Briggs And Stratton Engine Starts But Won’t Stay Running

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If your Briggs and Stratton engine starts but won’t stay running, it can be quite annoying and frustrating, especially if you have a lot of work to do with it. 

This means that something is wrong internally and you will need to identify the problem and fix it to get your engine running again.

In this article, I share the common reasons your Briggs and Stratton engine starts but won’t stay running and practical steps you can follow to diagnose and fix these problems.

Reasons Your Briggs And Stratton Engine Starts But Won’t Stay Running

Some of the reasons your Briggs and Stratton starts but won’t stay running include:

  • Old gas or water in the gas tank
  • Dirty air filter
  • Worn out spark plug
  • Blocked fuel vent
  • Stale oil
  • Clogged fuel lines/fuel filter
  • Clogged carburetor jets
  • Damaged carburetor diaphragm

Tools You Need For This Job

Some tools you may need to diagnose and fix your engine include:

  1. Work gloves
  2. Screwdriver
  3. A clean rag
  4. Wire brush
  5. Carb cleaner
  6. Compressed air
  7. Socket wrench
  8. Pliers
  9. Empty fuel can

Briggs And Stratton Engine Starts But Won’t Stay Running: Possible Causes And Fixes


1. Old Gas or Water In The Gas Tank

If your Briggs and Stratton engine starts but won’t stay running, check the gas tank to make sure that water hasn’t gotten into the fuel. Water in the fuel system may allow the engine to start but it won’t stay running.

Another possibility is that your gas is too old. As soon as 30 days after purchase, the fuel in your mower starts to degrade and lose its primary characteristics.

A sticky residue is left behind as the gas components degrade over time, and this residue might eventually cause issues in the fuel system. As a result of fuel limits, your engine may refuse to stay running.


Open up the fuel tank and pour the contents out into a container. Sticky residue in the fuel indicates that the fuel is old and should be changed.


  • Use a siphon to drain the old fuel into a container or fuel can.
  • If water or other contaminants are discovered in the fuel, clean the tank with carb cleaner
  • Then refill it with fresh fuel.

2. Dirty Air Filter


Your Briggs and Stratton engine may refuse to stay running due to a lack of airflow brought on by a clogged air filter.

For the engine to run, it needs a proper supply of air and fuel. To avoid overheating and malfunction, the engine needs cool air inflow from the air filter as it starts and warms up.

The air filter allows air to enter. When the air filter gets unclean, it blocks air from entering the engine, resulting in the engine shutting down.


Remove the air filter housing at the side of the engine. Search for dust and grime on the air filter.


  • If it is a paper air filter, replace it. 
  • If it is a foam filter, you can wash it but I would recommend replacing it with a new one.
  • Then clean the housing with a clean, dry rag to remove any residual dirt.

3. Worn Out Spark Plug

A worn-out spark plug is another reason your engine starts but won’t run.

If the spark plug isn’t producing enough spark, your Briggs and Stratton engine will shut off as it becomes warm. 

A malfunctioning spark plug may produce the initial spark needed to start the engine but won’t be able to produce enough spark to keep the engine running, which can cause the engine to die after starting.


Remove the spark plug using a socket wrench. Clean the plug with a wire brush. Dark ends or carbon residue suggests that the spark plug needs to be changed. 


Replace the damaged spark plug with a new one.

4. Blocked Fuel Cap Vent

The fuel cap of most mowers has a tiny hole that allows airflow into the engine. This vent can easily get blocked with grass or debris and as a result, creates a vacuum in the tank.

The vacuum created by lack of air intake results in a vapor lock and fuel stops flowing out of the tank to the carburetor. As a result, the engine may die after starting. 


Loosen the fuel cap and run the engine. If it runs without shutting down, the cap vent is blocked.


Replace the fuel cap or use a thin wire and compressed air to unclog the vent.

5. Stale Oil

Another factor that can cause your engine to not stay running is old engine oil. Once the machine has run for 25 hours, the oil needs to be changed.

The oil in an engine darkens and becomes stale with time due to friction and heat produced by the engine. Stale oil won’t be able to coat the engine’s moving parts, which require lubrication to work properly.

As a result, there is more friction between the moving parts and they may stop working, causing the engine to stall.

Also, make sure that the oil is not too little or too much as this can also cause the engine to stall after starting.


Insert a dipstick into the oil chamber to diagnose the issue. If the oil is dark and thin, it needs to be changed.


Empty the tank of the old oil and refill it with fresh oil. Ensure that you do not add too much oil or too little oil. The oil should be between the ‘Add’ and ‘Full’ marks.

6. Clogged Fuel Lines/ Fuel Filter


Once you have ruled out the possibility of bad fuel, clogged air filter, and a damaged spark plug, you have to check for fuel delivery problems.

The fuel lines and fuel filters are the paths through which fuel moves from the tank to the carburetor. If fuel sits in the tank for more than 30 days, the residue can clog the fuel lines and filter, causing fuel delivery to be reduced.

Lack of fuel in the carburetor and combustion chamber can make your engine refuse to run after starting up.


Check the fuel flow in each fuel line and fuel filter. If it is slow, the fuel lines or filter are clogged.


  • Simply replace the fuel lines or spray carb cleaner through the fuel lines to remove any debris clogging them.
  • Replace the fuel filter with a new one if it is clogged.

7. Clogged Carburetor Jets/Bowl

The state of your carburetor is the next thing you should check if your Briggs and Stratton engine still stalls after starting.

The jets and bowls of the carburetor may become blocked and cause the engine to stall after starting. 

If the fuel pathways become blocked by deposits from old fuel and engine dirt, the combustion chamber will have little fuel to work with and keep the engine running.

By restricting the right quantity of fuel and air from entering the engine, the engine could refuse to run and shut down after starting.


Check for slimy residue and particles in the carburetor bowl and jets.


  • Clean the entire carburetor with carb cleaner to remove all the build-up
  • Then allow it to dry and blow compressed air through the tiny jets to remove any remaining particles

If this doesn’t work, you can repair the carburetor with a repair kit or replace the entire carburetor. You can do this yourself if you have technical skills or visit a small engine repair shop to get it done.

8. Damaged Carburetor Diaphragm

The diaphragm of the carburetor helps to regulate the fuel in the mixture. It is made of a rubbery material that can stiffen with constant exposure to fuel and heat.

It needs to remain flexible to do its job properly. If the diaphragm becomes too stiff or cracked, it will put too much fuel into the engine and flood it. 

As a result, your Briggs and Stratton engine may start but won’t stay running.


The mower smoking excessively is an indication of a damaged diaphragm. 

Take out the carburetor and inspect the diaphragm. If it appears to be stiff or bent, it is damaged.


Replace the carburetor diaphragm with a new one.

How To Replace Carburetor Diaphragm

  • Take out the air filter and air filter housing 
  • Remove the carburetor by disconnecting it from the fuel tank and choke compartment
  • Spray the carburetor with carb cleaner if dirty to avoid dirt getting inside
  • Open up the carburetor screws and take out the rubber diaphragm
  • Inspect the spring supporting the diaphragm. If it is not popping up, replace it
  • Finally, place the new diaphragm on the mounting bolts and reinstall the carburetor


The reason your Briggs and Stratton engine starts but won’t stay running could be a clogged fuel cap, dirty air filter, carburetor problems, old fuel residues, dirty/faulty spark plug, or clogged fuel lines and fuel filter.

Follow the troubleshooting guide and recommended fixes in this article to identify and solve these issues. 

If the problem persists, take it to a nearby small engine technician who can further examine the engine.