Diagnosis
The combination of loud noise and severe vibration after hitting a rock indicates a loss of rotational balance. This is typically caused by a bent blade, a sheared blade bolt, or a damaged blade adapter (holder). However, the high-impact force of a rock can also bend the crankshaft or damage the engine mounts, which creates a systemic vibration that persists even if the blade is replaced.
How to Fix It
Stop using the mower immediately to avoid catastrophic engine failure.
- Safety First: Turn the fuel valve to the OFF position and disconnect the spark plug wire to prevent accidental starting.
- Positioning: Tilt the mower with the air filter/carburetor side facing UP. Never tilt it with the air filter down, as oil will leak into the carburetor.
- Blade Inspection: Remove the blade bolts. Inspect the blades for any slight bends, nicks, or cracks. Even a tiny bend disrupts the balance at high RPMs.
- Hardware Check: Inspect the blade holder (adapter) for warping or cracks. Ensure the bolts are not stretched or stripped.
- Crankshaft Test: With the blades removed, carefully rotate the engine by hand. If the center bolt or shaft appears to "wobble" or oscillate, the crankshaft is bent.
If That Didn't Work
If the blades are straight but the vibration continues, check the following:
- Engine Mounts: Inspect the bolts securing the engine to the mower deck; they may have loosened or sheared during the impact.
- Flywheel Balance: A severe impact can occasionally shift the flywheel or damage the crankshaft key, causing internal imbalance.
- Professional Alignment: If the crankshaft is bent, the engine typically requires replacement, as straightening a crankshaft is not a reliable permanent fix.
Heads Up
Always replace blades as a matched set to ensure weight equilibrium. Never attempt to "straighten" a bent blade with a hammer, as this alters the metal's integrity and creates an imbalance that can vibrate the engine bolts loose or crack the mower deck.