How do I make sure that my drive belt is correctly tensioned on my Petrol Lawnmower

If your lawnmower's self-propel function is not working correctly, one possible cause is a loose, worn, or detached drive belt.

This guide explains how to inspect the drive belt and gearbox tension spring to ensure they are functioning correctly.

Table of Contents

1. Understanding the Drive Belt System
2. Checking the Drive Belt
3. Checking the Gearbox Tension Spring
4. Common Faults
5. Visual Reference Guide

Understanding the Drive Belt System

The drive belt transfers power from the engine to the gearbox, allowing the self-propel system to drive the lawnmower forward.

The belt is kept under tension by a gearbox tension spring, which connects the gearbox to the mower chassis.

If this spring becomes detached or damaged, the drive belt may lose tension, slip from the pulley, or fail to drive the mower correctly.

Checking the Drive Belt

Tools Required

• No tools are normally required for inspection.
• Pliers may be required if the gearbox tension spring needs to be reattached.

Step 1 – Locate the Drive Belt

The drive belt runs between:

• The blade adaptor beneath the mower deck.
• The gearbox pulley located on top of the gearbox assembly.

Step 2 – Check Belt Tension

With the engine switched off and the spark plug cap removed:

1. Carefully press the belt sideways.
2. You should feel a small amount of movement.
3. The belt should feel secure and remain correctly seated on the pulleys.

Excessive movement may indicate insufficient tension.

Step 3 – Inspect the Belt Condition

Check the entire visible length of the drive belt for:

• Cracks
• Fraying
• Splitting
• Excessive wear
• Glazing or polishing

If any of these signs are present, the drive belt should be replaced.

Checking the Gearbox Tension Spring

The gearbox tension spring is responsible for maintaining tension on the drive belt.

Inspect the Spring

Check that the spring is:

• Securely attached at both ends
• Free from damage
• Not stretched or distorted
• Free from debris or obstructions

If the Spring Is Detached

If the spring has become detached, the drive belt may slip off the pulley or fail to maintain the correct tension.

The spring should be reattached securely.

If the spring is damaged or missing, it should be replaced before further use.

Common Faults

Belt Has Slipped Off the Pulley

This is often caused by:

• A detached gearbox tension spring
• Excessive belt wear
• Incorrect belt installation

Belt Is Damaged

A worn or damaged belt may:

• Slip under load
• Reduce self-propel performance
• Prevent the self-propel system from operating altogether

Replacement is recommended.

Spring Is Missing or Broken

Without the gearbox tension spring, the drive belt cannot maintain the correct tension and the self-propel function may fail.

Visual Reference Guide

Please refer to the accompanying images for guidance.

*View of the gearbox, pulley (white wheel) and GB tension spring (black spring)*

*Spring and belt fully fitted* 

*Spring detached, and belt slipped off the pulley*