Symptoms
- Handle shakes noticeably while mowing
- Vibration increases with engine speed
- Loose parts may rattle
- Started after hitting a rock, root, or curb
Likely causes
- CommonBent bladeEven a slightly bent blade throws off the balance and causes strong vibration.
- CommonUnbalanced blade after sharpeningA blade sharpened unevenly on one side spins out of balance.
- PossibleLoose blade boltA blade that isn't torqued properly can wobble as it spins.
- PossibleBent crankshaftA hard impact can bend the engine's crankshaft, causing persistent vibration that a new blade won't fix.
What to do
- Disconnect the spark plug wire, then inspect the blade for visible bending or damage.
- Remove the blade and check it on a blade balancer or a nail through the center hole; replace it if one side hangs lower.
- Check that the blade bolt is torqued to the manufacturer's specification when reinstalling.
- Inspect the deck and engine mounting bolts for looseness and tighten as needed.
- If vibration continues with a new, balanced blade properly torqued, suspect a bent crankshaft, especially after a recent impact.
- Stop using the mower if vibration is severe, since a bent crankshaft can worsen and damage the engine.
Tools and parts
- Wrench set
- Blade balancer
Replacement mower bladeSearch: “lawn mower blade balanced replacement”
When to call a pro
A small engine shop can check for a bent crankshaft, which is not a DIY repair on most walk-behind mowers.
Safety
Use caution
Stop mowing immediately if vibration is severe; a bent crankshaft can fail suddenly and is unsafe to keep running.