When closed, the choke helps provide a richer fuel-air mixture to the engine when starting it up cold for the first time in a while. Once the engine is running, the choke should be left fully open and may not need to be closed when starting warm.
If your lawnmower starts with the choke on but suddenly dies when it is off, it probably means that the carburetor is clogged, you are using bad fuel, or there are serious air supply issues in the engine.
A cold engine does not allow the proper flow of oil and fuel in its system. This way, starting a lawn mower can be hard after long storage. Fortunately, the choke mechanism helps you to start and run the engine smoothly for the first time in a while.
When closed, the choke reduces airflow into the carburetor’s throat. This reduces the internal pressure, which allows more fuel into the combustion chamber. The richer fuel-air mixture allows for quick engine starting and running until it warms up enough to continue running on an open choke.
The choke should be left fully open once the engine has started. Running the mower with the choke on for too long will result in a lawn mower smoking, reduced performance, and it would shorten the life of the engine.
Lawn Mower Will Not Run Unless Choked
If your lawn mower does not run unless choked, the following could be the main reasons.
1. Clogged carburetor
If your lawnmower starts with the choke on but suddenly dies when the choke is off, a carburetor is usually a suspect. A lawn mower carburetor can get clogged when fuel in its parts goes bad, especially during long storage. It can also happen when dirt and debris enter the carburetor.
The main function of carburetors is to mix air and fuel to provide a high combustion mixture to the engine. It increases or decreases the air-fuel ratio according to the engine speed and load changing.
2. Air supply problems
An engine highly depends on air for proper combustion of the fuel. However, when delivered in excess, there will be less volume available for fuel. This will result in the lawn mower sputtering or stalling. In this case, a lawn mower will depend more on the choke to run.
A choke restricts the air supply, which creates a vacuum for increased fuel suction. This enriches the fuel-air mixture that enters the engine, thus creating a smooth start-up.
While many things can interrupt airflow into the engine, the main suspects include clogged fuel injectors, faulty hardware in the carburetor, and vacuum leaks.
Air supply problems can also be linked to your clogged lawn mower air filter. In this state, the air filter can block the required air into the engine. It is common for air filters to become dirty or clogged up due to debris from the environment.
3. Using bad fuel
Bad fuel can be the reason why your lawn mower starts but won’t stay running without a choke. Old fuel in your mower can go bad in as little as 3 to 4 weeks, so it’s important to put as much fuel in your mower as needed and to add a stabilizer before storing it over the winter months.
Also, you might be using the incorrect fuel type in your lawn mower. Fuel with higher ethanol content above 10% can cause corrosion of metal parts in the engine, degradation of parts including carburetors, harder starting, and reduced engine life.
4. Spark plug problem
Worn-out spark plugs do not produce sufficient sparks, which is needed for fuel ignition. This results in the engine stalling and failing to start quickly. Spark plugs can also get damaged or become dirty over time which may also result in the same problem.
How to Fix a Lawn Mower That Won’t Stay Running without a choke On
Now that you know the main reasons why your lawn mower is not running without a choke on, let’s now look at how the problem can be solved.
- Start by cleaning your lawn mower carburetor to remove all the dirt and debris causing clogging in the air and fuel lines. Once you have disassembled the carburetor for cleaning, you may also learn that you need to replace some worn-out parts.
- Replace or clean dirty lawn mower air filters. Filters help to clean air before it enters the carburetor. Replace or clean them to prevent dirt and debris from entering the engine.
- Use the right kind of fuel for a lawn mower. Unleaded gasoline with an octane rating of 87 or higher with less than 10% ethanol is recommended for lawnmowers.
- Stabilize your lawn mower’s fuel before winter or any longer storage. A stabilizer extends the life of a fuel and stops it from going bad and clogging the fuel lines.
- Seal any air leaks in the fuel lines, or replace worn-out rubber seals in the carburetor. This will need the help of an expert or an authorized dealer.
- Clean or replace the worn-out spark plugs. Check for signs of bad lawn mower spark plugs, including excess carbon deposits or broken terminals, and take the necessary action.
The choke mechanism should only be used when absolutely necessary, as it can damage the engine if misused. Leaving the choke on after the engine has been successfully started could cause too much fuel to enter the combustion chamber. This flooding may cause the engine to lock up and stall.