Pulling the cord on a lawn mower and having it not function can produce frustration, fear and anger. But with a little investigation, you can figure out exactly what is wrong. The problem may be as simple as clumps of grass keeping the blade from moving or as serious as a rusted piston. There are steps you can take to help you determine how to fix a stuck lawn mower engine. Here are a few guidelines to follow to find a solution.
Lift up the mower to check that the blade is clear. Most modern lawn mowers are direct drive, meaning that when the blade is unable to turn, the motor also will not turn. Make sure that the blade can spin by removing any obstructions in its path. Pull the starter again. If the engine does not turn over, proceed to Step 2.
Remove the spark plug. Pull on the starter. If the engine turns over, and fuel comes from the spark plug's hole, the engine was hydrolocked. If no liquid comes out and the engine still does not turn over, skip to Step 3. The fuel's emergence means that the carburetor needs repair, or the primer bulb is allowing fuel to leak into the cylinder. Check that when the primer bulb is pushed, you encounter resistance and that it reseals when the bulb is released.
Check the carburetor or diaphragm. If the carburetor has holes, or the rubber is hard and brittle, replace it. If the diaphragm is misshapen, it will not fit the carburetor body properly.
If the engine remains stuck with the spark plug removed, the piston rings also may be rusted to the cylinder walls. Spray a small amount of marine fogging oil into the cylinder and allow it to sit for a few moments before attempting to start again. Try this two or three times, allowing the oil to soak into the rings for a little longer each time. If engine remains stuck, move on to Step 4.
Pour a small amount of penetrating oil, such as Marvel mystery oil, into the cylinder. Allow the oil to set overnight. Attempt to pull the cord again. If the engine turns over, change the oil, then replace the spark plug. If the motor turns over easily but does not start, the piston ring must be replaced to restore compression. This will require motor disassembly.
If penetrating oil does not free the cylinder, the piston is rusted in place. The engine will have to be rebuilt.