This how to guide is written to help you determine why your lawnmower is losing power while cutting the yard. There can be a number of potential causes.
Many of them are easy to fix!
Symptoms
The number one symptom of a lawnmower that is losing power is it’s bogging in conditions where it would not have bogged previously.
Otherwise if the mower is losing power when cutting, it may be that it is just underpowered for what you are doing with it.
If you’ve had your mower for a while, obviously you’ll have a good handle on whether the lawnmower is losing more power than normal.
Causes
| Issue | Possible Causes | Solutions |
|---|---|---|
| Clogged Air Filter | Dirt and debris buildup in the air filter. | Clean or replace the air filter. |
| Dirty or Faulty Spark Plug | Spark plug covered in carbon or malfunctioning. | Clean the spark plug or replace it if necessary. |
| Clogged or Dirty Fuel Filter | Fuel filter obstructed with debris or dirt. | Clean or replace the fuel filter. |
| Carburetor Issues | Carburetor clogged or improperly adjusted. | Clean the carburetor and adjust settings as required. |
| Dull or Damaged Blades | Blades are blunt, bent, or damaged. | Sharpen or replace the mower blades. |
| Oil Level Issues | Low or dirty oil in the engine. | Check oil level and refill or replace if dirty. |
| Fuel Problems | Using stale or wrong type of fuel. | Drain old fuel and refill with the correct, fresh fuel. |
| Overheating Engine | Engine running too hot due to various reasons. | Check for airflow blockages, and ensure proper oil levels. |
| Mower Deck Issues | Mower deck clogged with grass and debris. | Clean out the mower deck to ensure free movement of blades. |
| Restricted Airflow | Obstructions in the airflow system. | Remove any debris blocking air vents and pathways. |
Here are the most common causes of a lawnmower that is losing power when cutting. They are written in a diagnostic order that is a combination of easiness to check and likelihood of being the problem.
1. Bad Gas
If you have recently left your lawn mower out in the rain, without changing the fuel, it could very likely be that you have bad gasoline in there.
What you’re gonna need to do to check for that is get a flashlight and take your lawnmower someplace dark. Once you’re there, shine the light into the gas tank and see if you can see any bubbles. If you do see bubbles, that’s not air, it’s actually gasoline.
You’ll need to drain the tank. If it’s still running poorly after having fresh gas put in, you’ll need to take a look at the carburetor. Water is very corrosive, and it doesn’t take a whole lot of it to damage a carburetor.
This often happens after starting the mower over the off-season, or using old gas that made it through the off-season. If you do store a mower with the gasoline in there, a fuel stabilizer can go a long way in keeping this problem from happening again.
2. Fouled Plug
A fouled spark plug can cause a lawnmower to lose power while cutting. A spark plug is fouled when it can no longer provide the electric spark needed to properly burn the air/fuel mixture.
A spark plug can be fouled when the air/fuel mixture is wrong (cab issue, vacuum leak), or whenever oil slips past the piston rings (engine needs rebuilt).
It’s easy to check to see if the spark plug is fouled. Unbolt it, and take a look. It should be free of debris and oil. There should not be any buildup in between the electrode and the top of the plug. The spark plug can tell you a lot about the way the engine is running. Here’s a great chart that can help you determine if and why your plugs may be fouled.
3. Lack of Fuel
The fuel system on a lawnmower is gravity fed, which means that it doesn’t have a fuel pump. Because of this, the fuel filter needs to be really clean for gasoline to properly flow through it.
It’s also possible for something to have gotten into the fuel tank and block the fuel pick up line. Shine a flashlight in and see if there is anything blocking it.
I once fixed this problem on a riding lawnmower with a carpenter bee corpse blocking the hole.
Most fuel filters are see-through, take a look and make sure that the filter is full with the mower running. With the mower off, you can disconnect the fuel filter between the fuel filter and the carb. Fuel should freely flow out (have something ready to catch it with).
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4. Clogged Cutting Deck
If you have recently mowed the yard when it was wet outside, there’s a high likelihood that that is causing your mower to run poorly.
When it’s wet, grass will stick to the cutting deck to the point that it starts impeding the ability of the blade to turn freely. This puts extra pressure on the mower’s motor, which can cause you to lose power when cutting.
5. Belt (Rider Only)
The cutting deck belt can cause the motor to lose power. If the tensioner is too loose or the belt is too old it won’t have enough force to properly turn the blades whenever you’re in a particularly challenging part of the yard.
On the other side of the coin if a stick or rock gets wedged in there it can rub against the belt enough to siphon off a decent amount of horsepower.
6. Stuck Throttle or Choke
This is common on mowers that are stored outside. The throttle cable can get stuck or has some sort of debris keeping it from achieving full throttle.
Take a look and inspect to make sure that it is moving freely. If it is not, try spraying it with WD-40 and rocking it slowly.
If it has a manual choke, make sure that the choke mechanism is responding to the choke lever all the way. If it’s running with the choke on a little bit (or all the way) it’s going to have a hard time and lose power.
A choke mechanism that is not working properly can keep the air/fuel mixture too rich. Taking a moment or two to check this could very well solve your problem.
While you’re looking around right here, check the primer bulb if so equipped. Make sure that it is not dry rotted or has a hole or crack.
Careful not to overprime the bulb or you’ll flood your mower.
If it does it’s letting air in that is not supposed to be, changing the air fuel mixture. This will cause your mower to lose power.
Try covering the whole with your finger while the motor is running. If the motor suddenly seems like it is running better, there’s your problem.
7. Bad Carburetor
The carburetor on a lawnmower is simple enough. But just like the larger carburetors that were used in cars until the 1980s, they can be prone to problems. Is there a lot of white smoke coming from the engine? That can indicate a carb issue for sure.
The gasket between the carb and engine is a common failure point as well. A good carburetor rebuild kit is relatively affordable though.
Conclusion
We have listed seven of the most common reasons that a mower might have problems with losing power when cutting. If you find out what happened with your mower, leave a comment below. You might be able to help the next person who comes along. Good luck!