Lawn Mower Starts Then Dies
There are a number of possible causes why a lawn mower starts then dies. It could be a failure on the spark plug, lack of gas flow to the engine, a clogged air filter and the most common is a dirty carburetor. Actually, most of the problems that stall the engine while it is just starting come from the problem in the carburetor. But what is a carburetor and what is its function that it serves as an important part of an engine and yet can be the main cause of some lawnmower problems? 
All small gas-powered engines are equipped with carburetors particularly the 2- and the 4-stroke lawnmower engines. Its main function is to mix gas and air to convert it into a more combustible form of gas for the engine to use.
The carb, as we use to call the carburetor, is designed to allow air and gas and do the proper mixing. So whenever you start your lawnmower’s engine, fuel will flow from the tank and pass through a hose and goes inside the chambers of the carburetor. In the case of a 4-stroke engine, it has its bowl that stocks the gasoline and inside it is a floater that controls the inflow and outflow of fuel much like you have in your bathroom toilet float system.
What Happens When
When the engine starts, air will be sucked through the air vent and this will be pressurized inside the carburetor. The carburetor will then atomize the fuel and mix it with the pressurized air to turn it into a fine mist. 
This atomized gas will then be sprayed into the spark plug chamber. Because the spark plug is loaded with electricity, the air-gas mixture which we call air-fuel ratio (AFR) will generate a spark and that begins the internal combustion in the cylinder.
So as long as there is the continuous supply of gas and oxygen from the carburetor, the combustion process will go on and the rotation of the crankshaft will also continue. Unfortunately, lawnmower starts then stops if any of these processes are stopped.
Lawn Mower Starts Then Stops
Now we will talk about the usual problems why mower starts then stops due to carburetor issues. The carburetor is the main mechanical system that’s a separate part of the engine that mixes air and fuel so if there is a problem with it, your lawn mower will surely experience some issues particularly with stalling. Below are a number of the common carburetor problems and the solutions.
Loose
Carburetors have lots of screws so each of these should be tightened because the chambers inside are where the air and gas passes. If there’s a leak due to loose carburetor, there will be an insufficient flow of fuel and even the atomization of gas will be affected. With an unstable composition of the air-gas mixture, the mower starts then stops occasionally.
Solution:
Make it a habit to check your carburetor always if everything is secured and firmly fixed. Wiggle the carburetor a bit and test the tightness of the screws using a screwdriver
Sediments or residues clogging
One of the reasons why a lawn mower starts then dies is because of the blockage of the internal parts of the carburetor. This could be due to the gunk that has formed from old stored gasoline in the carburetor bowl.
Solution:
Drain out the old gas. Unscrew the bowl which you can find underneath the carb and check for trapped dirt. If there’s the gunk, scrape it out. Spray the inside of the carb with carburetor cleaner and reattach the bowl. Start the engine and observe. Put in fresh fuel and add a fuel stabilizer. This can improve the quality of the fuel and can prevent gasoline residue formation for up to 2 years.
Clogged screw pin
You can find in many carburetors the fuel bowl located underneath the carb cylinder. This bowl is where the fuel is stored. At the bottom of this bowl is a screw pin that has a hole and a seal in it and its function is to regulate the downward movement of gas so it’s some kind of a hole air inlet. If the hole in this screw gets clogged up due to gasoline sediments or dirt, the downward movement of gasoline from the inlet would be cut off and that will cause the lawnmower starts then stops.
Solution:
Carefully remove the screw. Clean out the bowl and check the hole on the pin. If you see it clogged or even if it’s not, use a thin wire to clear out the hole with possible debris lodged inside. Spray the inside of the carburetor with a carb cleaner to ensure that the remaining sediments will be dissolved. Recap the bowl but do not tighten the screw too much to avoid damaging the seal. 
Defective or dirty
Other reasons that a lawnmower starts then stops is a problem with the spark plug. Because the spark plug produces the sparks to complete the air-mixture combustion, once there is a carbon build up between the side electrode and the center electrode which are part of the spark plug that generate an electrical spark, there is no possible generation of spark as these two parts need to have a clear gap for the electricity to jump from one electrode to the other. 
Carbon deposits can be caused by an overly rich gas-air mixture or it could be due to clogged air filters. Another cause of spark plug failure is due to oil deposits can be caused by the breakage of spark plug tube seal. This seal or gasket prevents oil contamination when there is a spilling of oil during oil replenishment or oil over spilling. Dirt can also get into the seat of the spark plug and contaminate the electrodes. 
Solution:
The best thing to do with carbon deposits is cleaning it up by disconnecting first the plug from its seat using a plug socket. Clean the plug seat to ensure no soot remains. Use a wire brush in cleaning the gap between the side and the center electrodes and spray this with a plug cleaner or a carb cleaner. 
With oil deposits due to breakage of the seal, better replace the spark plug with a new one but clean the plug seat first. If oil continues to seep into the spark plug seat, this could mean the problem comes from inside the cylinder like a failing gasket or the O-ring of the piston has already worn out. In this case, better bring your machine to a shop repair.
