My Petrol Hedge Trimmer will not rev up

If your petrol hedge trimmer starts and idles correctly but struggles to accelerate, will not reach full speed, or cuts out when the throttle is applied, this guide covers the initial checks and adjustments you can perform.

Table of Contents

1. Preliminary Checks
2. Carburettor Adjustment – High Speed (H) Screw
3. Unable to Perform This Procedure?

Preliminary Checks

Before adjusting the carburettor, carry out the following checks:

Fuel

Ensure the fuel is fresh and mixed correctly according to the manufacturer's recommendations.

Old fuel or an incorrect fuel mixture can cause poor engine performance and prevent the engine from reaching full revs.

Air Filter

Inspect the air filter and ensure it is clean and free from debris.

A blocked air filter can restrict airflow and reduce engine performance.

Fuel Cap

Check that the fuel cap is fitted correctly and tightened by hand.

An incorrectly fitted fuel cap can affect fuel delivery and engine operation.

Carburettor

If the above checks have been completed and the problem remains, the carburettor settings may require adjustment.

Carburettor Adjustment – High Speed (H) Screw

Important: Carburettor adjustment requires a good level of mechanical knowledge. Incorrect adjustment may cause poor performance, engine damage, or invalidate your warranty.

Tools Required

• Carburettor adjustment tool ("Pacman" type)

Step 1 – Warm Up the Engine

Start the hedge trimmer and allow it to reach normal operating temperature.

Step 2 – Locate the Adjustment Screws

The carburettor contains two adjustment screws:

• L (Low Speed) – Controls idle and low-speed performance.
• H (High Speed) – Controls maximum engine speed and acceleration.

Step 3 – Check the Low-Speed Setting

Before adjusting the high-speed circuit, ensure the L screw is correctly set so that the engine idles smoothly without stalling and the blades remain stationary.

Step 4 – Adjust the High-Speed (H) Screw

With the machine running at full throttle:

1. Turn the H screw anti-clockwise until the engine begins to struggle to reach maximum RPM.
2. Slowly turn the screw clockwise until the engine runs smoothly and reaches maximum RPM again.
3. Test the throttle response to ensure the engine accelerates cleanly without hesitation or stalling.

Once adjusted correctly, the engine should rev freely and respond smoothly to throttle input.

Unable to Perform This Procedure?

If you are not confident carrying out carburettor adjustments, we recommend that you do not proceed.

Attempting to dismantle or adjust components incorrectly may invalidate your warranty and could result in further damage to the machine.