July 26, 2020
Lawn Mower Won't Start

"Help, My Lawm Mower Won't Start!" - There's hardly anything more annoying to homeowners than getting ready to mow your lawn, setting aside a time to do it, going through all the preparation, only to find out that your mower won't even start. If you're taking care of your lawn on your own then you're more than likely familiar with this situation. But, before you begin shouting the expletives, start planning a visit to the mechanic, and calculating how much the repair may cost, see if you can fix the issue yourself.

There are several problems that can prevent the engine from properly starting and some of them are easily fixable even if you don't have extensive technical knowledge and experience. Solving some issues doesn't even require any tools.

Lawn Mower Won't Start? Did You Check These 6 Things?

We'll go through the most common ones, see what causes them, and explain how to deal with them.

1.) Faulty Spark Plug

Lawn Mower Spark Plug

Believe it or not, the spark plug is probably the No. 1 culprit for your problems with starting the lawn mower.

If the spark plug is dirty, disconnected, or simply loose, the engine won't start.

If it's not put to use in a while, the spark plug can get corroded and the engine won't be able to ignite fuel. And, since half of its stem is exposed, it is rather susceptible to weather elements and outside dirt or grass spoilage.

Bear in mind that the dirt or looseness is not the only reason why the spark plugs may not work. They do have a finite life span and, as they start to degrade, their performance suffers.

The recommended period of usage of a single spark plug is around 150 hours, so have some backups handy.

What You Can Do

  1. Remove the spark plug from its socket. Inspect it carefully and see if there is any dirt or oil buildup (also check the electrodes and the gap between them).
  2. If there's any carbon buildup or if it's damaged, the best thing to do is to replace it with a new one. They're not that expensive. Pick them up from your local hardware store or Amazon.com.
  3. If it looks OK but is dirty, you can try to clean it with high-pressure air or some solvent in case there's is any oil on it.
  4. Don't forget to dry it before connecting it again.
  5. It's likely a good idea to test it before reinstalling it. You can do this with the spark plug tester, or, if you don't have one, clip the boot onto the plug and hold it against the cylinder while pulling the starter cord.
  6. If you see a strong spark, then it probably works, and the problem is elsewhere.

2.) Old Fuel

No matter how silly it may sound, make sure that there is gas in your tank. This is probably the most obvious problem, but people often forget to check it and go searching for a more complicated malfunction.

Gas may be the issue even if your tank is full. The gas that is sold nowadays in the US goes stale rather quickly, mostly due to the higher percentage of ethanol.

It's prone to phase separation and can leave some water in your tank.

Most manufacturers recommend replacing the old gasoline with the new batch after 30 or so days. So, if your mower was stored during the winter, the first thing you should do is drain the old fuel. Stale and old fuel clogs the carburetor, fuel lines, filter, and valves and causes corrosion.

Also, if the tank cap is a bit lose, fuel can be contaminated by outside dirt and moisture.

What You Can Do

This one is pretty simple. Drain the old gasoline and refill with fresh fuel.

Consider adding fuel additives or stabilizers to buy yourself some time and extend the usage of the new batch.

When buying gasoline, check that it has less than 10% of ethanol. If you are one of the rare owners of 2-stroke engine lawn mowers, make sure that you prepare the right mixture of gasoline and oil and to use the oil that is specifically intended for those purposes.

3.) Oil Issues

The poorly lubricated engine can cause friction damage to various moving parts. If this happens, not being able to start your mower can be the least of your problems.

Also, if the oil is left in the tank for too long it can thicken and perform much worse than it's supposed to.

Some engines, including recent models from Honda, Kohler, or Briggs & Stratton, have a built-in function that prevents the engine from working if the oil level is too low.

What You Can Do

First of all, make sure that you're using the right type of oil intended for air-cooled engines. If you're unsure which oil to buy, check the engine's manual.

Oil should be changed after 30-50 hours depending on a manufacturer, meaning that once you refill it, you should be good for a whole season.

