Why Your Brush Cutter Won't Start, and What to Do About It

Updated on December 14, 2017
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Mary is an organic coconut farmer. In her articles, she shares ideas on land management and how to increase the profit from a small farm.

Stihl Brush Cutter Motor
Stihl Brush Cutter Motor | Source

What to Check When Your Brush Cutter Doesn't Start

You go out to start your brush cutter, weed whacker, or string trimmer and it won't start. If you have tried a few times and it still won't start and you smell gasoline, you've probably flooded it. Wait about 10 minutes before you try again to give the gas time to evaporate.

If it still won't start, I'd like to tell you the procedure we use here on our farm when this happens and how we solve the problem with our Stihl brush cutter. Although we have a Stihl, most brush cutters and string trimmers will be similar to work with, use these steps as guidelines.

The first thing my husband does is swear. This step is completely optional but it seems to help him.

The problem may be:

  • The spark plug
  • The air filter
  • The fuel
  • The carburetor
  • The solenoid

Let's look at each of these steps, to pinpoint the problem and find the solution to get you back to work.

Checking the Spark Plug

Remove the spark plug and if there's gas on it, that's good, the gas is getting through.

The spark plug may be getting coked up if your engine isn't running at its best. Also if your air filter is dirty this will affect your spark plug.

If your spark plug is dirty use an emery board to remove any deposits around the electrodes. If you have a gapping tool which measures the distance, in a spark plug, it should be 0.02 inches (0.5mm). If the gap is too large use a pair of pliers to minimize the space.

The spark plug should be good for roughly 100 hours of use so if yours is getting close to those hours, this is an easy fix.

Checking the Air Filter

The next step should be to check your air filter. If you've been working in a dusty environment it could be clogged. Tap out the dust and if yours is a type which is felt you can wash it. Leave it to dry before returning it to the machine.

This is something which you'll want to check frequently if your cutting is kicking up a lot of dust. By cleaning the air filter, this will stop that coughing and sputtering noise you may be experiencing with your machine.

Carburetor Problems in a Brush Cutter

One of the last things to check will be the carburetor, this could likely be the cause if your brush cutter has been left standing for some time as gasoline may have clogged it up. As the gasoline begins to evaporate, you're left with a sticky residue.

Here on our farm, the carburetor is our main problem as the gasoline where we live is combined with alcohol and is full of grit. Although we use a funnel with a fine filter, small debris gets through and can clog up the jets in the carburetor.

The quality of fuel you use is important and a higher octane (89 or above) is suggested by most manufacturers of brush cutters. When the octane is lower, you'll get a pinging and your machine will run less efficiently.

If you don't know the recommended octane level for your brush cutter, check your manual which came with your string trimmer. If you don't have it any longer, most manufacturers offer this online and you can download it. In there they will tell you not only the recommended octane of gasoline to use but also the correct gas to oil mixture. Your manual will also show you where the high and low-speed adjustments are (if any), some less expensive models don't have this adjustment. These adjustments will you alter the amount of fuel/air going in so your machine will be working at its best. Instructions are also shown in the video below.

If your carburetor is dirty, wipe it over but don't use a carburetor cleaner, as the circuits may be damaged by this spray. You are best to use an ultrasonic cleaner.

*Note* I have included a video below where the man is using a carburetor cleaner, however, I have spoken with a Stihl repair center and they discourage using aerosol cleaners on the carburetor.

Replacing a Stihl Brush Cutter Carburetor
Replacing a Stihl Brush Cutter Carburetor | Source

Changing the Carburetor on a Brush Cutter

If your brush cutter won't start and you have had it for about 2 years, then in our experience the problem is likely to be your carburetor. Learn from our experience here, my husband has tried cleaning it and adjusting it as a way of saving money. This may work in the short-term and get it going for awhile but your time is valuable. This problem is going to reoccur and so now, when my husband knows the carburetor is on its way out, he gets a new one. For us, our nearest parts center is 40 miles away and when we know there is a problem, the time spent trying to repair the part just isn't worth it. Your time is too valuable and the grass and the weeds aren't going to stop growing because you have a problem with your brush cutter.

Buy the part and replace it yourself. Once you can do this simple repair job, you will become more confident and will begin to realize that you can, in fact, repair virtually everything on your brush cutter. Not only does this save you money, it allows to solve the problem quickly and keep your homestead or farm maintained and minimizes the downtime.

Solenoid on Brush Cutter

Recently we had to change the solenoid on our brush cutter. You may be wondering how my husband knew it was the problem and not something else. To understand this you need to know what a solenoid does. It simply changes an electrical current into mechanical energy. If you have changed your spark plug and you still aren't getting a spark, it's the solenoid that needs replacing. We bought the part, my husband replaced following the instruction regarding the size of the gap need and a few minutes later the machine was working.

Not only was it running again, it was almost like he had a new machine. Although we have had the brush cutter for 8 years, this was the first time he had to change the solenoid.

Replacing the Line on a String Trimmer and Brush Cutter

Now that you have got your machine working again, you'll need to wind your string on. If you're serious about your cutting and maintaining your machines, rewinding your string is something you'll need to learn.

Here on our farm, we don't mess about with those small packs seen hanging in the garden center or nursery, we buy a large roll as it works out so much cheaper.

