When your mower vibrates severely, start by checking whether the blade is damaged, imbalanced or loose. Tighten the blade bolt if it's loose. Replace the blade if it's damaged.
A rough-running engine can also cause severe vibrations so tune up the mower to cure engine vibrations.
If you have a hard time pulling the starter rope and the engine sputters when starting your lawn mower, the flywheel key might be damaged.
These repairs may help solve your gas walk-behind mower problem
Replace a lawn mower blade
A dull or damaged blade won't cut grass evenly. For safety and for a cleaner cut, replace a mower blade that's chipped, broken or otherwise damaged. If the mower vibrates too much, check the mower blade for damage because a bent or unbalanced blade can cause mower vibrations.
The flywheel key is a small metal rectangle that keeps the crankshaft and flywheel aligned when you tighten the flywheel nut. To protect more expensive components from damage, the flywheel key is designed to shear if the mower blade hits an object like a rock or tree stump hard enough to make the flywheel slip out of alignment with the crankshaft. If the flywheel key gets dented when the blade hits an object, the engine runs rough or vibrates severely. If the flywheel key shears when you hit an object, your lawn mower will stop running and won't start. If you inspect the flywheel key and find it dented or sheared, replace it.
If your lawn mower doesn't start after hitting a rock or a stump, the mower flywheel pin might have sheared to protect expensive components. This repair guide shows how to replace a lawn mower flywheel key. This guide is for older mower models that have a flathead engine.
Tune up a walk-behind mower when the engine runs rough or won't start. You may need to tune up the engine when it vibrates too much or won't stay running. Regular tune-ups prevent engine wear and improve engine efficiency. A walk-behind mower tune-up includes changing the oil, spark plug and air filter. It could also include sharpening or replacing the blade.