the engine for a few minutes. Then if you see oil appearing again in that area, then you probably have found the culprit!

6. Too much oil can cause a lawn mower to stall

Excess of oil in your mower’s oil reservoir may drown the engine and make it stop. If there is too much oil in your mower’s reservoir, you are probably going to see that your mower is smoking, leaking oil, or even blowing blue smoke. In this case, blue smoke coming out of the engine is a result of the excessive oil burnt by the engine.

If you realize that you have overfilled your mower’s oil reservoir, you will need to drain out the excess of oil. Once you have removed it and you’ve leveled up, it’s time to turn your lawnmower on again and check if it runs without stopping. After a while, the blue smoke will stop coming out of the engine. If it doesn’t, your mower’s engine has a serious problem that needs to be checked by a professional like us here at Canberra Mower Centre

7. A dirty air filter can cause engine to stall

A common cause for a mower blowing black smoke is a blocked air filter. A lawnmower might keep stopping because of a clogged air filter. Find your mower’s air filter and remove it.

If it’s too dirty, or if it looks clogged, wash it or replace it. It depends on whether your lawnmower’s model comes with a washable filter or a regular one. It’s great if this fixes the problem. If it doesn’t, it now has a clean filter, which is always good. If you need a new airfilter, we have a huge range, just give us a call on 02 6242 8996

8. Check you did not accidentally put diesel in lawn mower

A common problem indicated by white smoke is when diesel has been accidentally put into a lawn mower. If you lawn mower is blowing excessive white smoke you need to check what fuel you put into it. If you have put diesel in your lawnmower and tried to start it, the carburettor will need to be removed and drained and the spark plug may need to be replaced. At this point, we recommend you get a lawnmower service.

9. Old fuel can cause your lawn mower to sputter and die

If your mower has been stored for a while, often it will start and then stop or your lawn mower won’t start at all. If this happens you should check the fuel as old fuel can damage your lawnmower’s engine:

lawn mower keeps stopping
  • Fuel deteriorates over time. Old fuel won’t burn properly, and it can be the reason for the problem. If the fuel in your mower doesn’t seem to be ok, it’s always better to drain the tank and replace the old gasoline.
  • Fuel evaporation leaves residues that may clog your lawnmower’s engine moving parts, the fuel lines and, what is worse: evaporated fuel can create a varnish that sticks on vital parts like the carburettor.

If adding new fuel doesn’t fix the problem, you can try to use some fuel additives to remove the varnish and debris. You can check for obstructions in the fuel lines. If you find clogged fuel lines, replace them.

If you find bad fuel inside your mower’s tank and none of the things mentioned above fixes the problem, you may have to deal with your carburetor -the last point on this checklist.

10. A bad spark plug can cause a lawn mower to stall?

A bad spark plug can prevent your mower from starting, but it can also cause your mower’s engine to fail and to lose power. This power loss can make your mower stop when the engine is forced to cut tall grass, for example.

If you have tools at hand, you can remove the spark plug and check for cracks on the porcelain insulator, seek carbon buildup at the electrode, or search for damage or excessive light in the electrode.

If you have a spark plug tester, you can check whether the sparkplug is working and how strong the spark is. If you notice any of these symptoms or you haven’t replaced your lawnmower’s sparkplug recently, do so now. Having sparkplugs in shape always helps the smooth and proper working of any engine.

11. Leaving the choke on can stall your mower

What happens when you run a lawn mower with the choke on is that it will get too much gas and stall.

Your lawnmower can have a choke or a priming bulb. These are two different ways of helping your mower’s engine to start cold. Some lawnmowers can have an auto choke function. When the choke is set, less air enters the carburetor. This alters the air/fuel ratio and creates better conditions for a cold engine to work.

You can check if the choke is stuck close. You should check that the choke’s flap can move freely.

12. Check if your mower runs when you push the primer

If your lawnmower has a primer bulb, the final result when you push the primer button is the same: the air/fuel ratio will become richer. Instead of reducing the airflow coming to the engine, pressing the primer will send extra gas to the carburetor.

When lawnmower troubleshooting, if you find yourself asking why does my mower only run when I push the primer… This is because there is some kind of problem with the carburetor and as you push the primer button, the extra fuel allows the engine to run.

13. Check for problems with your lawnmower carburetor

This is the last item and the most difficult on this list. After you have checked all the items mentioned and you have made sure everything else is ok, the last thing that you can check by yourself is the carburetor. A clogged carburetor can cause the engine to stall. The first thing is trying to clean it with a carburetor cleaner. If simple carburetor maintenance tips don’t work, it can be a complicated task. So, if you have tried the things above you are probably better off using our lawnmower mechanics to repair your lawnmower. But if you want to try checking for signs of carburetor problems follow the next few tips…

Can you clean a lawnmower carburetor without removing it?

You can get any automotive carburetor cleaner and try to clean your mower’s carburetor. Make sure to spray the cleaner into the bowl of the carburetor and watch the throttle and choke cables; they should be easy to move with your hand. If you see they are hard to move, you should spray some lubricant like WD-40 to loosen them.

What is the best way to clean a carburetor?

If this doesn’t work, you can always try to go a bit further. You can remove the carburetor, take it apart and go for deeper cleaning. When you disassemble the carburetor, you will have access to the carburetor jet/s. Small carburetors usually have only one jet. The jet/s are tubes with tiny holes; make sure they are clean.

The carburetor has its own fuel bowl. It works as a buffer, preventing the engine from running out of gas. The carburetor sucks the fuel straight from the bowl. That’s why it’s very important to keep this bowl very clean. Sometimes dirt and grit coming from the fuel tank manage to pass through the fuel filter and stick into the carburetor jet.

The last two things you can inspect when you are working on your carburetor are the float and needle. The float is attached to the needle, and it’s located inside the fuel bowl. The float and needle control the fuel flow. The needle acts as a seal and can be worn out, and the float moves the needle according to the level of the bowl. The floaters can fail, puncture, or break, which will make them move the needle without the necessary precision.

Besides all the parts mentioned above, your carburetor has gaskets and other small parts. If you see a very worn-out part or a part in really bad shape, there are carburetor repair kits for almost every carburetor in the market. It depends on how much time you have and how confident you feel to do the job. Sometimes use, time and sludge have damaged the carburetor so badly that there is no other choice than to replace the whole carburetor.

Why does my lawnmower keep stopping?

If you have carefully followed the steps on this list and the problem persists; your lawnmower may be having more serious problems. This list intends to cover the most common and easy-to-fix problems. We covered some serious repairs like usual carburetor problems.

If you reached this far and you still can’t solve the problem, I would recommend taking your lawnmower to a professional to be checked and repaired. We provide professional lawnmower repair and service Canberra wide.

As a last word of advice, always remember that lawnmowers are pretty reliable machines, but they need regular maintenance. If you are not going to use them for a long time, it’s advisable to drain the fuel to avoid future problems. Always read the owner’s manual before trying any repairs and take all the safety precautions that appear in the owner’s manual to avoid any chance of accidents.

If you have another small engine like the Honda GX range, head on over to our Small Engine Troubleshooting article prepared for you by our Canberra Mower service team.

Troubleshoot your lawnmower

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