I want to show you what to do if your lawn mower runs for a while and then dies. It is one of the most common problems affecting older lawn mowers but it’s an easy fix.
What is happening is that the lawn mower is starving for fuel, spark, or air. You will need all three for a long-running engine. In this post, we will show you how to check for all three issues easily and fast.
Table of Contents
Lawn mower stops running when hot
A lawn mower will get up to running temp within a few minutes. This is about the length of time it takes for fuel to run out of the carburetor. This is a great indication that you have a fuel restriction in the carburetor.
What happens is the fuel is not getting from the bottom of the carburetor bowl into the engine. Often this is because of a swollen needle seat, sticky float, or a blockage. You will need to remove the carburetor to fix this problem.
Fuel restrictions can be either foreign material like grass, dirt, or air filter material or a swollen pilot needle seat in the carburetor.
A quick test is to start your lawn mower and remove the fuel tank cap. This will test if there is a problem with the one-way valve in the filling cap. If your filling cap is blocked a vacuum will prevent fuel from going into the carburetor.
If your lawn mower still runs for a while and then dies with the fuel cap off then check for blockages in the fuel lines. Sometimes the fuel lines can swell up and restrict fuel flow.
Inline fuel filters will be at the discharge to the fuel tank. Remove the bottom hose to check and inspect this fuel filter. If these are all good you will need to clean the lawn mower carburetor.
How to clean lawn mower carburetor
Lawn mower carburetors are very basic. They compose of 5 main parts.
- Outer body
- Internal float
- Needle jet
- Main jet
- Choke and throttle.
There is not much that goes wrong with them unless dirt gets into the carburetor blocking up the small holes or swelling the o-rings.
Start by turning off the fuel to the fuel tank or draining the lawn mower fuel tank.
Remove the carburetor by removing the two or three bolts that are holding it to the intake manifold. Be sure not to lose or damage the gasket seals. These can be reused if they are in good condition.
Remove the bottom bolt from the carburetor and remove the bowl casing. Inside you will see a float that will move up and down. This is like a valve that controls the fuel going into the bowl and the engine. It controlled a valve called the needle seat jet.
This rubber seat can swell over time and restrict the fuel flowing into the bowl. Remove the pin that holds the float in position. It should side out. Then remove the needle seat jet that the float is connected to. Pull out the seal that is under the needle seat jet and look for swelling, tears, compression, or deformities. Dig it out with a small screwdriver.
If this is good you will need to remove and clean out the holes in the main jet. These small holes will need to be rodded out with a very small piece of wire. Small paper clips work well for this task once straightened out.
Unscrew the main jet and clean out the hole with a small piece of wire. Once the hole is clean screw it back in.
My Briggs and Stratton engine runs then dies problem was gone once I cleaned out and replaced the pilot seal on my carburetor. They do look complicated to service but once you start pulling it apart it really is simple.
If you don’t want to service your carburetor, you can just replace the whole unit as they are quite cheap nowadays. Check out our post on different Briggs and Stratton carburetor types and compatibility.
How to prevent carburetor blockages.
Always drain your fuel from the carburetor after using your lawn mower. You can do this by turning off the fuel tap and running the lawn mower until it stops due to running out of fuel.
This makes the carburetor dry and it will prevent the swelling of the pilot and main jet seats.
If you don’t have a fuel tap just empty the fuel tank and run the lawn mower dry if you are storing your lawn mower for longer than a few months.
Be sure to clean your fuel tank filter every few years. These tend to block up over time if your fuel cap lid is faulty or cheap. Your fuel cap will have holes in it to equalize the pressure in the tanks. Dirt can creep into the tank over time.
Checking the spark plug
Spark plugs can fail when they get hot. Often this is because of a crack in the insulator. Spark plugs are cheap so it is best to replace this first if you have a lawn mower that runs for a while and then dies.
Check the spark plug lead that it is not worn or rubbing on some metal part. The breakdown of electricity is a sure way of stopping a lawn mower from running.
If the spark plug is dirty then clean it with some 300 grit sandpaper. Make sure there is no carbon between the insulator and the metal housing.
Checking the air filter
Be sure to check and clean the air filter. These will block up over time and will restrict airflow to the engine. If you have a blocked air filter it could cause your lawn mower to run for a while, then cuts out and dies.
