My Petrol Lawnmower is experiencing excessive vibrations

If your petrol lawnmower vibrates excessively during use, the problem is usually caused by damage to the cutting system following an impact with a solid object. In many cases, the blade, blade adaptor or crankshaft may have been damaged after striking a tree root, stone or other obstacle.

This guide explains the most common causes of excessive vibration and the checks you can carry out to identify the fault.

Quick Answer

Excessive vibration on a petrol lawnmower is most commonly caused by impact damage after the blade has struck a hard object.

The most common causes are:

• A damaged blade adaptor.
• A bent or damaged mower blade.
• A bent crankshaft.
• Loose blade fixings.

Stop using the mower as soon as excessive vibration is noticed, as continued operation can cause further damage to the engine and cutting system.

Before You Begin

Before inspecting the mower:

• Switch off the engine and allow it to cool completely.
• Remove the spark plug cap to prevent accidental starting.
• Empty the fuel tank or follow the manufacturer's guidance before tilting the mower.
• Wear suitable personal protective equipment (PPE), including gloves.
• Ensure the mower is positioned securely before working underneath it.

Never inspect or handle the blade while the spark plug cap is connected.

Steps

Step 1 – Consider Recent Blade Impacts

Think about whether the mower has recently struck a solid object such as:

• Tree roots.
• Stones or rubble.
• Drain covers.
• Paving slabs.
• Uneven ground.

A sudden impact can transfer force through the cutting system, damaging components and causing excessive vibration.

Step 2 – Inspect the Blade Adaptor

The blade adaptor is designed to absorb impact and help protect the engine from damage.

Inspect the adaptor for:

• Cracks.
• Chips.
• Missing shear pins.
• Distortion or deformation.

If any damage is found, replace the blade adaptor before using the mower again.

Using a damaged adaptor can lead to additional mechanical damage and unsafe operation.

Step 3 – Inspect the Mower Blade

Carefully examine the blade for signs of damage.

Look for:

• Bent or twisted sections.
• Cracks or splits.
• Dents.
• Uneven wear.
• A blade that appears out of shape when rotated by hand.

If the blade is damaged, replace it immediately.

Do not attempt to straighten, weld or repair a damaged mower blade.

Step 4 – Check for Loose Blade Fixings

Inspect the blade retaining bolt and any associated fixings.

Ensure:

• The blade is securely fastened.
• There is no movement between the blade and blade adaptor.
• The retaining bolt is tightened to the manufacturer's specification.

A loose blade can also cause excessive vibration and should be corrected before operating the mower.

Step 5 – Check for a Bent Crankshaft

If both the blade and blade adaptor appear undamaged, inspect the crankshaft.

With the spark plug cap removed:

1. Carefully rotate the blade by hand.
2. Observe the movement of the blade and crankshaft.
3. Look for any wobble or uneven rotation.

Signs of a bent crankshaft include:

• Severe vibration while mowing.
• The blade moving from side to side as it rotates.
• Uneven or off-centre rotation.

A bent crankshaft is usually the result of a severe blade impact.

Frequently Asked Questions

What causes excessive vibration on a petrol lawnmower?

The most common cause is impact damage after the blade strikes a solid object. This can damage the blade, blade adaptor or crankshaft.

Can I continue using my lawnmower if it is vibrating?

No. Excessive vibration usually indicates a mechanical fault. Continuing to use the mower can cause further damage and may create a safety risk.

How do I know if my mower blade is bent?

A bent blade may appear uneven when rotated by hand and often causes noticeable vibration while mowing. Cracks, dents or twists are also signs that the blade should be replaced.

What does a blade adaptor do?

The blade adaptor connects the blade to the engine and is designed to absorb impact during a blade strike, helping to protect the crankshaft from damage.

How can I tell if the crankshaft is bent?

With the spark plug cap removed, slowly rotate the blade by hand. If the blade or shaft wobbles instead of rotating smoothly, the crankshaft may be bent.

Can a bent crankshaft be repaired?

A bent crankshaft is a major engine fault. Depending on the mower model and the extent of the damage, replacing the engine is often more practical and cost-effective than repairing the crankshaft.