Lawn Mower Leaking
Gas from Air Filter

A hot mower leaking gas obviously isn't safe especially if you store your mower in a garage attached to the house. This is a problem that needs to be taken care of right now.

So why does the lawn mower leak gas from the air filter? The most common reason a mower leaks gas from the air filter is because it was tipped over with the carburetor side facing down. But there are other possible reasons:

  • Choke sticking
  • Worn float needle
  • Gas bowl seal worn
  • Perished gas line
  • Gas tank cap damaged

When a mower clogs with grass it's very tempting to just tip it over and clean it. This unfortunately allows fuel to get into places it shouldn't. If the mower is tipped over with the carburetor side down, gas will flood out of the carburetor and soak the air filter.

You may also find the mower hard to start, running rough and often blowing black smoke. This is a sure sign that the air filter is soaked in gas. The fix is simple - remove and replace the filter.

mower air filter

Air Filter

A gas covered filter will cause the engine to not run, run rough and often with black smoke.

Simply replace the filter.

This condition is known as flooding and it's easy enough to fix. We'll cover it right here in this post.

Tipping your Mower Over

The best way to tip your mower up is with the handlebars to the ground. If you find this unworkable, then tip the mower over but always with the carburetor side up.


The carburetor is typically on the opposite side to the muffler, it will have an air-filter cover that's usually rectangle or square and made from plastic.


But before you work on your mower, disable it first. Remove the plug wire and turn off the gas. Not all mowers will have a gas valve, just make sure you turn your mower over, carburetor side is up.

mower plug wire

Plug Wire

Pull the plug wire before  working on your mower, it prevents accidental starting.
mower gas tap

Gas Tap

If fitted to your mower, turn it off when mower not in use and before starting any maintenance.
mower maintenance

Tipped Up

You can tip your mower up like this by hooking the handlebars under a bench.
mower maintenance

Tipped Over

If you prefer tipping your mower to the side, that's fine just be sure the carburetor side is facing up.

The carburetor side is the same side as the air filter.

What Is A Float Needle?

A carburetor fuel supply usually consists of a fuel bowl, float and needle. The float is as its name suggests a float, attached to it, is a needle with a rubber tip.


The function of the float is to lift the needle as the fuel level rises in the fuel bowl. When the fuel bowl is full the needle will be pushed against the fuel feed port, sealing it. 


Failure commonly occurs in the carburetor when the rubber needle seal wears. This results in fuel continuing to fill the carburetor and eventually making its way into the air filter housing flooding your air filter.



In some cases the cylinder and crankcase will fill with gas, this stops the piston from moving, and is known as hydro locking. Removing the spark plug will release the fuel.


Other signs that your carburetor needle leaks, are: over full oil level; white smoke from the muffler; oil leaking from the muffler; gas leaking from carburetor; strong smell of gas in the garage.


If you have gas in the oil, don't run the engine, as the diluted oil offers little protection to internal components. First fix the issue by replacing the carburetor and then changing the oil. 


A leaking needle seal, is the carburetor telling you it's a little tired. You could replace just the seal but it's better to go ahead and replace the complete carburetor, they aren't expensive and it's a complete fix.


Check out this guide on "Carburetor Repair & Cleaning".


carburetor float


The float and needle control gas supply to the carburetor.
Carburetor needles


The tip of the needle is a seal and when worn turn pink. On some engines the seal is embedded in the carburetor needle seat.
carburetor float


When the float is in the dropped position gas should flow steadily.

Carburetor float


You can check the float and needle by removing the fuel bowl and lifting the float up, the gas should stop flowing.

If it doesn't - replace needle/seal or the whole carburetor.

Is The Choke Sticking?

As you know the function of a choke or priming bulb is to enrich the fuel mixture, so a cold engine starts smoothly. Your mower engine will be fitted with one or the other.


The choke enriches the mix by restricting the amount of air entering the carburetor. The priming bulb does it by injecting extra fuel.


Gas engines run best when the ratio of air to fuel is 14.7 to 1. Meaning 14.7 parts air to 1 part fuel, also known as air fuel ratio (AFR). Using the choke or priming bulb counteracts the lean condition caused by the dense cold air, on a cold start.


Choke Type

Most mowers will typically have a lever to control choke, if a choke plate is sticking in the on position, the mower will flood itself with fuel and the more you crank over the engine, the worse it gets. Choke cables will need adjustment from time to time.


More recent engines from Briggs & Stratton and Honda offer a thermostatically controlled automatic choke system. It's a simple set up - a thermostat, positioned beside the muffler pushes open the choke plate progressively as the engine heats. 


The system is a great idea no doubt, no more fiddling around with choke levers or priming bulbs. 


Thermostats can fail from time to time, if your choke isn't opening as the engine heats, it will flood the engine with gas. 


mower throttle lever mower throttle lever


When the choke is in the off position, the choke plate should be completely open.

Removing just the air filter will allow you see the choke plate.

If yours isn't opening, adjust cable or if yours is auto choke, check for binding.

What Is The Gas Bowl?

All carburetors will have a fuel bowl. This is where the gas enters the carburetor and it's from here the jet sucks the gas to feed the engine. You will find it behind the air filter, it's a distinct bowl like shape. 


The bowl is fixed to the carburetor with a single bolt usually. The fuel bowl will have a large rubber O-ring seal and the bowl retaining bolt will have a gasket seal also. The seals brake down over time and can also get pinched when refitting the fuel bowl.


Over tightening the retaining bolt will distort both seals and cause them to leak.


mower gas bowl seal mower carb seals

Bowl Seal

The bowl seal will leak if pinched, so care needs to be taken when refitting the gas bowl.

The seals become perished over time also.

Checking The Gas Lines

Fuel lines commonly leak at the connection clamps, replacing the clamps will fix the problem. In other cases the fuel line itself is braking down, it's not unusual to see a fuel line perished at the connection point and the quick fix is clipping the perished area off and refitting.


Some manufacturers say the increasing use of ethanol blended fuels is shorting the life of the plastic and rubber components of small engine fuel systems. Most manufacturers recommend regular gas or ethanol e10.


Fuels such as e15 and e85 are not recommended as they will damage your engine and void your guarantee.


mower gas line mower gas line

Gas Line

Check the line for cracks and leaks. Replacing gas line is an easy repair.

Bear in mind gas line comes in different sizes, so measure the inside diameter (id) of the old line before buying.

Leaking Gas Tanks

This is rare but I have seen it few times. The plastic gas tank develops a hairline crack and starts to split along the molded seams. No quick fix here, the tank must be replaced.


Gas cap seals perish and start to leak, only an issue when the gas tank is full and your mowing a steep hill.


Mower gas tank Mower gas caps


Tanks are pretty durable, but if left in direct strong sun can damage them.

Poor fitting or damaged gas cap will leak. A gas cap must be vented.

Related Questions

Lawn mower leaking oil from the air filter? The most common reason oil is leaking from a lawn mower air filter is too much oil in the engine, but it can also happen when the mower has been on its side.

Lawn mower leaking gas from the primer bulb? The primer bulb is perished and has split, mice like to eat them also. Remove the primer bulb retaining ring and replace the primer bulb.


Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.