- A petrol strimmer that will not start after months in the shed almost always has a fuel, air or spark fault, and you can check all three at home.
- Old petrol that has soaked up water is the first suspect, so drain the tank before touching anything else.
- Line that has vanished into the head needs a clean-out and a fresh length soaked in water for 24 hours, not a new strimmer.
The Strimmer Dies on the First Pull, and the Cause Is Usually Cheaper Than the Repair Shop
You pull the cord. Nothing. You pull again, ten times, and your shoulder starts to complain. Then you switch to the other fault: the machine runs, but the line has shrunk to a stub or disappeared altogether.
Both problems are fixable at home. The petrol engine in a strimmer works like the one in a car, and it needs three things: fuel, oxygen and a spark. A trimmer that will not start is nearly always missing one of them. Work through them in order: fuel first, then the air filter, then the spark plug. The line fault comes after that, and the cure is a clean head and a soaked length of new line.
Everything below is for petrol machines first. Battery strimmers get their own short section later, as their faults are much rarer.
Fuel Goes Off in the Tank While the Strimmer Sits
Start with the tank. Petrol usually contains ethanol, which breaks down over time, so the fuel should be fresh, ideally no more than 30 days old.
Here is the mechanism. Ethanol pulls moisture out of the air. That moisture works its way into the tank and collects as water droplets, which dilute the petrol. Dave Trezza, the Consumer Reports engineer who oversees string trimmer and outdoor power tool testing, explains the result: “That water-ethanol blend will separate from the gasoline, which can make a string trimmer tough, if not impossible, to start come spring.”
The machine sits for months, which gives the fuel a long time to soak up water. Craig Murray, owner of Liffco Power Equipment, gives the same warning about spare cans: “Don’t leave cans of gasoline sitting around, because the fuel will absorb water, which makes motors run rough. Funnel whatever’s left into your car’s tank, then buy fresh gas come springtime.”
So tip out whatever was left in the strimmer and fill with fresh fuel. If your machine has a two-stroke engine, oil goes into the petrol in the ratio the manufacturer recommends. Put the recommended amount of oil into the storage can first, then pump in one gallon (3.8 litres) of petrol and shake it thoroughly before it goes into the tank. The mix and fuel-type background is in this post on two-stroke and four-stroke fuel needs.
Check the Fuel Filter Inside the Tank
Once the tank is empty, look for the fuel filter. In most strimmers it sits inside the tank: a small mesh or plastic cylinder, just larger than a thimble, on the end of a rubber hose the size of a drinking straw. Dirt, leaves or grass in the tank clog it, and a blocked filter stops the engine drawing petrol at all. A full tank then does no good.
Pop the old filter off and fit a new one. Home centres and most hardware shops stock filters made for specific models, so take the model number with you. Check the manual, but typically a fuel filter needs replacing every two to three years.
Air Filter and Spark Plug Come Next
If fresh fuel and a clean filter do not fix it, move to the air.
The air filter usually hides behind a plastic cover next to the engine. Press a tab or two and the cover comes off. Inside you will find either a small paper filter about the size of a matchbook, like a miniature of the one in a car or furnace, or a ring of foam. Look for clogs and blockages. A filter that cannot pass air starves the engine of oxygen, so a blocked one is a fault worth ruling out even when the fuel is good. Trimmer air filters get dirty quickly in dust. A clean one gives the engine the right mix of air and fuel, so clean or replace it, and replace it annually for peak performance.
The spark plug is the last of the three. It is a porcelain-coated igniter, and each pull of the cord makes it spark and light the fuel and air. Over time a plug gets damaged, or coated in crud or grease, and then it cannot spark. Check it every year and replace it if it looks bad.
To remove it, follow the maker’s instructions. That usually means pulling the rubber boot off the plug and using a special socket wrench sized for that plug. The manual normally gives the socket size, and many makers sell the wrench. Buy a spare plug at the same time and fit it by reversing the steps. A new plug is a small cost against a dead machine.
The same three checks apply to a mower, as this post on a mower that is hard to start shows.
The Line That Vanished Into the Head
Now the second fault. The engine runs, but there is no line to cut with.
Strimmer line is meant to fail. It is designed to break if it hits something too hard, so it does not wrap around the object. It also wears down gradually and snaps when it gets too thin. That wear is why a feed system exists. There are three types:
- Bump feed: you bang the strimmer head on the ground to release more line.
- Automatic feed: more line is fed out when the strimmer starts.
- Manual feed: you stop the machine and pull more line out by hand.
Whichever you have, you need to feed out more line regularly. Forget to feed it out and the line becomes too short and disappears back inside the head. If that has happened, you will have to open the head and rethread it.
Before you do, clean the head. Dirt and debris build up in the trimmer head and clog the line mechanism, so the head spins the line less well. Switch the machine off, wait for it to cool and put on gloves. Use a soft brush to clear grass from the casing, then turn the strimmer over and clear grass from under the guard and around the line feed.
Restring With Line That Has Been Soaked
Line stays flexible and lasts longer when it has absorbed moisture before use. Put it in water for a minimum of 24 hours first. A spool that has sat unused for an extended period should come out of the head and soak in water for two days before use. Keep a spare spool on hand, as line needs regular replacement.
If your head needs manual restringing, work through these steps:
1. Remove the spool from the head.
2. Cut an 8 foot (2.4 m) length of new line.
3. Feed one end through the hole in the spool and pull the ends even.
4. Wind the line round the spool in the direction of the arrow printed on it.
5. Hold the line ends in the notches on the spool’s edge.
6. Refit the spool, with the line ends feeding through the holes in the head.
7. Tug each line to release it from its notches.
Heads that wind themselves are simpler: feed the line through the spool and turn the head to wind it. If the line keeps snapping once you are back at work, the cause is usually one of the six covered in strimmer line that keeps snapping.
What to Do If It Is a Battery Strimmer
Battery machines rarely have starting problems, but they do have them. Check that the head is on properly and the battery is fully charged and seated in the tool. If that fails, put the battery in another compatible tool from the same brand, or fit a different battery in the strimmer. The test shows whether the tool or the battery is at fault.
A dead battery still under warranty goes back to the retailer or maker. Otherwise recycle it at a Lowe’s or Home Depot store. To protect batteries for next time, bring them indoors over the winter, away from extreme temperatures. If the tool itself is broken, it needs servicing or replacing.
How to Stop It Happening Next Winter (and When to Give Up and Call Someone)
A little work at the end of the season prevents all of this.
- Clean the head and casing as described above, with the machine switched off and cool, and any battery removed.
- Spray the metal surfaces lightly with WD-40, leave it for 10 minutes, then wipe dry.
- Empty the tank. Keep spare petrol in a dedicated container mixed with fuel stabiliser, following the manufacturer’s recommendation. Stabilised fuel should be good for up to 18 months. Write the date on the can so you know its age.
- Store petrol only in a container made for the job, as far from your living space as possible, ideally in a detached garage or shed. Never keep it indoors, stabilised or not.
- Bring battery packs inside.
Know when to stop. If the engine still will not start after fresh fuel, a new fuel filter, a clean air filter and a new plug, take it to be serviced. Which? notes that dearer brands can be taken to a garden machinery specialist for repair, while cheap plastic trimmers will likely end up in landfill if they break. That makes the checks above worth doing first: they cost little, and on a cheap machine they can be the only repair you can justify.
