Lawn Mower Won't Start

So what we want to do here today is take a moment to explain the steps needed to troubleshoot your lawn mower in case it does not start after winter.

First, I'm going to attempt to start this lawn mower and see what it does. Typically, after about three pulls, if it hasn't started yet, we may already be faced with a problem.

Attempt to start lawn mower.

Step 1: Check for gas in the tank.

The first thing we want to focus on, and of course the simplest step, is making sure there is gasoline in the tank. I'm going to remove the gas cap and look inside the fuel tank. You can also move the lawn mower a little to see if the fuel is sloshing around and verify that you have a good quantity of fuel. The fuel level appears to be fine in this engine.

Step 2: Check the spark plug.

The next step in troubleshooting the engine is to remove the spark plug and see whether it is wet or dry to verify whether we're getting gasoline into the combustion chamber.

Ordinarily, before performing any service work, you would remove the spark plug wire so the engine doesn't accidentally start.

I'm also holding a special tool that we offer, which has both spark plug socket sizes on it. One end is 5/8 inch, which fits the smaller spark plugs, and the other end is 13/16 inch.

I'm going to remove the spark plug wire by simply pulling it off the spark plug and tucking it out of the way. Next, I'll engage the tool on the spark plug, make sure it's fully seated, and use the hook end for leverage to loosen it. Then I can simply remove the spark plug.

If we look at the end of the spark plug, it is very wet with fuel. That tells me a lot about what's going on. It indicates that the carburetor and fuel system are most likely working.

However, if the spark plug is dry, this is where you'll need to stop and investigate further. Your carburetor could be plugged with fuel deposits, or something else could be preventing fuel from reaching the combustion chamber.

Clogged carburetor.

If you determine that you're not getting any fuel into the combustion chamber, your carburetor may be plugged with fuel deposits. At that point, you'll probably need to take it to a dealer or a trained mechanic for service because it becomes fairly complicated to repair.

Let's take a look at what can happen inside the carburetor.

Notice the varnish buildup that forms in the float area and the carburetor bowl. This happens when gasoline evaporates, leaving chemicals behind that remain in the carburetor bowl and plug important small passageways, such as the jet that allows fuel from the bowl to enter the airstream and keep the engine running.

Fresh treated fuel.

To prevent fuel system problems like the ones shown in our example carburetor, begin by keeping the fuel in your gas can fresh. That starts with choosing a good-quality alcohol-free fuel additive like Sta-Bil. It's the official fuel additive of all Briggs & Stratton engines.

We recommend using Sta-Bil 360 Protection. It helps prevent corrosion caused by moisture in today's ethanol-blended fuels. While it's not widely known, gasoline can go bad in as little as 30 days. Sta-Bil 360 Protection keeps fuel fresh for up to 12 months.

Step 3: Check the ignition system.

Now that we've checked whether the spark plug is wet or dry, we've determined the direction to take with the fuel system. If the spark plug is wet and the engine still won't start, we need to focus on the ignition system next.

I'm going to use an inline spark tester, but I'll be testing it against a ground, such as the muffler, rather than directly on the spark plug.

I'll place the tester into the end of the spark plug wire, then attach the alligator clip to the muffler. In the tester's viewing window, we'll see whether a spark jumps the gap when I attempt to start the engine.

It looks like we do have spark.

Step 4: Check engine compression.

The next logical thing to check is compression. An engine needs three things to run: fuel, spark, and compression.

Compression is checked using a specialty tool called a compression tester. It threads into the spark plug hole, and then you pull the engine over to measure how much compression is being developed.

However, this is a more complicated test and requires a specialty tool, so at that point we recommend consulting your local dealer for repair assistance.

Now we're going to remove the original spark plug. I'll use the smaller 5/8-inch end of the spark plug tool and the handle to break it loose, then remove the spark plug completely.

Once it's removed, we'll discard the old spark plug and install a brand-new one.

Be careful when installing the new spark plug. It's best to turn it backward one turn until you feel the threads engage properly, then slowly thread it in until it seats at the bottom. You'll feel it stop when it's fully seated.

Reinsert the handle of the tool and tighten the spark plug with about a quarter turn. Then reconnect the spark plug wire because we're going to attempt to start the engine.

Conclusion.

It looks like we were successful in troubleshooting our engine. Hopefully, you found this information useful and are able to figure out what was going on with your lawn mower as well. If not, we suggest contacting your local dealer for repair assistance.

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