  • Older engines used to have a valve at the bottom through which you could easily drain the oil. 
  • Most of the newer models don't have it, so your best course of action is buying one of the specifically designed draining pumps which are very practical and usually cheap.

 Remember to regularly check the oil level in the tank using the dipstick.

4.) Clogged Fuel Filter

As we previously mentioned stale or duty gasoline can lead to clogging of the fuel filter. The sediment from the fuel builds up in the filter so it can no longer provide a good flow of the gasoline to the carburetor. This usually can prevent the engine from even starting.

What You Can Do

When the fuel filter is clogged, there's no cleaning. you need to replace it. The filter is commonly located near the bottom part of the tank, next to the gas line. If your engine has a fuel valve, you need to shut it.

Here's a good video (not ours) explaining the process, it's just 3 minutes, give it a quick watch if you're unsure!

Otherwise, clamp the gas line. Use pliers to carefully remove the metal clips that connect the filter housing to the line on both sides.

Then, simply slide the housing of the line. While doing this, place some old rag or towel beneath, since there will probably be some fuel spill.

Take the new filter and reverse the previous action. Make sure that the metal clips are tight and secure. If your engine has a primer bulb, pump it a few times to get the gasoline flowing and check for any leaks.

In case that your fuel filter is located inside the tank, the best course of action is to ask an expert for help.

5.) Dirty Air Filter

Dirty Lawn Mower Air Filter

In order to work and make the engine run, the carburetor needs air. Therefore, if the air filter is dirty or clogged the carburetor won't get the air it needs and your lawn mower may not even start.

What You Can Do

Regularly check your air filter to see the state of it. It generally needs to be replaced once a year, but you'll probably need to clean it more often.

Two types of filters are the most common on the mower engines. One is made of paper and covered with wire mesh on one side, and the other is made of sponge-like foam.

If the paper filter is not too dirty, you can probably clean it just by striking it a couple of times against the hard surface to "shake off" the dirt.

For dirtier filters, use compressed air, and blow it from the inside out to avoid the dirt sticking deeper into the body of the filter.

Foam filters can be simply cleaned with detergent and water, just don't forget to dry them before putting them back in. If cleaning doesn't do the trick, replace the old filter with the new one.

6.) Sheared Flywheel Key

The flywheel key is a small metal part that fits into the crankshaft and aligns the flywheel. If, while mowing, you hit a rock or similar hard immovable object, the blade and the crankshaft stop while the flywheel keeps spinning breaking the flywheel in half.

If the flywheel is broken the engine won't start in order to prevent further damage, especially to the crankshaft which is one of the most expensive parts on your lawn mower.

What You Can Do

If this happens, you'll need to replace the fly key.

  1. First, disconnect the spark plug and then remove the flywheel using a set of gear pullers.
  2. Inspect the flywheel key for damage. Even if it's not broken, but there are signs of shearing, you need to replace it.
  3. Put the flywheel back on the crankshaft and align the keyways.
  4. Place the flywheel key in the keyway and check that it fits properly.
  5. Once everything is in place, you can reattach the flywheel nut.

If you undertake all of the steps described above and your lawn mower still won't start, then it's probably the time to ask for professional help.

We recommend against tinkering with carburetor and other more sensitive parts of the engine.

What you can do is take all the necessary precautions to avoid this from happening in the first place.

Inspect the lawn before starting the job, mow carefully, use high-quality oil and fuel, check filters, keep lawn mower in a dry and safe place, and regularly service your engine.

Still Can't Fix It?

If you've gone through all of the above and STILL can't get your lawn mower working, it may be time to admit defeat and find a local small-engine repair shop.  Check Google or your Yellow Pages (do people still use those?)

If THEY can't get it working for you, it may just be time to pick up a new Lawn Mower.  We have several buyers guides on this website dedicated to helping you with that decision, so have a look around!


Daniel Simmons

About the author

TheLawnMowingKing.com brings my 25 years experience as a professional gardener and landscaper to you!

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