How to Fix a Lawnmower. Diagnose the most common problems with fixes.


So you are organizing a barbecue party at your home, but first, you want to make the lawn tip-top. You take out the lawnmower from your garage and start the engine. But it doesn’t start. Or it does start but dies out after some minutes. Many questions come into your mind at that moment. You start questioning your machine’s quality. Did I do the correct maintenance, or should I buy a new one? Our advice is first to investigate, find out what the problem is. Then try to fix it. We will provide you a list of potential reasons why you have a problem and how to fix them on your own.

How to fix a lawnmower. Diagnose the most common problems with fixes:

  • Dirty Air Filter: A dirty air filter blocks the supply of fresh air to the engine. Remove the air filter and clean it. If really dirty, replace it.
  • Dirty Carburetor: Remove the carburetor and open it to remove its components. Clean the parts, reassemble and reinstall the carburetor.
  • Ignition System Problems: Adjust the gap between the magneto and the flywheel to a width of two playing cards. Ensure that all connections are in good order and the wiring is not damaged. Replace damaged wires.
  • Dirty or Faulty Spark Plug: Check the spark plug for carbon deposit and moisture. Clean if dirty. Check the plug gap and adjust if needed.
  • Overheating: Clean the fins and maintain the correct oil level to prevent overheating.
  • Fuel and Oil Problems: Ensure that your mower uses fresh fuel. Check the oil level and add if the level is too low.
  • Fuel line and Fuel Tank: Smooth supply of fuel from the fuel tank to the combustion chamber is critical. The engine will run rough or is difficult to start with a clogged fuel line. Clean the fuel line and the fuel tank if it is dirty.
  • Dull Blades: Remove and sharpen the blades if they are dull. Replace damaged blades if they are chipped, cracked, or bent.

Most common lawnmower problems are not too difficult to fix. If you know the different potential issues preventing a mower from starting or keeping it running, it helps to diagnose the problem in your case. Before you start working on your mower, gather the necessary tools for the job. Given below are some common lawnmower problems and how to fix them.

Tools Needed:

You’ll need the following tools to perform general lawnmower repair and servicing:

  • A wire brush
  • A flat screwdriver
  • Needle nose pliers
  • Wrenches or a socket set
  • A spark plug wrench
  • Philip screwdriver
  • Degreaser
  • Crowbar
  • Petrol bottle
  • Clean cloth

Before You Start:

Every time you repair or service your mower, park your mower on an even open surface with the parking brakes applied. Turn off the engine and remove the key from the ignition. Depending on the job, disconnect the battery to keep it and the electrical system safe. Disconnect the spark plug cables to ensure an unintentional engine start.

● Lawnmower Fuel and Oil Problems:

Even though this may seem like an obvious one, fuel, and oil-related issues may prevent your mower from starting or cause it to die out some minutes after starting. Fuel and oil issues are severe when you are using your mower after a long time.

For your mower to work perfectly, there should be an adequate amount of fuel in the tank. The best practice would be to use fresh fuel each time you use your mower. If there is old fuel sitting in the tank, remove it and add fresh gas before starting. Don’t let gasoline sit in the tank for more than 30 days as it can go bad and harm vital components such as the fuel line and the carburetor. The use of fuel stabilizers is also recommended. It would be best to avoid gasoline with more than 10% ethanol, and the octane number should be best suited to the working environment.

Consult our articles titled, “Bad gas in a lawnmower, how to fix?” and “How long is gas good in a lawnmower?” for more details.

Using the correct oil and regularly checking the oil level is essential. Suppose the color does not look ok change the oil. Check the user manual what the recommended oil change time is.

● Dirty or Faulty Lawnmower Spark Plug:

After checking the fuel and oil level, it is not wise to directly dismantle the engine. If the lawnmower does not start, the second most important thing to do is remove and check the spark plug(s). Check if the spark plug is wet. Suppose it is moist, clean, and dry it thoroughly with a clean cloth. If there is carbon buildup around the electrodes, remove it with a metal brush, file, or sandpaper. The spark plug gap should also be checked and adjusted.

If the spark plug has a broken insulator sheath or too dirty to be used again, replace it with a new one.

If the spark plug is acceptable and you want to check other components, leave it disconnected and reinstall it when the work is finished.

For more details on the spark plug topic, consult our blog posts titled, “How to tell if a lawnmower spark plug is bad?” and “What spark plug to use for a lawnmower?”

● Clogged Lawnmower Fuel Line and Fuel tank:

Think about all the fine debris and small grass particles flying around the mower while trimming a lawn. These can clog the fuel line, block fuel supply to the carburetor, and prevent the mower from starting. If your engine runs rough and does not have the power you expect, the fuel line should be checked and cleaned. To clean the fuel line on your mower, you need first to turn the fuel valve to the off position so that more gasoline will not come into the line. If the fuel line is cracked, replace it with a new line but if it is only dirty and can be used again, clean it very carefully and reinstall it into the system and turn on the fuel valve.