Blocked fuel
Did you know that even the cap of the lawn mower can make the lawn mower starts and dies? In most lawn mowers, their gasoline caps have holes to stabilize the pressure inside the gas tank. Once this hole is clogged by dirt, the pressure will develop a vacuum that can disrupt the flow of gasoline to the carburetor. Fine dirt can easily penetrate this hole so every filling of gas, you should check this hole thoroughly.
Solution:
The hole can be easily cleaned out with a thin wire. But if the cap has already seen better days, better buy a new cap from your lawn mower’s manufacturer.
Pouring too much oil
The one common sign of lawnmower starts then stops is putting too much oil more than what your lawn mower needs. So if you have troubleshooting your carburetor and cleared all the holes and you start your mower and there’s some white smoke coming out from the engine, the most possible cause of a stalling lawn mower is oil overloading. This means that some of the engine parts are being drowned by too much oil and some excess is dripping out and burning which produces the white smoke. 
Solution:
If you suspect that too much oil is the culprit is draining about 1/4 of the content oil of the machine then use the dipstick to measure the oil level. Be aware of the level at the end of the dipstick and the oil level must not be over the upper line. Drain excess as necessary. To drain oil, locate the drain plug which is usually underneath the mowing deck. If there’s no drain plug, open the oil cap and tilt the machine towards the direction where the oil reservoir is located and catch the oil with an oil pan.
Clogged air
A lawn mower will always need air to sustain its combustion for the air-fuel mixture and it needs clean air to do this. If there is insufficient air coming to the engine because of dirty air filter, this can cause the air-to-fuel ratio to be overly rich thus can cause sputters or stalling of the machine. So most likely when the mower starts then stops while the carburetor is in good condition, the problem can come from a clogged air filter. 
So before you start the engine, don’t forget to check the air filter which is very easy to remove and clean. There are 3 types of air filters for lawn mowers and each type requires a specific cleaning method. These are the foam, paper and dual-elements.
Solution:
A dirty foam air filter only needs washing with soap detergent and water and squeezing it until all the dirt and oil residue is gone. Then dry it by squeezing it again while wrapped in a clean cloth. You can put 2 or 3 drops of engine oil on the sponge and squeeze it again to spread it. This is to make the sponge more effective in filtering out dust and dirt coming from the air and soil. 
The paper type filter can be cleaned using compressed air but if it is already worn out, replace it with a new one. These types are disposable. The dual-elements have sponge and paper cartridge in them. Clean the sponge as described above and use compressed air on the paper filter.
Mower Starts Then Stops
If you noticed, we always recommend the use of carburetor cleaner in every step of the way in cleaning the carburetor of lawn mowers. But did you know the benefits of using these cleaners? Here are some of the things they can be good for.
It can clean your carburetor inside which helps in the distribution of the right amount of fuel necessary in achieving the exact air-fuel ratio. 
It can dissolve stubborn built-up dirt inside and outside the carburetor and there’s no need for scrubbing or even disassembling your carburetor. Just unscrew the carb bowl and do a quick spray inside until you see the dirt dripping out and that’s how easy this stuff works.
Not only it is effective in cleaning and degreasing your carburetor and prevent the problem of a mower starts then stops but it can also be used in cleaning out the inside of your cylinder if you are disassembling it. So carb cleaners are actually all-purpose degreasers and dirt dissolvers.
Always a handy stuff in achieving better performance for your lawn mower engine. Since dirt is the usual culprit getting trapped inside the carburetor, the carb cleaners are the only solution to clean this dirt out without detaching the carburetor completely.
Types of Carburetor
There are different types of carburetor cleaners and these are the chlorinated,non-chlorinated, aerosol spray, and dipping can. Some of the chlorinated types contain volatile organic compounds which unfortunately are banned in some states. However, these compounds are what make the carb cleaners more effective in clearing out dirt. 
The non-chlorinated carb cleaners are less toxic but these are more flammable. So being flammable means you don’t use them in a very hot environment but are effective cleaners if used inside the garage. They don’t, however, dry out easily unlike with the chlorinated that dries in seconds but are still safe to use on materials with plastic components. 
The aerosol types which are usually in spray cans are the most common and effective in reaching even the inside of your carburetor especially in cleaning up passages, crevices and holes. The carb cleaner dip can types are best preferred for those who want to soak their engine parts in a pan for overall cleaning.
Our
Now that you have learned a lot about carburetors and how they work including their usual problems that makes the lawnmower start then stops, hopefully, you will know what to do once your machine dies on you after a few seconds of starting. And we have provided you also the best information about how carb cleaners work. 
So the next time you work with your mower and it begins to show starting then stalling, try these troubleshooting tips. These may save you a lot of money going to the repair shop. A simple fix with a professional mechanic can mean a good amount of money. However, if you have done everything you have learned here and still your lawn mower starts then dies from time to time, try to call your mower’s manufacturer for some advice. Also, watch Youtube videos that provide clearer hands-on explanations.
But if you don’t have time for these and want quick repair attention with your mower, then a professional mechanic can do an excellent job for you plus you will be given some tips for proper maintenance.
