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Lawn Mower Leaking Gas From Air Filter – Causes and fixes

By: Author John Cunningham. Published: 2019/01/31 at 6:59 am

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 lawnmower leak gas from the air filter? The most common reason a mower leaks gas from the air filter is that 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 just to 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.

Air Filter – A gas-covered filter will choke the engine, causing rough running and black, and often results in nonstarting.

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

Correct Way To Tilt Mower

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.

Carburetor location – The carburetor is typically on the opposite side of the muffler; it will have an air-filter cover that’s usually rectangular or square and made from plastic.

Disable engine – Before working on a mower, best to disable it first. Remove the plug wire and turn off the gas. Not all mowers will have a gas valve; turn your mower over, and the carburetor side is up.

Of course, tipping the mower over incorrectly isn’t the only cause of leaking gas from the air filter; a faulty float needle is common, too, and that’s what we’ll look at next.

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, and 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 overfilling the carburetor and eventually entering the air filter housing, flooding your air filter.

Snowblower float needle

Float – The float and needle control gas supply to the carburetor.

Need more info on the fuel system, carburetor components, and how they work, you can check them out here.

Small engine carburetor components


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: overfull oil level, white smoke from the muffler, oil leaking from the muffler, gas leaking from the carburetor, and a 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.

Carburetor needle valve replacement

​A leaking needle seal is a 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” or check out the carburetor repair video here; it covers removing and refitting the float and needle valve.

Needle – The tip of the needle is a seal and, when worn, turns pink. On some engines, the seal is embedded in the carburetor needle seat.

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

Up – 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 the 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 an air-fuel ratio (AFR). Using the choke or priming bulb counteracts the lean condition caused by the dense cold air on a cold start. Check out the choke test video here.

Choke Type

Most mowers will typically have a lever to control the 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 setup – 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. 

Throttle – When the choke is in the off position, the choke plate should be completely open. Removing just the air filter will allow you to see the choke plate. If yours isn’t opening, adjust the 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, 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 break down over time and can also get pinched when refitting the fuel bowl.

Jet and bowl clean

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

Bowl Seal – The bowl seal will leak if pinched, so care needs to be taken when refitting the gas bowl. The seals perish over time, also.

Check out lawnmower carburetor kits on the Amazon link below.

Amazon Lawnmower Carburetor Repair Kit

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 breaking down; it’s not unusual to see a fuel line perish at the connection point; the quick fix is clipping the perished area off and refitting.

Some manufacturers say the increasing use of ethanol-blended fuels is shortening 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.

Gas Line – Check the line for cracks and leaks. Replacing a 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 a 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 you’re mowing a steep hill.

Leaks – Tanks are pretty durable, but if left in the direct strong sun can damage them. Poor-fitting or damaged gas caps will leak. A gas cap must be vented.

Related Questions

Lawnmower 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.

Lawnmower leaking gas from the primer bulb? The primer bulb is perished and split; mice like to eat them also. Remove the primer bulb retaining ring and replace the primer bulb.