It is ok if your mower smokes a little from time to time, but excessive smoke coming from your lawn mower is never a good sign. Usually, it’s a signal that something isn’t quite right with your mower’s engine and that it’s time to give it a bit of TLC. So, what does it mean if your lawn mower has black smoke, and is it something you need to be concerned about? Let’s find out.
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Black Smoke From Lawn Mower Engine (Overview)
Black smoke coming from your lawn mower engine is an indication that your fuel-to-air mixture is too rich. In other words, your lawn mower is burning too much fuel and not enough air. Some elements that cause rich fuel are the choke being on, a clogged air filter, or an incorrectly adjusted carburetor.
What to Do When Your Lawn Mower Starts Smoking
Whenever your lawn mower starts smoking, it’s always a good idea to turn off the engine until you understand what is going on. Black smoke is a red flag signaling that your lawn mower has a mechanical issue that needs attention. The last thing you want to do is ignore the warning signs of the black smoke and allow your mower to suffer and get worse. If you do, your mower could develop irreparable damage or need an expensive repair.
What Would Cause a Lawn Mower to Start Smoking Black?
If you notice black smoke, the first thing to do is switch off your mower. Now, let’s figure out why your engine is blowing black smoke. Here are the components you’ll want to inspect when diagnosing the cause of the black smoke.
The Lawn Mower Choke is Still On
The purpose of the choke is to reduce the airflow into the engine to enrich the fuel mix when cold-starting the engine. Reducing the air allows the carburetor to send a higher percentage of fuel into the cylinder head, which makes it easier for the engine to start. Once the engine has started, the enriched fuel is no longer needed and is not ideal for good performance. Therefore, the choke needs to be switched off so the air can enter the engine and produce the necessary fuel mix ratio.
Pretty much every gas-powered lawn mower has some type of choke, whether it’s a manual choke or an auto choke. If you forget to switch the choke off or the auto choke mechanism fails, your lawn mower will continue to run rich, creating black smoke.
The Air Filter Is Clogged
A dirty air filter limits the amount of air that can enter that carburetor. As a result, it alters the fuel-to-air, making it rich. When this happens, it has the same effect as leaving the choke switched on. Basically, it creates black smoke and makes the engine run rough. So to restore the airflow through the carburetor, you’ll want to either clean or replace the air filter.
The Carburetor is Incorrectly Adjusted
The last item you will want to check is the carburetor. Depending on your specific mower and carburetor, you may or may not have the option to adjust the air-to-fuel mixture. Typically, smaller lawn mowers like push mowers have simple carbs that don’t have a fuel mix adjuster, whereas larger lawn mowers such as tractors and zero-turns do. First, find the carburetor on your mower and check whether or not it has two or three adjuster screws.
If you have the ability to adjust the air-to-fuel mixture, then this is where you should begin your repair. However, if you don’t have the ability to adjust the carb, then you’ll want to move on to cleaning it. Sometimes dirt and debris can work their way into the carburetor, which affects its performance and the airflow.
Lawn Mower Black Smoke Summary
Here is a quick overview of how to diagnose a lawn mower blowing black smoke. After figuring out the root of the issue, you can then move on to making the appropriate repair. Let’s take a closer look.
| Cause | Diagnosis | Action |
|---|---|---|
| Manual Choke ON | Check Choke Position | Switch Choke OFF |
| Auto-Choke ON | Check Choke Mechanism | Repair Auto Choke |
| Air Filter Clogged | Check Air Filter Condition | Clean Air Filter Replace Air Filter |
| Carburetor – Adjustable | Check Choke & Air Filter | Adjust Air-to-Fuel Mix |
| Carburetor – Non-Adjustable | Check Choke & Air Filter | Clean Carburetor Replace Carburetor |
Is Lawn Mower Black Smoke Dangerous?
You will notice issues other than poor performance if you run your lawn mower for a long time while it blows out black smoke. The black smoke means carbon is being produced faster than the engine is designed to handle. If the engine is exposed to high carbon smoke from running rich, then it will start to accumulate carbon on its exposed internal surfaces. As a result, it will develop issues with carbon buildup on the valves, spark plug, and exhaust. So, it won’t be long before you will need to change the spark plug, clean the valves, and potentially replace your lawn mower’s muffler.
How to Fix a Smoking Lawn Mower (Rich Fuel)
Now let’s go over how to fix a smoking lawn mower when the engine is running rich and blowing out black smoke.
Using & Fixing a Lawn Mower Choke
If you have a choke lever on your lawn mower, then all you need to do is disengage the choke, and the black smoke should stop pretty much instantly. Also, your lawn mower should perform significantly better. But if your lawn mower has an auto-choke, like a thermostatic choke that works off the exhaust muffler, you’ll need to carry out a repair.
Normally, the small bar designed to engage and disengage the choke gets a little bent out of shape, and you’ll need to straighten it out. So, find the small arm that moves back and forth depending on the heat of the exhaust pipe and check that it is working as it should. If it’s not, you should be able to straighten out the bar once it has cooled down.
It’s also worth noting that if the bar is bent and preventing the choke from engaging, you’ll have trouble cold starting your lawn mower.
Clean the Air Filter
There are different methods of cleaning a lawn mower air filter. For example, if your air filter is made of paper, you can blow out the dirt using an air compressor. On the other hand, you can wash a foam air filter with a degreaser. Remember not to wash a paper filter, as it will likely fall apart.
Head over to my blog on how to clean a mower air filter if you want more details.
Tuning the Carburetor
If you have a carburetor with an air-to-fuel mixture screw, you’re in luck. This is by far the easiest way to tune your lawn mower carburetor. Before you start your adjustment, you’ll need to know what you want to achieve. So, the goal is to find the middle ground between rich and lean.
If your mower is blowing black smoke, then your carb is running rich. So, you want to adjust the mix to reduce the fuel and increase the air. But if you go too far with your adjustment, your mower will run lean and begin to smoke white. As you can see, you are aiming for that middle ground. So, you need to adjust the adjustment screw in both directions until the mower runs well and there is no black or white smoke.
Here are the steps to adjust the fuel-to-air mix on your lawn mower.
- Confirm that the air filter is clean and in place.
- Start your lawn mower.
- Allow your mower’s engine to warm up.
- Confirm that the choke is working and disengages once the engine warms up.
- Adjust the fuel-to-air mixture screw until the smoke stops and the mower is running well.
What You’ll Need to Adjust a Lawn Mower Carburetor (Fuel-to-Air Mixture)
What to Do if You Can’t Adjust the Carburetor
It is common for carburetors to lack adjustment screws. Alternatively, you can try cleaning your carburetor. Sometimes dirt buildup in the carb can cause an issue and make your mower smoke black.
While you may need to remove the carburetor to clean it, it’s worth trying a carburetor spray first. Sometimes this can be enough to stop the smoke. Failing this, you might need to install a replacement lawn mower carburetor.
