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Weed Eater Smoking: Causes and Solutions


What causes a weed eater to smoke (and how to fix it).

I ran into some trouble recently when my weed eater started smoking. I decided the best way to go about this is to research what had caused my weed eater to smoke and try to find ways to prevent this from happening again. 

A weed eater smoking is mainly caused by poor combustion. When the choke is not turned off after ignition, it can cause black smoke to emit as a result of incomplete burning of the fuel. Some of the solutions to preventing any kind of smoke are to turn off the choke, letting the engine cool down, and using a different type of fuel. 

Knowing basic repairs like these if something goes wrong can save you money in the long run. Let’s look at the causes and solutions of a weed eater smoking.

Inefficient Combustion Causes a Weed Eater to Smoke

Weedeater engines usually comprise a chamber where the fuel interacts with the air, which results in a chemical reaction known as combustion. In some cases, there can be too little or too much oxygen circulating in the engine, and too little air may cause inefficient combustion. 

When the fuel is too rich, there is not enough air to fulfill the chemical reaction completely, thus causing black smoke. 

Inefficient combustion itself is usually caused by:

  • Not turning off the choke on the engine. The choke is a valve that lets air into the engine, and closing this valve increases the ratio of fuel to air; thus, forgetting to turn it off after the engine starts as you are supposed to can cause your weed eater to smoke.
  • Fuel that is too old. As you can probably figure out on your own, older fuel tends to burn less efficiently because it has already been exposed to oxygen, and it can even oxidize your engine and cause even more damage. 

Luckily, fixing your black smoke issue is rather easy, no matter what the cause is. 

How to Fix Black Smoke from Your Weed Eater

Firstly, ensure that you have read the manual for your weed eater – it will have thorough instructions on how to close the choke and run the engine to avoid any damage. If you have done that, then you will be quick to realize if you have been working with your weed eater incorrectly and not closing the valve properly – this is an easy fix, of course.

As a general rule, it is wise not to leave fuel in your engine for more than 30 days because that is how long it takes it to oxidize, and you don’t want that! 

If your engine starts emitting black smoke and you are certain that the closure of the choke is not the issue, your next best bet is to replace the fuel. Although this may seem like a waste, you will know what to do in case you run into this problem again. 

Pro Tip: Reduce the risk of damage and clogging by using an ethanol-free fuel. Ethanol causes serious issues for small engines. As a bonus, ethanol-free fuel has a longer shelf life than gas from the pump.

The Gas/Oil Balance is Extremely Important

Weed eaters usually contain a mixture of both oil and gas unless you have a four-stroke engine, which tends to produce fewer emissions and avoids the issue of an incorrect gas to oil ratio altogether. 


If you start noticing that your weed eater is either leaking oil or emitting blue or gray smoke, then this is an issue of the oil to gas ratio. 

  • Too much oil. The gas serves as the fuel, which powers the motor while the oil lubricates the piston and ensures that the engine runs smoothly. However, too much oil can also cause some problems. 
  • Too little oil. Conversely, too little lubrication can cause gray smoke to come from your engine, and this usually means that your weed eater is overheating. Gray smoke can be the most dangerous because it can result in an engine seizure, which cannot be repaired. If adding more oil does not fix the issue, then you should replace the motor brushes. 
  • Water damage. Additionally, if you have a cordless weed eater, gray smoke can be a sign of water damage; be careful because there is a risk that your weed eaters short-circuit. 

Blue or gray smoke also has the potential to damage your weed eater unless you take the right repercussions and turning it off immediately after you start noticing the smoke is the best first step you can take. 

How to Find the Perfect Gas to Oil Ratio

Finding the perfect gas to oil ratio starts with figuring out the motor type of your weed eater and getting the proper directions straight from your manufacturer. Always start by carefully reading the manual you have already been provided with. 

There is three common gas to oil ratios to choose from 50 to 1, 40 to 1, and 32 to 1, with the most common one being 40:1; however, absolutely always double-check the model of trimmer you have. Here are a couple of standard measurements that you might end up using (128 oz is equal to 1 gallon). 

For instance, if the gas-to-oil ratio is 32:1, the volume of gas will be 32 ounces for every one ounce of oil or 128 ounces of gas to 4 ounces of oil.

Pro tip: You can save yourself a lot of headaches by using commercial premixed fuels, plus they have a much better shelf life. See my comparison of homemade premix vs. commercial premix.

Dirty Mufflers and Carburetors can also Cause Issues

The muffler is the part of the weed eater where the exhausts from the fuel usually end up exiting the machine in order to avoid overheating and damage. 

After using your trimmer for a couple of years, you may notice smoke emitting from the mufflers, which is a sign that there is a build-up of carbon exhaust that has essentially caught on fire

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Luckily, mufflers can be cleaned easily, and this issue will be gone in no time. 

Dirty Carburetor

Another potential cause of smoke can be a dirty carburetor, which is where the oil and gas usually mix in a weed eater. Carburetors, like mufflers, can get clogged. However, the process of cleaning these can be a bit more complicated. 

It is recommended that if you have determined a dirty carburetor to be the issue of your weed eater, then it is best to visit a professional mechanic to get it cleaned out

Even if you pay someone to get this fixed for you, it is still relatively quick and inexpensive, and after all, if it makes your weed eater last longer, it is worth it in the end.  

How to Properly Clean Weed Eaters

The first step to cleaning weed eaters is, of course, disconnecting them from the power source and taking out the batteries to lessen the likelihood of damage. 

To clean out the mufflers:

  • Use a tube brush and scrub out all of the excess carbon waste. 
  • You can also use a brush, water, and soap to clean all the visible dirt off of the weed eater. 
  • Make sure that you dry all parts of it after cleaning. 
  • Mufflers should be cleaned after 30 hours of use.

To clean the filter, rinse it with water and let it dry. Filters should be cleaned after 10 hours of use.

To clean the carburetor: 

This is the most challenging part of this process. For some, it’s better to turn this part over to a professional but if you are up to the challenge, this YouTube video provides a solid walk-through of the process:

How to Clean a Two-Cycle/Two-Stroke Engine Carburetor

Since cleaning the carburetor involves taking apart your machine, it is best to have somebody experienced to take care of this task if you aren’t comfortable with it. 

Taking Care of Your Weed Eaters

Your weedeater can be smoking for a variety of reasons and being able to recognize what the cause of the smoke is, is the first step to repairing them and making sure it doesn’t happen again. 

From adjusting the levels of the oil and gas to cleaning the carburetor and muffler, to simply letting the weed trimmer cool down – there are many things you can do to ensure your weed eater is in good working condition. 

Paul Brown

Paul has a two-acre yard on red clay soil in Southeast Texas. He knows exactly what the challenges are to nurturing a thriving yard in difficult soil. He takes a practical approach to yard improvement and enjoys putting best practices and “golden rules of lawn care” to the test. Click here for Paul’s author page

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