Skip to main content

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

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

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've 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 gasoline time to evaporate.

If it still doesn'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 FS85 brush cutter. Since writing this article, we have updated our machines. We have a Stihl FS350 and a Branco 52A. The advice given here pertains to most brush cutters and string trimmers, use these steps as guidelines in conjunction with your manual.


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 cutting grass and weeds.

Checking the Spark Plug

Remove the spark plug, and if there's gas on it, that's good, it means 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 fine emery paper to remove any deposits around the electrodes. If you have a gapping tool that measures the distance, it should be 0.02 inches (0.5mm) in a spark plug. If the gap is too large, use a pair of pliers to minimize the space. Other brands of brush cutters may have a different recommended gap, so check your manual. If you no longer have your users manual, these are often available online.

The spark plug should be good for roughly 100 hours of use, so this is an easy and inexpensive fix if yours is getting close to those hours. It's a good idea to have a spare one so you can replace it and get cutting again.

Checking the Air Filter

The next step should be to check your air filter. It's easy to think that cutting grass and weeds shouldn't clog it, but if you've been working in a dry and dusty environment, it could be the problem. Tap out the dust, and if yours is felt, you can wash it. Leave it to dry completely before returning it to the machine.

This is something you'll want to check frequently if your cutting is kicking up a lot of dust. Cleaning the air filter will stop that coughing and sputtering noise you may be experiencing with your machine. By periodically checking the air filter, you'll be helping your machine work to its best. It's an easy thing you can do as a preventative measure to avoid more significant problems.
We live near dunes and have strong winds, so dust particles are a common problem for us. As such, our air filter gets check regularly.

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 the gasoline may have clogged it up. As the gasoline begins to evaporate, a sticky gummy residue is left behind.

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 that came with your string trimmer. It 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 allow you to 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

Changing the Carburetor on a Brush Cutter

If your brush cutter won't start, and you have had it for about two years, then, in our experience, the problem is most likely your carburetor. Learn from our experience here; my husband has tried cleaning it and adjusting it to save money. Doing this may work in the short term but, this problem is going to reoccur. So now, when my husband knows the carburetor is on its way out, he gets a new one. Our nearest parts center is 40 miles away, so we try and keep a spare carburetor on hand. The time spent trying to repair the part isn't worth it.
Buy the part and replace it yourself. Once you can do this simple repair, you'll become more confident and begin to realize that you can repair virtually everything on your brush cutter. Not only does this save you money, but it also allows you to solve the problem quickly and keep your homestead or farm maintained and minimizes any 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 it following the instruction regarding the size of the gap needed and a few minutes later the machine was working.

Not only was it running again, but it was also almost like he had a new machine. Although we have had the brush cutter for eight 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 which works out much cheaper.

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

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 to 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 the 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 relatively poor quality, and many times, although we use a filter in our gasoline funnel, tiny 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 want to avoid the potential problem come spring when you need to use it again. This is an easy way to prevent future problems.

Another preventive measure regarding your gasoline is when you are refilling your tank. Be aware of where you're doing this. When you remove the cap, don't allow sand, cut grass, or anything to enter your tank. Work cleanly to avoid any potential problems down the road. If necessary, take a small sheet of plastic or cloth to put the machine on. You'll likely have tiny particles of grass or debris on your gloves. Again, avoid this going in your tank.

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.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: When I tried to start my two stroke brush cutter, it has no sparks and fuel started coming out of the exhaust pipe. What do I do?

Answer: The first thing is to check your spark plug. In the article I mention things to look for. Clean it, and check the gap is correct. Or buy a new spark plug. Changing the spark plug is an easy fix. The gas coming out is because it isn't being burned.

If you change the spark plug and it doesn't rectify the problem, it's likely your solenoid.

Question: I used a 2-stroke line trimmer without oil now the machine won't start. What's the problem and how do I start it again?

Answer: I am sorry to be the one to give you the bad news but it's very likely you've seized up the engine. My advice is to go and buy another machine.

Question: My Stihl fs160 keeps dropping power in the process of running. What could be the problem?

Answer: Here are a few things to check:

It could be the exhaust outlet is blocked with carbon deposits.

Remove exhaust and check and scrape away carbon if necessary.

It may also be dirt in the carburetor. Remove the carburetor and clean. Clean or change gaskets and diaphragm if necessary.

Check your fuel lines are clear and clean.

Check your fuel tank filter is clean.

Question: When I tried to start the Weed Eater, it blows gasoline out of the carburetor, why is that?

Answer: It sounds like you have a blockage. Check the carburetor for debris and if that doesn't solve it, check your fuel lines.

Question: When I tried to start the brush cutter I noticed that the spark plug was becoming dry, so what is the cause of that?

Answer: The gasoline isn't getting to the spark plug. It may be a clog in your tubes or in the carburetor.

Question: I have GMC GL25 Model line trimmer, and it's hard to get parts for it. Now I think I need to change the carburetor. How do I find a suitable replacement?

