Most often a chainsaw refuses to start because the engine is short of fuel or air. The cause is usually simple: an empty tank, the wrong mix of petrol and two-stroke oil, a clogged filter or a fouled spark plug.
Most of these faults can be fixed yourself in about half an hour. Regular care and correct storage prevent most stoppages.
What you will need
Important: Work only on a cold engine, away from naked flames, in a well-ventilated place. Pull the HT cap off the spark plug before removing it to prevent an accidental start.
Step 1
Find the cause
Start with the tank: there must be enough fuel, and the mix must be fresh and made in the ratio the manufacturer specifies. Petrol that has stood for several months loses its lighter fractions and the saw struggles to fire.
Next check the air supply: a blocked air filter starves the engine of oxygen. A dirty fuel filter in the tank, a dirty or badly adjusted carburettor and a sooted spark plug also prevent starting. If the saw still will not start after all these checks, the engine may be damaged and a service centre is needed.
Step 2
Clean the filters
Remove the cover and take out the air filter. Blow it out with a compressor from the inside outwards, or wash it in warm soapy water and let it dry. Replace a torn or fuel-soaked filter.
Fish the fuel filter out of the tank with a hook and rinse it in clean petrol. If it has gone dark and hard, fit a new one.
Step 3
Clean the spark plug
Unscrew the plug with the spanner. Polish the electrode bright with fine sandpaper and brush the thread with a wire brush.
Then spray the plug with WD-40 Specialist Contact Cleaner: it dissolves carbon and dirt, evaporates quickly and leaves no film. Wipe with a cloth, check the gap and refit. Often this alone is enough to get the saw running.
Step 4
Drive moisture out of the ignition
In cold damp weather, especially in autumn and winter, condensation forms on the HT lead and plug cap and weakens the spark. Spray WD-40 Multi-Use onto the cap and lead: it displaces moisture and restores the contact.
Then start following the manual: switch on, close the choke, prime the bulb and pull the cord in smooth, even strokes.
Step 5
Lubricate and protect against rust
To make the next start easy, clean the body after work and treat metal parts, cables and pivots with WD-40 Smart Straw. The fine straw reaches tight spots, while the product lubricates, displaces moisture, removes grime and frees seized parts.
Step 6
Store the saw correctly
Before a long break, drain the fuel and let the saw idle until it stops. This keeps the carburettor diaphragms from sticking with old fuel residue. Keep the chain lubricant tank full instead, so the pump does not gum up.
Store the saw in a dry, ventilated room out of direct sunlight, in a cover or case and out of reach of children.
Tip: Keep a spare spark plug and air filter in your toolbox. A swap takes two minutes, while hunting for the fault without spares can take half a day.
What next
Clean the saw after every use, check the filter and plug every 10-20 operating hours, and give it a full inspection before the season and after winter storage. See our separate guide for chain care.
These uses come from the experience of WD-40 users in many countries. Always follow the instructions and warnings on the product label.
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