10 Reasons Your Lawn Mower Loses Power (Solved!)

Listen to your lawn mower. You can tell there are times when the engine just doesn’t sound as strong. You can hear and feel your mower begin to lose power. There are many different reasons why your mower can experience a loss of power.

A lawn mower will lose power when there is too much load on your engine resulting from mowing wet, thick, or tall grass; debris building under the mower deck; restricting air and fuel flow, or being low on engine oil.

I’ll explain more about the loss of power and what you can do to diagnose and prevent this condition in your lawn mower.

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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

Reasons Why Your Mower Loses Power

Too Much Load on Your Engine

Your lawn mower can lose power when too much load is being put on the engine. Too much load is when the mower is expected to work harder than normal because you are operating under tougher conditions due to one of the following:

• Too Much Ground Speed – Slow down the speed of your mower so it can better handle the grass cutting. Also, slow the ground speed when operating on inclines.
  
• Mowing wet, tall or thick grass – Expect your mower to have to work harder when you are cutting wet, tall, or thick grass. I recommend mowing dry grass because you will receive a better cut, you’ll have less buildup of grass under the deck and you’ll put less load on the engine.
  
  You should avoid cutting very tall grass with your lawn mower. Keep your lawn well maintained by performing regular cuttings. If your grass does get a little long, move your mower deck to the highest position to do your first cutting. Lower the deck to do your next cutting.
  
• Grass and dirt build-up under the mower deck – Grass and debris stuck under your deck can cause your engine to work harder because your mower blades can jam up and not spin freely.
  
  Keep grass clippings from collecting in your deck by scraping the deck frequently, especially if you are mowing damp grass. A silicone spray can be added to the bottom of the deck to help prevent grass from sticking.
  
• Dull blades – Dull, rounded blades can also cause your engine to work harder. Keep your blades sharpened. Replace any damaged blades. Read this article for steps on sharpening and balancing your mower blades.

Plugged Air Filter

A plugged air filter can cause your lawn mower to lose power. Your engine needs air to run so when air is restricted through the filter, the engine suffers and is not able to run at its best. Check your air filter, clean it, or replace it with a new filter if necessary.

How to Clean a Foam Air Filter

• Remove your filter from the air filter housing being careful not to knock dirt into the engine’s air intake.
• Wash the filter with dish soap and water to remove the dirt. Rinse until all soap is removed and let air dry. Placing the filter in the sun will help it dry faster.
• Once the foam filter is dry, use a clean engine oil to lubricate the filter. You will just want to get the foam filter lightly dampened with oil. You do not want to saturate it in oil. Wring out the excess oil.
• If the filter seems dry or brittle and has dark spots on it, you must replace your filter.

How to Clean a Paper Air Filter

• Remove the filter from the air filter housing being careful not to knock any dirt into the engine’s air intake.
• Tap your air filter against a hard surface to knock the loose dirt out of your air filter. Do not use an air compressor to blow the dirt out of your filter.
• Hold your air filter up to the light. If you can see light shine through the paper in your air filter, you can place it back in the air filter housing.
• If you are unable to see light through the filter, you must replace it.

Bad Fuel

A big issue that seems to cause a lot of problems in lawn mowers is the use of bad or old fuel. A lot of people don’t realize that gasoline becomes less effective after about 30 days because it begins to break down.

The fuel breaking down can leave deposits in your fuel system that can create fuel blockages.

Most lawn mowers require unleaded gasoline with an octane rating of 87 or higher. It is very important that the gas contains no more than 10% ethanol. Ethanol is corrosive and attracts moisture.

The lower the ethanol content, the better. Learn more about the effects of ethanol and the best fuel choices in my article This is the Type of Gas Lawn Mowers Use.

Drain bad fuel out of your lawn mower and add fresh fuel with a fuel additive like Sea Foam to reduce moisture buildup and clean your fuel system. Click here to read about the advantage of using Sea Foam as a fuel additive.

Dirty Carburetor

A carburetor’s function is to regulate the correct amount of air and gas your lawn mower’s engine requires to run. Without the right amount and gas or air, your lawn mower may lose power.

The carburetor is likely to become dirty and form gummy and crusted spots where deposits from bad fuel have formed.

If you are somewhat mechanical, you can attempt to clean your carburetor. Most of the time, a carburetor is able to be cleaned. On some occasions, the carburetor must be replaced because it has been damaged or is too dirty to clean to effectively work as designed.

Cleaning Your Carburetor

• Spray carb cleaner to minimize carbon buildup. Remove the air filter and spray some carb cleaner in the air intake. Start the engine to see if it will run. If your lawn mower fires up and still won’t stay running then we need to get inside the carburetor.
  
• Gather pliers, screwdrivers, sockets, and ratchets so you don’t destroy parts while taking the carburetor apart.
  
• Take a photo for reassembly. These days most people have a handy camera on their phones. It’s a very good idea to take a picture of the carburetor so you can refer to it if you don’t remember how to reassemble it after tearing it apart. You will want to make sure you get a photo showing how the linkage and springs go back on the carburetor.
  