Maxpower 333695 Residential Grade Round .095-Inch Trimmer Line 855-Foot Length
Maxpower 333695 Residential Grade Round .095-Inch Trimmer Line 855-Foot Length

Because we can't afford any downtime here on our farm, we buy a large roll. For us, we don't tend to buy the brand names because they are expensive. However, cheap lines can break more easily. The Maxpower is a good compromise for us. We use ours on grasses and small weeds including some dense clumps of grasses. If we have shrubs with a thick woody stalk and there are loads of them in one section, my husband would forego the line and put the blade on instead. For 98% of the cutting on our farm, we use this line.


Brush Cutter, Weed Whacker, String Trimmer

Although they are basically the same type of cutters, they are known by different names. The brush cutter is a more robust version of a string trimmer. On some models, it will come with a blade, chainsaw or flailing chain attachment to use on scrubland or small coppices. The motor capacity will also be greater as will the weight of the machine.

Some models will also have different styles of support systems. For example, ours has a crossbody strap, others have a backpack and some, which are lighter weight, have no support.

Some of the names you may have heard are:

  • String trimmer
  • Brush cutter
  • Strimmer (UK)
  • Bush Cutter
  • Weed Whacker
  • Weed Eater

Using our Stihl brush cutter
Using our Stihl brush cutter | Source

Learn to Troubleshoot

If your brush cutter or string trimmer won't start you need to act quickly. Here on our farm, our Stihl brush cutter is a crucial piece of equipment and we can't afford it have it not working. Therefore, my husband has learned to troubleshoot quickly.

We know that whether we're clearing a large piece of land or simply cleaning up an area around the lawn, being without the right equipment at the right time can make the job more difficult.

It isn't a case of putting it down and hoping it will work the next time it's picked up, the problem needs sorting immediately so we can carry on with our work on our farm.

Are You Using Poor Quality Gasoline?

If it has been some time since you started your machine the first thing to check is that the fuel lines aren't blocked. Where we live, our gasoline is rather poor quality and many times, although we use a filter in our gasoline funnel, small bits of debris find their way into our gasoline tank. This doesn't only cause problems with the hoses, but also the tubes.

Before we leave the subject of gasoline, if you have finished using it for the season, make sure you leave the tank empty, gas can bung up and you don't want to have to solve the problem come spring when you need to use it again. This is an easy way to prevent future problems.

String Trimmer Keeps Cutting Out

If you can get your brush cutter going, but it keeps cutting out and stalling, you might be wondering why this is occurring. This too can be caused by problems with your carburetor. Follow the guidelines above in the video and in your manual to find the perfect balance of air/gas for your machine.

When using the brush cutter, try and ensure you vary between low and high revs as using it only on low is likely to cause carbon problems and get coked up. You don't have to run it with the throttle wide open, they are made to be working with a load.

Do you do your own repairs on your brush cutter or string trimmer?

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© 2017 Mary Wickison


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    • Blond Logic profile image

      Mary Wickison 7 weeks ago from Brazil

      Hi Anika,

      In our experience, it is likely to be a fuel issue. You would be best to clean your carburetor and start with fresh fuel which you have passed through a filter. It's a good idea to check your air filter as well.

      I can't see the rain affecting anything because it is a closed system. That said, if you were caught in a downpour, your air filter could be damp.

      Open the cover and let things dry out.

      In this article, I've highlighted several reasons why it may not start but for us, it usually comes down to poor quality gasoline.

      Buy a high octane gasoline if you can and pass even that through a filter.

      Hope that helps.

    • profile image

      Anika Clarke 7 weeks ago

      I usually use my brushcutter to cut sometimes it gets are little wet from the rain I don't know if its because it gets wet but it won't start can you help.

    • Blond Logic profile image

      Mary Wickison 3 months ago from Brazil

      Hi Dhanaji,

      I hope it does help you. When one thing doesn't work it affects other things. Nowadays people are so busy and just need their machines to work as intended.

      Thanks for your comment.

    • profile image

      Dhanaji Sakya 3 months ago

      Great input, it will definitely help me. I will try & let you know if any problem.

    • Blond Logic profile image

      Mary Wickison 9 months ago from Brazil

      Oh, the air can get quite blue here on our farm. Repairs always seem to be needed at the worst possible time. The weeds just keep on growing and any delay can make the job more difficult.

      Thanks for reading Shauna.

    • bravewarrior profile image

      Shauna L Bowling 9 months ago from Central Florida

      I got a chuckle out of the first step of troubleshooting: swearing.

      Great step-by-step instructions, Mary!

    • Blond Logic profile image

      Mary Wickison 10 months ago from Brazil

      Hi Suhail,

      Pleased you found it useful. The brush cutter is such a useful piece of equipment on our farm.

      Thanks for reading.

    • Suhail and my dog profile image

      Suhail Zubaid aka Clark Kent 10 months ago from Mississauga, ON

      Great advice here, Mary.


      Suhail and my dog K2

    • Blond Logic profile image

      Mary Wickison 10 months ago from Brazil

      Mr. Holland,

      I think you underestimate your DIY skills.

      I suspect you are more than capable of repairing a weed whacker.

      Thanks for reading.

    • billybuc profile image

      Bill Holland 10 months ago from Olympia, WA

      I don't know if I mentioned this before, but we call them weed whackers here, and I love using one. Now, would I disassemble one and try to fix it? Maybe, if I didn't care about the outcome. LOL Thanks for the tips and instruction, and have a great Thursday.