After starting up your lawn mower, you may find it doesn’t have the power it usually does. Maybe the mower was running fine when you put it away, but when you’ve tried to use it again, there’s an issue. This isn’t uncommon and can happen to the best of us, so don’t be too concerned. Let me share with you some common causes that could be making your lawn mower engine to run slowly.
Reasons for a Lawn Mower Running Slow (The Short Answer)
Several reasons can cause a lawn mower to run slowly, including serviceable issues with the engine. Additionally, any potential obstructions in the lawn mower deck that put stress on the engine will cause it to run slowly.
Why Is Your Lawn Mower Running Slow?
As you can see, there are quite a few reasons your lawn mower engine could be running slowly and for its unusual, rough performance. Luckily we have them covered here in more detail. So let’s take a look through the list of potential reasons.
Old or Damaged Spark Plug
The spark plug is a good place to start your diagnosis if you think your engine is running slowly. If the spark plug is not able to generate the ignition spark as intended, then the engine will never be able to achieve the necessary engine speed. In addition, spark plugs can become sooted up, wear with age, or have a burnt-out electrode. As a result, it’s best to pull out the plug and check it for damage and wear.
Here are a couple of other posts I wrote that should help you determine if your spark good needs replacing or not:
Like the spark plug, the carburetor is an essential part of the combustion process. Its purpose is to process a regulated air-to-fuel mixture to the combustion chamber at a specific rate relevant to the throttle demand.
Problems with gasoline impurities and general debris that has made its way through the fuel system can dirty up the carb. You’ll find that the problem gets worse when you try to increase the throttle.
Clogged Mower Deck
If your mower deck happens to be full of clippings, then this can be the reason for your slow engine. This is because the blades are bogged down and are not able to spin freely. A quick test you can carry out is to take a look under the mower’s deck and check for the obstruction.
Dirty Air Filter
Dirty air filter symptoms are similar to that of a dirty carburetor. The air-to-fuel ratio is going to be affected since not enough air will be drawn into the carburetor. To test for a dirty air filter, you can pop off the air filter’s cover and take a look at its condition.
The fuel is the last item on my list of reasons your lawn mower is running slow. If fuel has been left unprotected long enough, it can go bad. The result of bad fuel is that it loses its combustibility. Basically, this means it no longer works as it should, and the engine struggles and can’t achieve a high speed.
Additionally, this is another cause of a dirty carburetor. The easiest way to check fuel is to compare a new fuel sample with the sample in the mower. If the old gas is discolored, it has gone bad and needs to be removed. This is one of the easiest symptoms of bad gas in a lawn mower to spot.
How to Fix a Lawn Mower That Runs Slow (+ What You’ll Need)
So, you can see there are several common reasons your mower engine may run slowly. Now let’s look at how you can resolve the problems quickly and get your mower back up and running smoothly.
Replacing the Spark Plug
There are just a few steps to follow to replace the spark plug. Let’s take a look.
Remove the Ignition Cable
First, locate the spark plug and remove the ignition cable.
Remove the Spark Plug
Next, using your plug wrench, remove the existing spark plug.
Replace the Spark Plug
Take your new replacement spark plug and insert it into the engine. Next, take your plug wrench and tighten up the spark plug. When tightening the plug, be careful not to over-tighten it as they are quite brittle. It just needs to be a snug fit.
Attach the Ignition Cable
Lastly, reinstall the ignition cable and give the mower a test. If the spark plug was old or sooted up, then replacing it will certainly make a difference in both the engine’s performance and the effort needed to start the mower.
Tools & Parts Required to Replace the Spark Plug
- Plug Wrench
- New Spark Plug
Cleaning the Carburetor
There are two ways in which you can clean a carburetor. The first is to clean it when it is attached to the engine, and the second is to remove it and strip it down completely. In this guide, we’ll look at how to clean the carburetor without removing it.
Remove the Air Filter
First, locate the air filter and remove its cover. Next, remove the air filter and set it to one side. Now you will see that you have access to the inside of the carburetor.
Remove the Carburetor Fuel Bowl/Cup
Next, located at the bottom of the carburetor is the carburetor fuel bowl. To remove the cup, take your socket wrench and remove the bolt at the bottom. With the bolt removed, the bowl can be taken off. Now you will have access to the carburetor float and jet.