Answer: It can be frustrating when that happens. If you have a service center, they may be able to advise you if parts are available. Sometimes other models will fit, and a service center should be able to give you that information. If you've checked online on sites such as eBay and Amazon, and still can't find the required part, then it may be time to replace the machine for one which has parts readily available.

It is poor customer service when that happens, and it makes people less likely to buy from that brand again.

I hope you find a resolution to your problem.

Question: I have a wolf garden brush cutter which I've had for a year now. When I start it, it idles fine. The problem arises when I rev it, it stops running. Its as though it doesn't have enough power to run at full capacity. What's the problem, and how can I fix it?

Answer: You should clean or change your carburetor. It sounds as though the jets are clogged. This could be due to dirty fuel or leaving the machine for a time without using it.

Question: I have a brand new brush cutter which I used the day I bought it and it was working well. But, when I tried restarting it, it won't start. I have tried all of the above but it still won't start. How can I troubleshoot my brushcutter's problem?

Answer: Take it back to the store you bought it from. It will be under warranty.

Question: Is there a way of checking if the spark plug is sparking?

Answer: Yes, there is. Take the spark plug out but leave it connected to the cable. You have to ground this by placing it on a metal part of the machine. Ideally on top of the cylinder head. DO NOT put it anywhere near the gas tank!

Set the machine to start and pull the cord. You should see a spark. If it is 'white/blue' that is good. If it is looking yellow/orange it's getting weak.

Changing the spark plug is an easy fix. Also, check to see if the gap is correct by using a gapping tool. For a Stihl brush cutter, the correct gap is 0.02". Check your manual or online for your specific measurement.

Question: How do I make strings go back in?

Answer: I am not sure if you are referring to the cutting nylon or the pull cord. If it is the cutting cord, you have two options. Just cut off the excess and continue or remove the bobbin and rewind the cutting line.

If it is the starting pull cord, you will need to remove the cover and rewind the pull cord. Check for any signs of wear or an obstruction that could have caused it not to recoil as normal. Occasionally one of the small plastic pieces inside can snap. This is something that occurred on our Stihl after many years. It would be best to replace it with a new piece other than try and glue it back together.

© 2017 Mary Wickison


Mary Wickison (author) from USA on May 03, 2020:

Hi John,

I have the same issue with a larger machine we have here on our farm. Ours is a Branco which is more powerful than our Stihl and for me, more difficult to start because as you say, it pulls the starting cord back.

Only recently have I been able to start it, to tell you the truth. What I do, First I pump the primer bulb. Then I also move the throttle to get the gas into the carb. I open the choke for the first pull only.

Sometimes the cord will only come our about 6 inches. There is a lot of compression in some of these larger models.

I have found standing up and placing my left hand on the head gives me the right angle. Occasionally I flood it and have to leave it and return in a few minutes.

You'll find a position, either kneeling or standing that works best for you.

Keep your elbow tucked in as you pull so that the cord is running straight out of the machine not on a slight angle.

John Joseph Sakep on April 21, 2020:

I have hipowa string cutter, and it's hard to start as I pull because it pulls up abit harder. What do I do now.

Matthias Goodson on March 19, 2020:

Thanks very much! because I was wondering but now is working

Mary Wickison (author) from USA on May 20, 2019:

Hi Livingstone,

Yes it could be the cause. Clean your spark plug and carburetor and then begin again with the correct ratio of gasoline and oil. The ratio will be in your manual. If you don't have one, they are normally available online for free.

Livingstone on May 20, 2019:

Does too much oil cause my brushcutter not to start fs160

Mary Wickison (author) from USA on March 24, 2019:

You should take it to a repair center and speak with them. When they hear it, they may be just as happy to sell you the parts and let you do the work. If the cost is too high for a repair job, then you'll need to purchase a new machine.

Joe Cardoso on March 24, 2019:

I have a Ryobi 4300A brush cutter that has run well for two years. We use it about 25 hours per month in the summer months. We stopped using it recently when it started making a rough noise at high rpm, but this goes away at idle. Could this be a big end bearing and is this an easy repair.

aliki taria on October 13, 2018:

my rayban brushcutter not supplying fuel to the oil cup

Mary Wickison (author) from USA on January 02, 2018:

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.

Anika Clarke on December 31, 2017:

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.

Mary Wickison (author) from USA on November 18, 2017:

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.

Dhanaji Sakya on November 18, 2017:

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

Mary Wickison (author) from USA on May 25, 2017:

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.

Shauna L Bowling from Central Florida on May 24, 2017:

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

Great step-by-step instructions, Mary!

Mary Wickison (author) from USA on April 22, 2017:

Hi Suhail,

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

Thanks for reading.

Suhail Zubaid aka Clark Kent from Mississauga, ON on April 22, 2017:

Great advice here, Mary.


Suhail and my dog K2

Mary Wickison (author) from USA on April 20, 2017:

Mr. Holland,

I think you underestimate your DIY skills.

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

Thanks for reading.

Bill Holland from Olympia, WA on April 20, 2017:

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.