Check out our post on how to service your 4-stroke lawn mower every year to prevent starting and running issues.
How to start a lawn mower with a choke
A choke makes starting lawn mowers easy and fast. It reduces the airflow into an engine which in turn creates a bigger vacuum so more fuel is pulled from the carburetor into the cylinder. This fuel-rich mixture will quickly ignite from the spark plug but will cause a rough idle until the choke is pushed back into the normal running position.
These are two different types of chokes on a lawn mower.
- Manual Choke
- Automatic Choke
Manual chokes are older and are usually connected from the throttle cable via springs or via a push-pull lever on the side of the carburetor. To start a lawn mower with a manual choke pull the throttle cable to the start position and start the lawnmower. After a few minutes of warming up the lawn mower, you can put the throttle choke into the normal running position.
With the air filter off you should see the choke close off to the intake of the carburetor. Check that the choke is moving by pushing and pulling the choke throttle cable.
An automatic choke works via a temperature-controlled device called a thermostat. This piece of metal will swell and moves a lever that will open and close the choke depending on the temperature of the lawn mower.
If you are running a lawn mower with an auto choke and you have trouble starting it when it is cold or hot you could have an issue with the auto choke. Make sure all of the links and leavers are connected and not broken.
The lawn mower only runs on choke.
If your lawn mower only runs on the choke then you may have a too lean fuel mixture or you have a blockage in the carburetor. Both will need to be investigated why this is the case.
If your fuel mixture is too lean you can richen it up by adjusting the fuel screw or idle screw on your lawn mower carburetor. Check the make and model of your grass cutter online for how this is done.
I had this problem with my Briggs and Stratton engine runs then dies. Using the choke fixes it temporarily, I had to remove the dirt blockage in the main jet.
Why does my lawn mower only run for a few seconds and then dies?
This problem affects all lawnmowers both 4-stroke and 2-stroke. It all comes down to fuel flow either into the carburetor or from the carburetor. It affects, Briggs and Stratton, Honda, Victa, Husqvarna, Rover, Bolens, Masport, John Deere, and many other brands.
What is happening is fuel is getting into the engine from the primer bulb when you press it 3-4 times. This fuel is then burnt off and the engine is stopping after a few seconds. The cause is either a faulty fuel pump or a blocked main jet.
On Briggs and Stratton engines the fuel pump is inbuilt into the fuel tank via a diaphragm and spring. Replace this diaphragm if it has a hole in it. They can get tough and brittle over time so always replace this diaphragm when cleaning the plastic carburetor.
Clean the air filter, replace the spark plug, and get new fuel.
Make sure the choke is working correctly and that you have no fuel restrictions.
I would also check the engine oil level. Some more expensive brands incorporate a low-level switch that will cut the power to the lawnmower when it detects a low engine oil level. This only applies to 4-stroke engines so check the oil level.
If your small engine starts with starter fluid and then dies you defiantly have a fuel delivery problem. Check the fuel filter, carburetor, and any air leaks.
Lawnmower Only Runs On Starting Fluid
If your lawn mower only runs on starting fluid then there could be an issue with the fuel delivery or the air-to-fuel ratio.
Often I find it is bad fuel, a blocked fuel filter, a stuck float in the carburetor, or a blocked main jet. The spark is fine because the lawnmower will run with starting fluid.
Check out how you can make your own starting fluid.
Conclusion
If you flow the above tips you will be able to fix a lawn mower that runs for a while and then dies. It all comes down to fuel 99% of the time. But occasionally it’s the spark plug or an air filter issue.
Always run your lawnmower with a clean air filter. These will block up and deteriorate over time. Once they fall apart they will block up your carburetor and you will need to take it apart to clean it.
Fresh fuel is also helpful to keep your lawn mower running in tip-top condition.
With the world moving to electric lawnmowers may be the trouble of starting them will become a thing of the past.
I am a qualified Industrial Electrician for the past 20 years and I love to share my knowledge on home repair and maintenance jobs.
I love fast toys like Motorcycles, Cars, Jetskis, Boats, and Computers so writing about them is easy. Working on them is fun.
To keeps costs down I do all my own mods, repairs, and servicing. These skills I want to share with everyone. DIY is a skill everyone can learn.
