Lawn Mower Acts Like It Is Running Out Of Gas

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If your lawn mower starts to sputter and act like it is running out of gas during a mowing session, you check your fuel tank and discover that you have enough fuel. What could be the cause of this problem?

In this article, I share common reasons your lawn mower acts like it is running out of gas and recommend simple fixes that you can apply to solve this problem.

Why Does My Lawn Mower Act Like It Is Running Out Of Gas?

The reason your lawn mower acts like it is running out of gas may include:

  • Clogged gas cap vent
  • Stale fuel
  • Fuel in the gas tank
  • Clogged fuel filter
  • Split/blocked fuel line
  • Faulty fuel pump
  • Clogged carburetor jets
  • Stuck float needle valve

Tools You Need For This Job

Some tools you may need for this job include:

  • Work gloves
  • Screwdriver
  • Pliers
  • Fuel siphon
  • Empty container
  • Carburetor cleaner
  • Carburetor repair kit 
  • Clean rag

Lawn Mower Acts Like It Is Running Out Of Gas: Causes, Diagnosis, and Fixes

1. Clogged Gas Cap Vent 

The gas cap vent is a tiny hole on the fuel cap which allows air into the fuel tank.

As the mower runs and uses fuel, the fuel level decreases. After about 20 to 30 minutes, a vacuum will form in the gas tank if the air is not circulating via the vent hole on the cap.

As a result of this, a vapor lock may occur in the tank which will prevent fuel from leaving the tank and getting to the carburetor.

This can make your lawn mower run rough and act like it is running out of gas.

Diagnosis:

Slowly open the fuel cap. You will hear air being drawn into the tank if the vent is clogged.

Then, start the engine with the cap slightly open to allow air in. If it continues to run smoothly, the vent is blocked.

Fix:

I recommend replacing the fuel cap, but you can unclog the vent with a thin wire and compressed air.

2. Stale Fuel

Fuel that has stayed in the tank for over 30 days has become stale and may begin to form a sticky residue.

This sticky residue can clog the fuel tank outlet and other fuel delivery systems of the engine, causing the mower to act like it is running out of fuel.

Diagnosis:

If you have left fuel in your mower for more than 30 days, drain the fuel into a fuel can or any clean empty container, using a fuel siphon.

Allow the fuel to settle and examine it. The presence of slimy or sticky residue indicates that the fuel is stale.

Fix:

Drain out all the old fuel and clean the fuel tank and its outlet using a carb cleaner. Then, refill the tank with fresh fuel.

To prolong the life of the gas and avoid moisture buildup, fill the tank with fresh fuel and a fuel stabilizer.

3. Water In The Fuel Tank 

When water is present in the gas tank, it will enter the engine and mix with the fuel-air mixture. Your lawn mower may act like it is running out of gas because of this.

Water impedes proper ignition because it does not ignite when it enters the cylinder. As a result, whenever the water tries to ignite, the engine sputters, runs rough or acts like it is running out of gas.

Diagnosis: 

Empty the tank of all fuel. The presence of water in the fuel indicates contaminated fuel. 

Fix:

Empty the gas tank and carburetor bowl. Then, refill the tank with fresh fuel.

4. Clogged Fuel Filter

A clogged fuel filter will make your lawn mower act like it is running out of gas.

The fuel filter cleans the fuel as it moves from the fuel tank to the carburetor. If the filter becomes clogged with debris, fuel delivery may be slowed down.

The engine may run as if it is running out of gas due to insufficient fuel delivery caused by a clogged fuel filter.

Diagnosis:

Using your pliers, remove the fuel filter. Tilt it to allow fuel to spill out from both sides. If the filter is good, the fuel should flow out easily but it will drop out very slowly if clogged.

Fix

Purchase and install a new filter. When installing the new filter, be careful to go in the direction indicated by the arrow. 

5. Split/Blocked Fuel Line

If your lawn mower acts like it is running out of gas, the fuel lines should be checked as well.

The fuel line may clog, which will reduce fuel flow . It can also split directly where it touches the engine. 

If this happens, fuel vaporizes and does not enter the combustion chamber as liquid fuel. This can cause a vapor lock to occur after a while.

Additionally, check to see that the fuel lines are not touching the hot engine as this could potentially cause fuel evaporation and vapor lock.

Diagnosis:

Check for the fuel flow in each section of the fuel line to pinpoint the location of the obstruction if you want to find a clogged line.

The fuel lines should also be carefully inspected for cracks and rips. Inspect the lines for contact with the engine’s body as well.

Fix:

Replace damaged or clogged fuel lines with new ones.

6. Faulty Fuel Pump 

Your lawn mower will have a fuel pump if the fuel tank sits lower than the carburetor. A faulty fuel pump will prevent fuel from reaching the carburetor. 

This could be why your lawn mower acts like it is running out of gas.

The fuel pump has a pulse port, gas in port, and gas out port. The engine crankcase is connected to the pulse port by a little rubber tube.

If the engine oil is overfilled, oil can enter the pulse port line through the crankcase, preventing the pulse port from functioning properly.

The pump also contains check valves that can wear down over time and cause the pump to malfunction.

Diagnosis:

  • First, ensure that fuel is flowing to the pump.
  • Once you’ve established that you’re getting enough fuel to the fuel pump, remove the fuel line from the carburetor and place it in a container.
  • Next, start the fuel flow and your mower. If you see a stream of fuel coming out of the fuel line, it means your fuel pump is working properly.
  • If not, the fuel pump is defective.

Fix:

Replace a defective or damaged fuel pump.

7. Clogged Carburetor Jets 

Your lawn mower may act like it is running out of gas due to fuel distribution issues caused by a clogged carburetor jet.

Lawn mower carburetors have tiny apertures called jets that can become clogged with dirt and residue from stale fuel and engine debris over time.

If this happens, the mower’s engine may receive little to no fuel from the carburetor, making it act like it is running out of fuel.

Diagnosis: 

Examine the carburetor jets for dirt and debris. Also, inspect the carburetor bowl for any sticky residue from old fuel.

Fix:

Disassemble the carburetor and soak the metal parts in the carburetor cleaner for a few hours.

Then, inspect all the vital parts including the needle seat, carburetor gasket, and diaphragm, and replace any worn/damaged parts.

8. Stuck Float Needle Valve

The float needle valve of the carburetor can get stuck in the closed position and prevent sufficient fuel from getting into the engine.

The float needle regulates the amount of fuel going out of the carburetor. It closes to stop fuel flow when there is enough in the carburetor. 

It can get stuck in the closed position with debris from stale fuel. The needle valve will not allow fuel to flow to the engine properly if it is stuck causing the mower to act like it is running out of gas.  

Diagnosis:

  • First, remove the carburetor using a screwdriver
  • To access the carburetor interior, remove the float bowl from the carburetor. 
  • Try moving the float and needle valve with your hands up and down; it should move easily and with little resistance.
  • If there is resistance, it is stuck.

Fix:

Replace the float needle. The float usually doesn’t need to be replaced, but I recommend giving it a good cleaning.

Use a carb cleaner to clean it. Simply soak the object and let it sit in the carburetor cleaner for a few hours. After that, wipe it off and let it air dry.

Conclusion

Your lawn mower may act like it is running out of gas due to: a clogged fuel cap vent, stale fuel, water in the gas, clogged fuel filter, damaged fuel line, bad fuel pump, clogged carburetor jets, or a stuck float needle valve.

Follow the troubleshooting guides in this article to diagnose the problem, then apply the recommended fixes to get your mower running smoothly again.