Why Is My ATV Carb Leaking Gas? 

When your ATV carburetor or cab leaks gas, you could be thrown for a loop. Why is this happening and how do you fix it?

Your ATV carb could be leaking gas for these reasons:

  • Improper float height
  • Damaged carb bowl gasket
  • Old needle valve O-ring seal or brass fitting
  • Dirty gas
  • Stuck carb float
  • The tank vent is blocked

In this article, we’ll go through each of these reasons for ATV carb leaks to rule out the culprit. By the time you’re done reading, you’ll know how to diagnose your problem so your ATV carb can run without issue after a few small repairs!

6 Reasons Your ATV Carb Is Leaking Gas + How to Fix Them

Improper Float Height

The first reason that your ATV carburetor could leak gas is that the carb float is not at the right height. 

Before we can get into what height the carb float should be, allow us to briefly explain what a carb float is for the uninitiated. 

A carb float is a metal or plastic foam component with a needle. The float sits inside either a bowl or a float-feed chamber where it floats in fuel. It’s installed on the carb’s side. 

The needle in the carb float determines how much fuel the float chamber receives.

When the float height is misaligned, not only can the carb begin leaking gas, but you’re wasting whatever gas is in your ATV. This is quite an expensive problem then and one that’s worth fixing immediately.

How do you check the float height? First, access the ATV’s drain valve. You’re looking for the area where the drain pipe and float bowl diverge.

Take some rubber pipe and place it beside the float bowl. Then gauge how much distance is between the current fuel level and the float bowl flange from the top.

Your ATV’s owner manual will tell you what the correct float height should be. Align the float bowl so it’s level.

After all, if the bowl is at even a slight angle, the fuel you pour into your ATV could end up in the carb overflow pipe.  

The Carb Bowl Gasket Is Damaged

If your ATV’s carb float is at the correct height, then the next area to troubleshoot is the carb bowl gasket. 

This gasket goes between the carb bowl and the carb housing. The longer you’ve had your ATV, the better the chances the gasket is in bad shape, as wear and tear can cause it to degrade.

Accessing the carb bowl too often is another way to wear down the gasket.

You’d first have to disassemble your ATV’s carburetor to reach the carb bowl gasket. Once you take it out, you can assess it more closely.

To uninstall the ATV carburetor, you must turn the fuel valve off. You can also use a clamp to set the fuel line into the off position.

Next, remove plastic covers, the air filter box, and the air filter, as they all block the carb. 

If your ATV has a two-stroke engine that utilizes an auto-lube system, you’ll have to first detach the two-stroke injection pipe, then the clamp that keeps the carb attached to the rubber intake manifold. 

For other engine types, you can skip this step. 

Finally, you should loosen all the screws keeping the carb choke intact, and then the carb is free. Now you can look at the gasket. 

Repairing the gasket isn’t really an option, so you’ll have no other choice but to replace it. The good news is that the gasket isn’t very expensive.

As you install the new gasket, don’t rush. The components that surround the carb bowl gasket are fragile. If you bump or damage those parts, then you will have to spend considerably more money to get your ATV in operable condition. 

The Needle Valve O-Ring Seal or Brass Fitting Needs to Be Replaced

Your ATV carb also has a needle valve, which determines how much gas or fluid the carb receives. 

The needle valve is usually sealed to an O-ring so you don’t have to maintain it, even if you own and use your ATV for many years.

The O-ring itself is between the seat and the actuating threads. It’s supposed to fit very closely around a cylinder. This limits thread contamination and keeps the needle valve at a lower internal volume. 

You’ll have to take out your ATV carburetor to get a closer look at the needle valve O-ring. It could be that the ring dried up over time, which can happen. 

Perhaps the needle valve isn’t sealed when the brass fitting is securely closed. 

You can determine if the problem is one of these two issues with a bit of troubleshooting.

First, you want to confirm that the float valve is set into the correct position and that it’s also clean. 

Next, you can reattach your ATV’s fuel line. Set the fuel valve into the on position. If you see gas coming through the ATV float valve, even if it’s not a lot of gas, then everything is working as it should so far.

Raise the float a little and wait until the gas trickle stops. Well, it should stop, but it might not even if the float valve is closed. That tells you it’s an O-ring problem. 

Now that you suspect that the O-ring could be your culprit, take off its plate and screw. This will separate the brass fitting from the rest of the O-ring. 

