Lawn Mower Only Starts Without Air Filter

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Running your lawn mower without an air filter can allow a lot of dirt to enter the mower’s engine and cause serious damage to other engine parts. 

Because of this, it is important that you find out the cause of the problem and fix it to prevent your engine from getting damaged.

In this article, I talk about some of the reasons your lawn mower may refuse to start with the air filter installed, helpful steps to diagnose and fix the problem, and also some of the risks associated with running your engine without an air filter.

Reasons Why Your Lawn Mower Only Starts Without Air Filter

The reason your lawn mower starts only without an air filter may include: 

  • Clogged air filter
  • Carburetor running too rich
  • Gas in the oil
  • Dirty/damaged breather tube

Tools You Need for the Job

Some tools you may need to help you examine your engine and diagnose the problem include:

Lawn Mower Only Starts Without An Air Filter: Causes, Diagnosis, and Fixes

1. Clogged Air Filter 

An air filter purifies the air that goes into the engine of the lawn mower to prevent dust, grasses, and other debris from entering it. 

When the air filter gets clogged with too much dirt or oil, air will be restricted from entering the lawn mower. This can prevent the engine from starting with the air filter installed because the combustion process of an engine requires air.

Removing the clogged air filter will facilitate airflow into the engine and allow the engine to start and run smoothly.

Diagnosis:

Open the air filter housing and inspect the air filter. If it is dirty or clogged with oil, it is restricting airflow into the engine.

Fix:

  • Wash/replace the air filter if it is a foam filter
  • Replace the air filter if it is a paper filter

2. Carburetor Running Too Rich

A damaged carburetor can also make the engine refuse to start with the air filter on. The carburetor might be running too rich meaning that the engine is receiving too much fuel and too little air. 

The engine running too rich will make it require more airflow to balance the fuel supply and the air filter might be limiting the amount of air allowed into the engine which will stop the engine from running.

Removal of the air filter allows air to flow continually into the engine, balancing the excess fuel supply and improving the combustion process. Although this may work temporarily, it can be harmful to the mower in the long run.

Diagnosis:

Inspect the carburetor float needle and fuel lines to see if they are damaged or stuck. 

Fix:

The damaged parts should be fixed or replaced if the damage is severe. 

3. Gas In The Oil

Another reason your mower may start only when the air filter is removed is that gas has found its way into the oil tank. This could be a result of the engine running too rich.

When the fuel is supplied in excess, it can escape from the carburetor and the combustion chamber into the oil compartment and mix with the oil.

This will make the oil too thin and ineffective and can cause the engine to overheat and require extra airflow. 

Diagnosis

Pour out some oil from the oil compartment. If it smells like gas or appears to be too thin, it has mixed with gas.

Fix:

Drain out the bad oil and refill the tank with a new one.

4. Dirty/Damaged Breather Tube

A blocked or damaged breather tube can also be the reason your lawn mower refuses to start with an air filter. 

The breather tube collects the hot gases that accumulate in the engine from the fuel-air combustion process and directs the gases and hot air out of the engine through the exhaust system and muffler.

If the breather tube gets clogged, the engine overheats and will require more airflow for it to start and run effectively. This could be why your lawn mower only starts without the air filter.

Diagnosis:

  • Remove the muffler to access the breather tube
  • Inspect the holes on it to determine if they are open or clogged 
  • If they are open, inspect the breather tube. If you notice oil leakage, cracks, or clogged tube, the breather tube is most probably the cause of the problem.

Fix:

  • Clean the holes if they are clogged with dirt or debris
  • Replace the breather tube if cracked or seriously damaged

Risks Of Running A Lawn Mower Without An Air Filter

Running your lawn mower without an air filter is never a good idea. You might be tempted to run your lawn mower without an air filter especially as it may run great without it but this can affect your lawn mower in so many negative ways. 

Some risks of running your mower without an air filter include:

1. Damaged Engine Components 

In addition to controlling the air supply, air filters are responsible for preventing dust and dirt from getting into the internal engine components.

These particles enter freely into the engine and clog, block, and damage various engine parts such as the carburetor, fuel lines, and cylinder heads over time. Fixing/replacing these parts would cost more than just replacing the air filter.

2. High Fuel Consumption  

Air filters help to save money spent on fuel by preventing dirt from getting into the mower engine. Accumulation of dirt in the engine causes increased friction which leads to high fuel consumption.

The excess airflow into the engine will increase the fuel consumption by the engine, making you spend more money on gas.

3. Mower Runs Rough 

A clean air filter is responsible for the engine running smoothly. The absence of an air filter also causes your lawn mower to run roughly as a result of dirt accumulation, friction and overheating.

Mowing your yard with a mower that runs roughly can make your work harder and more frustrating. This is why you should always install a clean air filter before starting your engine.

Conclusion:

Although your mower might start and run smoothly without an air filter, it is not advised to do so as it can damage vital components of the engine.

Thoroughly inspect the carburetor, air filter, breather tube, and other parts mentioned in this engine to diagnose the problem, and then apply the recommended fixes to avoid severe engine damage.