This #diy video shows how to #diagnose and #repair a lawn mower that won't start.
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For an engine to start and run when you pull the recoil starter rope, it needs 3 things. The first is fuel, which is a combination of gasoline and air that mixes in the carburetor. Then it needs a spark from the spark plug to ignite that fuel in the cylinder. Finally, it needs the right amount of compression in the cylinder to drive the piston. To fix your problem, we’ll have to figure out which of these three is missing.
A clogged air filter can keep air from getting to the carburetor and mixing with the gas.
While the air filter is 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 it doesn't start, that eliminates a fuel problem, leaving spark and compression on our list of suspects. If the engine starts and runs briefly, click here to skip to the section about diagnosing fuel problems.
Check the spark plug first. Pull off the spark plug wire and remove the spark plug, using a ratchet fitted with a deep socket. Look for carbon or oil buildup on the spark plug electrode that could prevent sparking. If the spark plug looks good, reinstall it and then connect a spark plug tester to check the ignition system. Release the rope from the mower handle so it's in reach when testing the spark plug. Connect the tester boot to the spark plug and connect the spark plug wire to the other end of the tester. Clamp the bail control bar down to release the blade brake. Pull the starter rope and see if the tester sparks. If the tester shows that there’s no spark, follow the steps in our video about troubleshooting the ignition system.
If the spark plug is good and the ignition system works, check the cylinder compression. You need a compression gauge for this test.
Remove the spark plug from the cylinder and pull the rope several times to purge fuel or oil from the cylinder. Insert a compression gauge in the spark plug hole and push the button to zero-out the gauge. Pull the starter rope repeatedly, the gauge should measure between 40 and 60 psi of compression. If it’s lower than 40 psi, have a service technician examine the engine and fix the compression problem.
Now, if the engine started and ran briefly during the starting fluid test, you know the spark and compression are okay and the problem is in the fuel system. Old gas can keep a mower from starting. In the future, if you're going to leave gas in the tank for a while, add fuel stabilizer.
To drain the tank, place a clean, durable container under the fuel line connection on the bottom of the tank. Release the fuel line clamp and pull the fuel line off to drain the gas into the container.
If you find debris in the drained gas, check for a contaminated fuel tank.
Check the filter screen at the bottom of the gas tank for damage. On this style of engine, the screen isn’t available as a separate part, so if the screen is damaged, you have to replace the tank. Some tanks have a removable fuel filter that you can clean or replace. Check your owner's manual for more information.
You should also check the fuel line for a crack or clog. Disconnect the other end of the fuel line from the carburetor. Examine it for cracks or cuts and replace it if it’s damaged. If you don’t see any damage, look through the fuel line and clear any clogs with a pipe cleaner.
If you’ve ruled out the quality of the gas and the fuel line, it’s time to turn to the carburetor, which mixes gas and air that is drawn into the cylinder.
Let’s start with the carburetor choke plate. A choke plate that’s stuck open could prevent the cold engine from starting. Check the choke plate by looking through the carburetor air inlet on the air filter base. The choke plate should be fully closed. If the choke plate is open, check the choke thermostat and linkage.
Examine the choke linkage and return spring to see if they're connected properly and moving freely. Reconnect the choke linkage and return spring and replace any broken parts. If the choke linkage and return spring looked good, then your choke thermostat is likely stuck. Replace the choke thermostat if it's stuck in the open position.
Instead of a choke plate, some engines have a primer bulb that you press to force more fuel into the cylinder when starting a cold engine. Replace the primer bulb if it's damaged.
If the choke plate or primer bulb work, it’s likely you need to replace or rebuild the carburetor. You can rebuild the carburetor, which takes more effort but costs less. Order the carburetor rebuild kit and then clean and rebuild the carburetor if you want to save money on parts. Here's a video that shows how to rebuild the carburetor.
Once you restore the fuel flow to the engine cylinder by replacing or rebuilding the carburetor, the engine should start.
