If you’re out cutting the grass, the last thing you want is to send up a smoke signal letting everybody know that something is wrong with your lawn mower. Still, you can’t always prevent it. Smoke coming from your lawn mower is a telltale sign that something is going (or has gone) wrong. Mowers can produce either white/bluish smoke or black smoke. If your lawn mower is blowing black smoke, there are usually just a few major reasons for it.
Why is My Lawn Mower Blowing Black Smoke?
A lawn mower blowing black smoke means that the engine is running rich and is usually caused by the choke set/stuck “on”, a blocked air filter, or a carburetor that isn’t working correctly. Combustion engines depend on a mixture of fuel and air, and when there is too much fuel in the mix, it is considered rich (when there is too little fuel in the mixture it is considered lean).
Let’s take a look at each reason why your engine could be getting a rich mixture resulting in your lawn mower blowing black smoke.
Choke Set or Stuck “On”
Lawn mowers use either a choke or primer bulb to help get them started when the engine is cold. The choke on your lawn mower is used to enrich the gas and air mixture that the engine gets when first starting the mower. It’s called choking because the fuel/air mix is enriched by restricting the amount of air that can enter the carburetor. A primer bulb enriches the fuel/air mix by pushing extra fuel into the engine. Once the engine is running or warmed up a bit, the choke should be turned off. If it isn’t turned off, or it gets stuck on, the rich fuel and air mix will produce black smoke.
Only manual and automatic chokes can cause black smoke, primer bulbs will not. Double-check that your choke setting isn’t the issue first. If it isn’t, then run the mower and see if changing the choke settings affect how the lawn mower runs. If the choke seems to be working correctly, you’ll have to look into the air filter or carburetor.
Blocked Air Filter
Your mower’s air filter is super important and something that should be checked and cleaned off a couple of times a season. Technically it’s recommended that you clean your air filter every 25 hours of operation, and replace it every 100 hours. If your air filter is overly dirty or wet it can block air from getting to the engine. This is another way that you can end up with a rich fuel and air mixture creating black smoke. Luckily, it’s pretty easy to get at your air filter on most lawn mowers.
To tell if this is your problem, take a look at the air filter and if it has a bunch of dust or debris on it, try and clean it off. If it is wet or oily you should probably replace it. Then, once you have your cleaned or new air filter reinstalled, run the mower and see if the black smoke clears. Some people recommend taking the filter off and running the mower without an air filter to see if it’s the problem, but I wouldn’t advise doing that. It’s an easy way to suck a bunch of gunk into your carburetor or engine.
Carburetor Problems
This usually isn’t the case but if you know that your choke and air filter are working correctly, the cause of your black smoke is probably due to the carburetor not working properly. The job of the carburetor is to mix fuel and air and shoot it into the engine. If the fuel and air aren’t being combined in the right ratio, the engine won’t run as it should. In the case of black smoke, the carburetor isn’t supplying enough air.
How to Fix a Lawn Mower Blowing Black Smoke
Fixing a lawn mower blowing black smoke means bringing the air and fuel mixture that the engine burns back into balance. Each of the three causes of a rich fuel mix has a different fix, so let’s check them out.
Choke Set or Stuck “On”
If your lawn mower has a manual (lever) choke, fixing the choke may be as simple as turning the choke off when the mower is running. But if that doesn’t work, or you have an automatic choke, you’ll need to dig a bit deeper.
Manual Choke:
Adjust the choke when the mower is running to see if it changes how the engine runs. Turning the choke off should make the engine run smoothly (and without black smoke) and leaving the choke on should cause the engine to run roughly. If the choke doesn’t seem to work as it should, you will have to adjust the cables that control the choke flap opening and closing. This isn’t a super complicated job but usually requires the removal of the carburetor.
Automatic Choke:
If you have a newer lawn mower, it could have an automatic choke. These adjust themselves using a thermometer reading. As the engine warms, the thermometer should force the choke flap to open (turning it off). If it isn’t working properly it is usually because the thermometer is broken, or the links attached to the choke flap aren’t allowing it to open and close. Replacing these parts can be a bit more tricky than dealing with a manual choke.
Blocked Air Filter
Cleaning off your filter can be done relatively easily. Usually, there is just a plastic clip or a set of wingnuts or small bolts holding the filter in place. Blowing off the air filter with compressed air, or tapping it down on a clean surface will usually shake off any loose dirt or debris.
But if the filter has gotten wet or soaked up any oil, you’ll need to replace it. The part isn’t expensive, and installing a new filter just requires reversing the steps you take to remove it. It’s worth maintaining a clean air filter because it will help your mower in more ways than one.
Carburetor Problems
Again, if the choke is turned off, and the air filter is clean, the carburetor is now your prime suspect. Fixing a carburetor that is mixing too much air in means two things: adjusting the airscrews, or replacing the carburetor. The airscrews on carburetors are always screwed in a precise amount to control the air intake. You’ll need to reference a service manual or manufacturer’s specifications to determine how far the screw should be set. Usually, you need to turn the screw all the way and then back it out a set number of turns.
If you go through the trouble of removing the carburetor and find that the screw was already set correctly, you will have to replace the carburetor. Depending on what type of lawn mower you have, it shouldn’t be too costly to do yourself. If you don’t have the tools or know-how to tear into your carburetor, you should have a small engine mechanic tackle the project for you.
Why is My Riding Lawn Mower Blowing Black Smoke?
Riding lawn mowers blowing black smoke do so for the exact same reason that push mowers do: a rich fuel and air mixture. Check the choke, and air filter first to see that they are functioning as they should. If they are, you will have to check for carburetor malfunctions.
Causes of 4 Stroke Lawn Mower Blowing Black Smoke
Unlike a lawn mower blowing white or blue smoke, 4-stroke and 2-stroke mowers blow black smoke for the same reasons. The type of choke is essentially the only thing that will differ between push mowers. Again, if your mower has a primer bulb, the choke is not the issue and you’ll need to look at the air filter or carburetor.
Causes of 2 Stroke Lawn Mower Blowing Black Smoke
As I mentioned in the section above, a 2-stroke mower will blow black smoke for the same reason as a 4-stroke mower. Because black smoke is related to the fuel and air mixture, all lawn mowers produce black smoke for the same reasons. But, to be clear, this isn’t always the case for white or blue smoke.
Is Black Smoke from Lawn Mower Dangerous?
Black smoke from a lawn mower isn’t only bad to look at, it can actually be dangerous for a number of reasons. First off, black smoke is basically just soot from unburned gas/fuel being pushed out through the exhaust. If inhaling gas fumes is bad for you, you definitely don’t want to breathe in a bunch of sooty fuel that has run through your engine. Also, black smoke coming from your lawn mower can signal damage occurring within the mower. A lawn mower burning a rich fuel/air mixture can lead to you needing to replace choke parts, an air filter, spark plugs, or even the carburetor itself. If you see black smoke, be sure to address the problem right away.
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