My Petrol Strimmer/Brush Cutter will not rev up

Quick Answer

If your petrol strimmer or brush cutter starts and idles correctly but struggles to accelerate, will not reach full engine speed, or cuts out when the throttle is applied, the most common causes are stale fuel, a blocked air filter, restricted fuel flow, or incorrect carburettor settings. Follow the checks below before attempting any carburettor adjustments.

⚠️ Safety Notice

Before carrying out any maintenance or adjustments:

• Switch off the engine and allow it to cool completely before inspecting the machine.
• Wear suitable personal protective equipment (PPE), including gloves and eye protection.
• Keep the cutting head clear of people, pets, and obstacles.
• Carburettor adjustments should only be carried out using the correct tools and by someone with suitable mechanical knowledge.

Incorrect carburettor adjustment can affect engine performance and may cause engine damage.

Why Won't My Strimmer Rev Up?

If the engine starts but struggles to accelerate or reach full speed, possible causes include:

• Old or incorrectly mixed fuel
• A blocked or dirty air filter
• Restricted fuel flow
• Incorrect carburettor adjustment
• A blocked fuel filter
• A worn or fouled spark plug

Begin with the basic checks below before adjusting the carburettor.

Preliminary Checks

Check the Fuel

Ensure the fuel is:

• Fresh
• Mixed to the correct ratio recommended by the manufacturer
• Free from contamination

Old or incorrectly mixed fuel can reduce engine performance and prevent the engine from reaching full RPM.

Check the Air Filter

Inspect the air filter for dirt and debris.

A blocked air filter can restrict airflow, causing:

• Poor acceleration
• Reduced engine power
• Engine hesitation
• Excessive exhaust smoke

Clean or replace the air filter if necessary.

Check the Fuel Cap

Ensure the fuel cap is fitted correctly and tightened by hand.

A loose fuel cap can affect fuel delivery, while an overtightened cap may restrict tank ventilation on some models.

Carburettor Adjustment

If the preliminary checks have been completed and the engine still will not rev up, the carburettor settings may require adjustment.

Important

⚠️ Carburettor adjustment requires mechanical knowledge and the correct adjustment tool.

Incorrect adjustment can:

• Reduce engine performance
• Cause engine damage
• Increase exhaust emissions
• Invalidate your warranty

If you are unsure, please contact our Technical Support Team before proceeding.

Tools Required

• Carburettor adjustment tool (Pacman type)

Adjustment Procedure

Step 1: Warm Up the Engine

Start the engine and allow it to reach normal operating temperature.

Carburettor adjustments should always be made with a warm engine.

Step 2: Locate the Adjustment Screws

The carburettor contains the following adjustment screws:

Step 3: Check the Low-Speed Setting

Before adjusting the high-speed circuit, ensure the L screw is correctly adjusted so that:

• The engine idles smoothly.
• The engine does not stall.
• The cutting head remains stationary while idling.

An incorrect low-speed setting can affect acceleration.

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

With the engine running at full throttle:

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

Avoid over-adjusting the H screw, as excessive engine speed can damage the engine.

If the Problem Persists

If the engine still struggles to rev after completing these checks:

• Inspect the fuel filter for blockages.
• Check the spark plug for wear or carbon deposits.
• Inspect the fuel lines for leaks or damage.
• Ensure the carburettor is clean and free from contamination.

If the problem remains:

• Contact our Technical Support Team for further diagnosis.
• If appropriate, a replacement carburettor may be recommended.
• If your machine is within its warranty period, our team can advise on inspection and repair options.

Frequently Asked Questions

Why does my strimmer idle but not rev up?

This is commonly caused by stale fuel, a blocked air filter, restricted fuel flow, or incorrect carburettor settings.

What does the H screw adjust?

The H (High Speed) screw controls the fuel mixture at higher engine speeds and affects maximum RPM and acceleration.

Should I adjust the H screw before the L screw?

No. Always ensure the engine idles correctly before adjusting the high-speed circuit.

Can old fuel affect engine performance?

Yes. Old or incorrectly mixed fuel can cause hesitation, loss of power, and poor acceleration.