Strimmer Won't Start? 5 Causes (And How to Fix Them Yourself)

Nine out of ten petrol strimmers that won't start come down to one of five causes: stale fuel, a fouled spark plug, a perished primer bulb, a blocked air filter, or a flooded engine from over-priming. Paul fixes the same five problems all season long at the Hyde workshop — and most can be tackled at home in under thirty minutes with basic tools.

A petrol strimmer that won't start is one of the most common problems Paul sees at the workshop. The good news is that most causes are simple and often fixable at home with basic tools.

Here are the five most common reasons, in order of how often Paul encounters them.

1. Stale Fuel (The Number One Cause)

If your strimmer has been sitting for more than a few weeks with fuel in it, this is almost certainly the problem. Petrol degrades quickly in small engines — the lighter compounds evaporate, leaving behind a varnish that blocks the tiny jets in the carburettor.

Fix it yourself: - Drain all old fuel from the tank - Remove the fuel filter (usually inside the tank on the end of the fuel line) and check it's not blocked - Refill with fresh fuel mixed at the correct ratio (usually 50:1 or 40:1 — check the manual) - Pull the cord 10-15 times with the choke on

2. Flooded Engine

If you've been pulling the cord repeatedly with the choke on, you've probably flooded it. There's too much fuel in the cylinder and the spark plug can't ignite it.

Fix it yourself: - Turn the choke OFF (open position) - Hold the throttle wide open - Pull the cord 10 times quickly — this clears excess fuel from the cylinder - If it still won't start, remove the spark plug, dry it off, and pull the cord a few times with the plug out to clear the cylinder

3. Spark Plug Issues

A fouled, worn, or incorrectly gapped spark plug won't produce a strong enough spark to ignite the fuel-air mixture.

Fix it yourself: - Remove the plug with the correct socket (usually 19mm or 21mm for most strimmers) - Check the electrode — it should be light brown. Black and wet means it's fouled - Clean off deposits with a wire brush, or simply replace it (most strimmer plugs cost £3-5) - Check the gap matches the specification in your manual

4. Blocked Air Filter

A clogged air filter restricts airflow and throws off the fuel-air ratio. The engine may turn over but won't run, or it starts briefly then dies.

Fix it yourself: - Remove the air filter cover (usually one or two screws) - If it's a foam filter, wash it in warm soapy water, squeeze dry, and lightly oil it - If it's a paper filter and it's dirty, replace it — they can't be effectively cleaned

5. Fuel Line or Primer Bulb Perished

The rubber fuel lines and primer bulb on two-stroke engines become brittle over time, especially in machines stored outdoors. Cracked lines let air into the fuel system, and a split primer bulb can't draw fuel into the carburettor.

Spot the signs: - The primer bulb doesn't spring back when you press it, or it stays depressed - Visible cracks in the fuel lines - Fuel leaking from the machine

This is an easy repair but requires the right replacement parts. Paul keeps fuel line kits and primer bulbs in stock for most common brands.

When to Bring It In

If you've tried fresh fuel, a new spark plug, and a clean air filter and it still won't start, the carburettor likely needs stripping and cleaning. This is the most common professional repair for strimmers — and it's what Paul does every day.