Spray with Carburetor Cleaner
Use your carburetor cleaner to give the inside of the carburetor a good spray. Then let the cleaning liquid sit for five minutes to soak in and break down the grime.
Spray with an Air Line
Next, blast the inside of the carburetor using an air line from a compressor or a can of compressed air. This will blast off as much of the grime as possible without completely stripping down the carburetor. For the best results, I always like to repeat this process and give the carburetor another spray with the cleaner and air line.
Put the Carburetor Back Together
Once you are happy with your cleaning, you can go ahead and reinstall the carburetor fuel bowl and air filter.
Test the Lawn Mower
Finally, give the mower a test. It may take a few extra pulls on the starter cord as the carburetor will be emptied of fuel after cleaning it.
Tools & Parts Required to Clean the Carburetor
- Socket Wrench
- Air Line
- Carburetor Cleaning Spray
Cleaning the Mower Deck
If you have already looked under the mower’s deck and have seen that it’s full of clippings, you’re going to want to empty it out. Let’s take a look at how you can do this.
Shake the Mower
First, raise the front of the mower off the ground and shake it out. This is going to get rid of a lot of the clippings.
Tilt Over the Mower
Next, tilt over the lawn mower the correct way, so oil doesn’t get in the filter or carburetor. You want it to be able to support itself so that you can work on the underside.
Scrape Out the Old Clippings
Using a scraper and a flathead screwdriver, scrape out the old clippings. It’s worth taking some extra time to get right into the corners.
Spray the Deck
Finally, use a PTFE spray on the underside of the lawn mower’s deck. PTFE spray, like a silicone spray, will keep grass clippings from sticking under the mower deck and thus reduce the chances of your lawn mower running slow in the future.
Tools & Parts Required to Clean the Mower Deck
- Flathead Screwdriver
- PTFE Spray
Replacing the Air Filter
Now let’s look at how you can replace your air filter and clean out the housing.
Remove the Filter Cover
First, remove the air filter cover and set it to one side. This may require you to remove a mounting screw if it’s not just clipped on.
Remove the Old Filter
Remove the old air filter and discard it in the trash unless you want to try cleaning it.
Clean the Housing
Next, use your carburetor cleaner to spray the inside of the housing, including the air filter cover. Allow the cleaner to sit for a few minutes so that it can break down the dirt. Next, take a cleaning cloth and clean out the dirt. Doing this is going to preserve the life of the new filter.
Install the New Filter
Once the housing is clean, you can go ahead and install the new filter. Just make sure to install the filter and pre-filter in the correct way if you happen to have both.
Install the Cover
Finally, reinstall the air filter cover and give the mower a test.
Tools & Parts Required to Replace the Air Filter
- Carburetor Cleaner
- Clean Cloth
- New Air Filter
Removing the Bad Fuel
If you have discovered that you have bad fuel by carrying out a visual comparison, then you are going to want to remove the fuel. Additionally, for better results, it is going to be best to also clean the carburetor as described earlier. So, let’s take a look at how to drain gas from your lawn mower.
Position Pump/Siphon & Fuel Can
Start by positioning a fuel can next to the lawn mower. This should be a fuel can you do not mind dumping the old fuel into. Then, insert the pump/siphon into both the lawn mower’s fuel tank and the spare can.
Transfer Bad Fuel
Finally, start the pumping/siphoning process and allow the tank to drain. Make sure to keep the siphon towards the bottom of the fuel tank to get all of the fuel out.
Disconnect the Fuel
Next, locate the fuel line that goes into the carburetor. Remove the spring clip using a pair of pliers and remove the fuel line.
Drain Remaining Fuel
Once the fuel line is removed from the carburetor, the remainder of the fuel will drain from the fuel system. As a result, you will want a small container to catch the fuel. An old cup should work fine.
Reconnect the Fuel Line
Once the fuel has finished draining, you’ll need to reinstall the fuel line. Start by installing the line onto the carburetor, then reinstalling the spring clip using your pliers.
Fill with Fresh Gasoline
With a completely drained fuel system, you can go ahead and fill the lawn mower with fresh gasoline and start the engine. However, this will probably take a few pulls of the starter cord as the entire system is empty. To avoid this problem in the future, it would be a good idea to use a fuel stabilizer that will prolong the life of the gasoline and stop it from going bad.
Tools & Parts Required to Remove Bad Fuel
- Fuel Can
- Old Cup
- New Fuel