How to Fix a Lawn Mower that Won’t Start: Fuel, Ignition and Compression Problems in Denver Metro

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Hello, this is Wayne with Sears PartsDirect. Today, we’re discussing troubleshooting tips to help you discern why your lawnmower won’t start. Even though your mower may look different from this Craftsman 21-inch mower, they all work pretty much the same.

Here are the tools and materials you may need, depending on the issue. Since we’ll be working with gasoline, choose a well-ventilated area that is free of open flame or sparks. For the engine to start and run, three things are required when you pull the starter rope.

– Firstly, fuel, which is a combination of fuel and air, combines in the carburetor.
– After that, it requires a spark from the spark plug to ignite the fuel inside the cylinder.
– Finally, it needs the right amount of compression in the cylinder.

To fix the problem, we need to figure out which of these three things is missing. We’ll start by taking some simple steps. A clogged air filter can prevent the air from getting to the carburetor and mixing with the fuel.

Remove the air filter cover and pull out the air filter. This mower has a pleated paper filter, but some mowers have a different type of paper or foam filter. If you have a slightly dirty paper filter, you can tap it on a hard surface to clean it. If it’s entirely blocked, you’ll need to replace it. If your mower has a foam air filter, you can find cleaning instructions in your owner’s manual.

With the air filter removed, let’s do a quick test to see if the engine has a fuel problem. Spray starting fluid through the air filter opening to the carburetor and then try to start the engine. If the engine doesn’t start, that eliminates a fuel problem, leaving spark and compression suspect. If the engine starts and runs briefly, skip to the section on diagnosing fuel problems.

First, check the spark plug. Pull off the spark plug wire and remove the spark plug using a ratchet fitty with a deep socket. Look for carbon or oil buildup on the spark plug electrode that could prevent sparking.

Also, check for a crack in the ceramic insulator. If you see excessive buildup or a break, replace the spark plug. If the spark plug looks good, reinstall it and connect a spark plug tester to check the ignition system.

Connect the tester boot to the spark plug and attach the spark plug wire to the opposite end of the tester. Release the rope from the mower handle so it’s in reach when testing the spark plug. Clamp the bail control bar all the way down to release the blade break. Pull the starter rope to see if the tester sparks.

If the tester shows no spark, follow the steps in our video on troubleshooting the ignition system. If the spark plug is good and the ignition system works, check the cylinder compression. You’ll need a compression gauge for this test. First, remove the spark plug from the cylinder and pull the starter rope several times to purge gas or oil from the cylinder.

Insert a compression gauge into the spark plug hole and push the button to zero out the gauge. Pull the starter rope repeatedly until the needle on the gauge stops rising. The gauge should measure between 40 and 60 pounds per square inch (psi) of compression. If it’s lower than 40 psi, have a service technician check the engine and fix the compression problem.

If the engine started and ran briefly when you performed the starting fluid test, you know the spark and compression are okay, and the…