• Remove the throttle cable and choke cable if your lawn mower has one.
  
• Undo the filter housing and nuts or screws that hold the carburetor.
  
• Slowly remove the springs so you don’t stretch them out too much. You may have to twist the carb a bit to get the springs off. Also, watch the gasket at this point so you don’t tear it. This is the gasket located between the engine block and the carburetor.
  
• Remove the bottom screw from the float bowl. The float bowl is where gasoline is stored inside the carburetor. It should have gas in it so have a rag ready to catch the gas.
  
• Remove the bowl being careful to not damage the o-ring around it. Caution: Do not get any carb cleaner or any other chemical on the o-ring. It will stretch out and you won’t be able to reuse it.
  
• Inspect the stem for clogged holes. This stem hangs down from the center of the carburetor and has holes in it. If these holes get plugged from old fuel it will not draw fuel up to the jet. If the holes are plugged, take a thick wire to clean them out. It’s easier to see what you’re doing if you use a flashlight. Once you get the holes clean you can rinse them with carb cleaner.
  
• Inspect the carburetor for hard crusty white buildup. This white buildup is fuel additives including ethanol. You need to try to get as much of the white power material out as you can. It’s nearly impossible to get it all out.
  
• Reassemble the carburetor now that the carb is clean. Put it back to together in the reverse order you took it apart. Remember to refer to the photo you took of the carburetor when reassembling so all parts are reinstalled in the right places.
  
• Add fresh fuel plus a fuel stabilizer before you start your lawn mower. Pour the fuel into the tank and give it a chance to fill the bowl of the carburetor. Start your engine. If you are starting with a pull cord, give the rope a yank. It may not start on the first pull, but it should start after several pulls and continue to run.

Clogged Fuel Filter or Fuel Lines

A fuel restriction in your fuel filter or fuel lines can cause your run ability problem. Check for fuel flow by isolating areas of your fuel lines. Using clamps to start and stop the flow, check to make sure fuel runs through each fuel line.

To clear a clogged fuel line, remove the line from your mower and spray carb cleaner into the line. Use a can of compressed air or your air compressor to blow air through the line to remove the clog. Replace a fuel line if you are unable to remove the clog or if your fuel line is dry, cracked or brittle. Replace a dirty fuel filter.

Bad Spark Plug

A fouled spark plug can cause an intermittent spark that can cause a loss of power. Inspect your spark plug for signs of carbon, dirt, and oil buildup on the tip.

If you find a dirty or damaged spark plug, I recommend replacing it with a new one to ensure you’re running a good plug in the mower.

Alternatively, if it’s only dirty and not very dark in color, you can attempt to clean it with a wire brush and reuse it.

Low Engine Oil Level

Engine oil is used in your lawn mower to keep internal components lubricated. When there isn’t enough oil in the engine crankcase, increased friction will build due to a lack of sufficient lubrication.

The engine will get hot and cause a loss of power. If this problem isn’t caught soon enough, the heat of the engine will begin to melt the internal engine component which can result in having to replace the engine.

It is crucial to check the engine oil in your lawn mower prior to operation. Creating a lawn mower checklist and going through these procedures before each mowing may add a few minutes to your mowing time, but it can save you from significant damage to your lawn mower.

Too Much Engine Oil

Overfilling the crankcase with engine oil will cause your engine to smoke. Increased pressure builds as a result of too much engine oil and oil can be pushed into the cylinder through the valve train.

When this happens, a bluish-white smoke is emitted when the oil burns in the cylinder.

This thick cloud of smoke can plug your air filter causing running issues because your engine isn’t able to get the clean air it needs. Check your air filter and your spark plug, and clean or replace them if needed.

Continuing to run your lawn mower with too much oil can cause seal damage, the engine to hydrolock, and a bent piston rod. Correct an engine with too much oil by removing a little oil.

You can do this by using an oil evacuator, a drain plug, or a turkey baster. Read more about the effects of too much engine oil here.

Engine Air Circulation Passages Clogged

Good air circulation around your engine is necessary to keep the engine cool and your mower from losing power. Remove the engine shroud and remove any grass clippings, leaves, and other debris that may be collected around the engine.

Cooling Fins are Dirty

Like the reason why you need to clear any debris from the engine area, the same is true of cleaning your cooling fins. The fins need to be clean so they can use the air circulating around the engine and the surface area of the fins to dissipate heat.

The fins can often collect a buildup of dirt and debris that can compromise air movement. Make sure your fins are in good condition without any broken fins and remove any buildup.

Working Your Engine Harder Can Cause Your Mower to Overheat

Many items listed above not only can cause a loss of power, but it can also cause your lawn mower to overheat and cause extensive damage to your engine. Learn more from my article “7 Things That Can Cause a Lawn Mower to Overheat”.

In Summary

Most lawn mower power loss issues are due to a lack of air or fuel. If you have exhausted all of these troubleshooting steps and haven’t resolved your problem, it is a good time to bring your lawn mower dealer.

The mechanic can diagnose the problem to determine if it’s the charging system, an electrical issue, or an internal problem in your engine.