Lawn Mower Will Not Run Unless Choked? How to Fix

When closed, 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, choke should be left fully open and may not need to be closed when starting warm. If a lawn mower will not run unless choked, it is a sign of problem. In this guide, let’s look at why this happens and how to fix.

If your lawnmower starts with the choke on but suddenly dies when the choke is off, it probably means that the carburetor is clogged, you are using bad fuel or there is serious air supply issues in the engine.

Lawn Mower Will Not Run Unless Choked

A cold engine does not allow proper flow of oil and fuel in its system. This why it can be hard starting a lawn mower after a long storage. Fortunately, choke mechanism helps you to start and run the engine smoothly for the first time in a while.

When closed, choke reduces the flow of air 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 starting and running of the engine until it warms up enough to continue running on open choke.

Choke should be left fully opened once the engine has started. Running the mower with the choke on for too long will result to a lawn mower smoking, reduced performance, and it would shorten the life of the engine. If your lawn mower will not run unless choked, the following could be the main reasons.

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 a long storage. It can also happen when dirt and debris enter the carburetor.

The main function of carburetors to mix air and fuel to provide a high combustion mixture to the engine. It increase or decrease the air-fuel ratio according to the engine speed and load changing. This will not take place when there is debris partially blocking air and fuel lines.

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 make your lawn mower to sputter or stop running. In this case a lawn mower will become more depended on the choke to run.

A choke works by restricting air supply, which creates a vacuum for increased fuel suction. This enriches the fuel-air mixture that enters the engine thus a smooth start up. While there are many things that can interrupt the flow of air into the engine, main suspects include clogged fuel injectors, faulty hardware in 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 amount of air required into the engine. It is common for air filters to become dirty or clogged up due to debris from the environment.

Bad fuel

Bad fuel can be the reason why your lawn mower start 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 only put as much fuel in your mower as needed, and to add stabilizer before putting it into storage over the winter months.

Also, you might be using incorrect type of fuel in your lawn mower. Fuel with higher content of ethanol above 10% can cause corrosion of metal parts in the engine, degradation of parts including carburetors, harder starting, and reduced engine life in small engines.

Spark plug problem

Worn out spark plugs does not produce sufficient sparks, which is needed for starting and running the engine. This results in the engine to stall and fail to start quickly. Spark plugs can also get damaged or become dirty overtime which may also result to 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 choke on, let’s now look at how the problem can be solved.

  1. Start by cleaning your lawn mower carburetor to remove all the dirt and debris that are 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.
  2. Replace or clean dirty lawn mower air filters. Filters help to clean air before it enters the carburetor. Replace or clean them to prevent chances of dirt and debris from entering the engine.
  3. 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 lawn mowers.
  4. Stabilize your lawn mower’s fuel before a winter or any longer storage. A stabilizer extends the life of a fuel and stops it from going bad and clogging the fuel lines.
  5. 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.
  6. 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.

Final Thought

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 into the combustion chamber. This flooding may cause the engine to lock up and stall.

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