Check for any debris in the fuel tank as well. If the fuel tank is dirty, it should be removed and cleaned with a brush, soap, and water. It would be best if you immediately replaced a cracked fuel tank.

For detailed fuel line cleaning procedures, consult our blog post titled, “How to clean a lawnmower fuel line.”

● Dirty Lawnmower Air Filter:

A dirty air filter can be the prime reason preventing the combustion chamber from receiving enough air from the atmosphere. Dirt, dust, and oil residue can jam your air filter. This is simple to check and fix. Remove the air filter and check it. If dirty, clean it with water, soap, and a brush. If you have a paper air filter, or the foam one is broken or too dirty, you need to replace it. Make it a habit to check and clean the air filter regularly. If the air filter is too dirty and greasy, it should be replaced with a new one.

● Lawnmower Ignition System Problems:

The most commonly used ignition system in lawnmowers is the magneto ignition system. This acts as a step-up transformer. The most important thing about a magneto ignition system is that there should be a little gap between the flywheel magnets and the magneto itself. The gap between the flywheel magnet and magneto should never be greater than the size of a playing card so that the transmission of electricity can be done easily. If the gap between the magneto and the flywheel is too large, you will not be able to start your mower. So, you should adjust this gap.

Furthermore, you should check the wiring and look for any disconnection, wear, and grime that might be the problem. Replace the broken and corroded wires to start your mower.

● Dirty Lawnmower Carburetor:

As said earlier, gas sitting in the tank for a long time can harm the carburetor by producing a gum-like substance that jams the carburetor. A jammed carburetor fails to provide fuel-air charge to the combustion chamber, which either prevents the mower from starting or reduces its power and efficiency. Another reason is if the fuel is dirty (see earlier point). For these cases, you need to check and clean the dirty carburetor.

First of all, remove the carburetor from the engine by removing the screws holding it in place and disconnect it from the fuel line. Make sure that the fuel valve is closed while you are working on the carburetor. You may need to twist and wiggle the carburetor to remove it.

The next step is to open the carburetor. For this purpose, unscrew the carburetor’s top cap, and you will have a removed spring and throttle slide. Unscrew the four screws to dismantle the upper cap of the carburetor. You’ll see a greasy dusty float connected with the float bowl but do not start cleaning it yet. Now remove the float pin and place the removed float pin, and float into a small container with the other parts. Carefully remove the main jet and pilot jet along with the float needle valves, gaskets, and O-rings.

– Cleaning the lawnmower carburetor:

The container now contains all the parts from the carburetor that you disassembled. The quickest and easiest way is to soak the gas or carburetor cleaner items to remove greasy lumps and dust. After washing the parts for a while, take a wire brush and scrub the pieces very gently and carefully. Spray the carburetor cleaner into the float cap’s holes and on the main carburetor body and valves; then air-dry the pieces with an air compressor’s help. When cleaning is done, install brand new O-rings and gaskets, and reinstall the fuel jets, pilot jets, float, and float pin one by one. You may reuse the old O-rings and gaskets if they are in good condition. Clean them, too, before reinstalling.   

● Lawnmower Overheating:

Overheating causes engine stalls and is very undesirable. Overheating is usually caused by excessive friction between the moving parts because of too little oil or wrong oil grade or fins covered in dirt and debris.

If your lawnmower engine is overheating, check the oil level and add more if the level is low. Always use the right oil grade for your mower. Also, clean the fins if they are covered in dirt and grass debris.

For more details on overheating and what to do if your mower stops running when hot, consult our article titled, “Lawnmower stops running when hot.”

● Dull or Damaged Lawnmower Blades:

Suppose your lawnmower does not cut as well as before. You should check the blades. Maybe they are dull or damaged. You should check and sharpen lawnmower blades regularly for the best cutting performance and grass health. You can sharpen blades using a metal file or grinder. Chipped, cracked, and bent blades should be replaced.

Consult our articles, “How to sharpen lawn mower blades?” and “When to replace lawnmower blades?” for detailed info on the topic of lawnmower blades.

Final Remarks:

If your lawnmower suddenly stops working or does not keep running, there can be different causes for this. Grease, dirt, and dust on engine parts, shortage of gas and oil, etc., can cause the engine to stop working. If you are not familiar with the causes behind a halted engine, you might get worried and rush to a technician’s shop. However, with basic info on this topic, you will be able to diagnose your mower and fix this problem yourself. With the information provided in this blog post and related blog posts, you’ll be able to fix common lawnmower problems yourself, which will save you time and money.

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