lawn mower leaking gas

While lawnmowers are a boon to a property owner who wants to keep the garden looking presentable, when they go wrong, it is very frustrating. You can keep the frustration levels to a minimum by attempting an accurate diagnosis and following a structured repair procedure.

The possible reasons a lawnmower may leak gas from the air filter may include any of the following causes.

  1. The lawnmower tilted on the wrong side.
  2. A broken gas tap.
  3. A sticking choke.
  4. Worn carburetor float needle.
  5. The carburetor gas bowl seal is worn.

Although it may seem dramatic at the time, gas leaking out of the lawnmower engine may result from a simple error, such as tilting it to the wrong side. Even if there is an issue in the carburetor, it can generally be fixed without much difficulty.

The Most Common Reasons Why Gas Leaks From The Air Filter

Gas leaking out of a lawnmower represents a fire risk which needs to be eliminated urgently.

If the lawnmower starts to leak gas out of the air filter, you must find the cause, and if it is due to a fault, ensure you rectify it as soon as possible.

The most common reasons for gas leaking out of an air filter are;

  1. The lawnmower tilted on the wrong side.
  2. A broken gas tap.
  3. Choke sticking.
  4. Worn float needle.
  5. The gas bowl seal is worn.

The Lawnmower Tilted On The Wrong Side

While it is understandable that you may need to tilt the lawnmower on its side, it can damage the lawnmower if you tilt it to the wrong side.

The right side to tilt a lawnmower is with the carburetor (and air filter) pointing up.

Tilting the lawnmower the wrong way may cause gas to leak out of the carburetor and into the air filter.

While it does not have long-term consequences for the lawnmower, you should avoid this situation for the following reasons.

  1. Letting gas drip on a hot engine is a potential fire risk.
  2. Gas pouring onto grass will compromise it.
  3. The oil in the engine may run back up into the cylinder and compromise the spark plugs.

If the air filter is saturated, it is unlikely to start; however, if it does start and there is an issue with the valves, it may cause a flame to shoot out, damaging the various components.

There are two options to rectify this.

  1. Remove the air filter and allow it to dry in the sun.
  2. Replace the air filter with a new unit.

A Broken Gas Tap Can Cause Gas To Leak Out The Air Filter

The gas tap is installed between the gas tank and the carburetor.

Its purpose is to stop gas flowing between the tank and carburetor when switched off.

After a few seasons, these, often plastic, parts begin to degrade and will ultimately fail.

Replacement is very easy and involves the following simple steps.

  1. Empty the gas tank.
  2. The gas tap will be attached to the gas line or directly onto the gas tank, depending on the lawnmower.
  3. Loosen the pipe clamps attaching them to the gas tap. (slide them up the pipe for reuse after the new gas tap is installed).
  4. Remove the pipe from the gas tap.
  5. Depending on where the tap is installed, one or two tubes will be attached.
  6. If the gas tap is installed on the tank, use a long thin rachet or a pair of needle nose plyers to undo the mounting bolt inside the tank.
  7. Replace the gas tap and reverse the procedure.

A Sticking Choke Can Cause Gas To Leak Out The Air Filter

The purpose of the choke is to enrich the gas/air vapor enabling the cold lawnmower to start.

Most lawnmowers’ optimal air/gas mixture is 14.7 parts of air to one part of gas. The choke is needed because as the air gets colder, the air density rises while the gas quantity remains at the same level, so the mixture is too lean.

The choke forces more gas into the mix, and therefore the mixture becomes optimal for starting.

Once started, the engine heats up, and the air density reduces, so the throttle settings need to be changed to the running mode, which changes the mixture to the optimal running setting.

Different chokes work in different ways.

  1. A separate choke jet opens up; when the choke is set, it bypasses the gas inlet port and needle, causing the carburetor to flood.
  2. Other chokes work by restricting the amount of air that enters the carburetor.
  3. Some chokes work by creating a vacuum that sucks in more gas.
  4. Some lawnmower engines are fitted with a priming gas bulb instead of a choke.

While most lawnmower engines are fitted with a manual choke lever or a setting in the throttle lever for starting, some of the later models come fitted with an automatic choke.

Automatic chokes use a spring mechanism that uncoils at lower temperatures. As the temperature increases, it contracts, and the coils tighten, pulling the choke to a closed position.

If the choke plate is sticking, the mower will flood with gas. The more the engine is cranked over, the worse the flooding.

Eventually, it gets to the point where the gas leaks out of the air filter.

If the choke is sticking, the repair may be as simple as releasing the stuck choke cable, or it may require the carburetor to be serviced.

Worn Float Needle Can Cause Gas To Leak Out The Air Filter

A typical carburetor consists of the following components.

  1. The carburetor gas bowl.
  2. The carburetor float.
  3. The carburetor float needle.

A carburetor works with the following steps.

The Gas Bowl

Gas runs down the gas line for the gas tank and into the carburetor, where it fills the carburetor bowl.

The Carburettor Float And Needle

The float has a rubber needle attached to the top of the float.

As the gas runs into the carburetor bowl, the carburetor float rises, and when full, it pushes the rubber-tipped needle against the gas inlet port and seals it.

The carburetor mixes the gas in the bowl with air and sprays it through the inlet valve into the piston chamber.

As the gas in the bowl is used, the needle drops, and more gas is allowed in, so the cycle repeats itself.

A common issue is the rubber tip at the end of the needle begins to degrade, which compromises its seal to the gas inlet port.

If this happens, the needle does not stop the gas from entering through the inlet port, and the bowl overfills, with the excess gas being dispersed into the air filter.

Another possible consequence is that excess gas may enter the piston chamber and, if it is not all consumed, may mix with the oil, causing the oil to dilute and become ineffective.

If you see any sign of this, it is important to stop the lawnmower immediately and repair it.

On some model carburetors, the needle changes color when it wears, indicating that it needs replacing.

If this is the problem, you need to service the carburetor and replace the needle.

A Worn Bowl Seal May Cause Gas Leaking From The Air Filter

The carburetor bowl seal is installed between the carburetor bowl and the carburetor itself.

If the nut holding the bowl in place has been tightened too much, or the seal was incorrectly placed, fuel will leak out. Over time the rubber naturally degrades, and it will eventually fail.

As the air filter attaches to the carburetor, any leaking fuel will flow into the air filter and, when saturated, will leak on the ground.


Gas leaking out of a lawnmower air filter may be as simple as tilting the lawnmower onto the wrong side. It could also involve an issue inside the carburetor and require the unit to be serviced and repaired.

Apart from a possible fire risk, against which all precautions should be taken, the most serious consequence may be the lawnmower gas mixing with the oil in the piston chamber, which dilutes and therefore degrades the efficacy of the oil.

Sharing is caring!

Similar Posts