You’ll probably require needle-nosed pliers to get the brass fitting all the way off.

The O-ring should jut out from the brass fitting’s diameter. The ring should also have a degree of flexibility. If neither is true, then it’s time to replace the old O-ring with a fresh one.

Be sure to measure the cross-section and inner diameter of the O-ring to ensure you get one that’s the proper size.

You’ll need calipers to match the appropriate size of the O-ring to the current one. With the calipers, calculate the brass fitting diameter along the groove where the O-ring goes. That’s the inner diameter.

The outside diameter is usually larger than the inner diameter. If it’s at least two millimeters larger, then the new O-ring should be a single millimeter bigger than the inner diameter to ensure the ring creates the proper seal.

As you place the new O-ring, using silicone grease or another lubricant will make it a lot easier. 

You’re Using Dirty Gas

As we always say in these troubleshooting articles, sometimes a problem isn’t as complex as measuring a cross-section and inner diameter of an O-ring.

Think for a moment about the type of gas you filled your ATV with last. Is the gas fresh? Is it clean?

If you’re not quite sure or if you answered no, then you could have solved your own problem. Dirty gas in the tank is not good. 

Even though your ATV has a fuel filter, that filter cannot catch everything, especially very small particles. Those particles linger in your ATV’s internal system, breaking down the gas lines little by little.

As the gas line’s inner wall disintegrates, rubber particles flake off. These particles are small too so they can get through fuel filters. 

The particles eventually accumulate in the ATV’s valve and force it into an open position. Since the valve is always open, the bowl becomes full and then overfull with fuel.

The excess fuel drips into the carburetor.

To determine if this is your problem, open up your ATV and peer in at the gas lines. 

Lines that are more than a decade old should not remain in your vehicle, as they’re going to break down sooner than later if they haven’t already.

While you’re at it, you might as well change out the gas filter as well. It’s more than done its job and isn’t clean enough to effectively catch most particles anymore. 

The Carb Float Is Stuck

If you’re still struggling to figure out what may be wrong with your ATV carb, it could be time to go back to the float.

The float, as you know, has a needle that gauges fuel levels. Well, in instances where the fuel levels are excessively low, the float begins to drop. This applies pressure on the needle valve or float valve.

From there, more fuel would be allowed into the carb bowl to ensure it isn’t empty for long. 

Once the fuel rises to a certain point, the valve will close and the amount of fuel in the carb should be regulated.

So what would happen if the fuel valve couldn’t close? Well, fuel just keeps coming in and coming in. Eventually, the carb bowl is overwhelmed by all the fuel, so it exits out of the carburetor overflow pipe.

If you often put your ATV on flatbed trucks or you park it on a steep incline, this can be enough to cause the carb float valve to get stuck. Sometimes, it’s just simple wear and tear that does it. 

In other cases still, over-winterizing the ATV with an empty fuel bowl will make the floats get stuck the next time you fuel up your vehicle. Start using a fuel stabilizer in the future to prevent that issue.

To unstick the carb float valve, take your ATV and park it on a flat, level surface. Access the carb bowl and gently tap on it. You can bounce your ATV too, but not too hard.

These motions could be enough to dislodge the stuck float.

If not, use a carb cleaner. Old gas could have made the carb gunked up, which is why the float valve is sticking. 

It’s not a bad idea to disassemble the whole carb and use some compressed air and a brush in addition to the carb cleaner to get your carb in tip-top shape. 

The Tank Vent Is Blocked 

Finally, the gas cap in the fuel tank vent could be blocked up, which is what’s causing the ATV carb to leak gas.

The fuel tank in your ATV features an air vent. The purpose of the vent is to let air in, especially as the fuel levels deplete. When the fuel tank gets hot, the vents can prevent it from overheating. 

The fuel tank vents may be part of a gas cap. If that’s the case for your ATV, then a blocked vent will allow air pressure in the tank to accumulate. 

The air pressure can get so immense that it sends the fuel to the carb and out of the overflow tube.

You’ll have to take the gas cap off your ATV and allow it to rest for a few minutes. The gas might stop leaking immediately, which suggests that the abovementioned pressure accumulation is the issue.

Try replacing the gas cap just to be on the safe side. 

Final Thoughts

If your ATV carb is leaking gas, you’re going to feel rightfully concerned. Determining the source of the problem isn’t always easy, but we hope the pointers in this article give you a place to start.

Our best recommendation? Start with the easier, more obvious fixes first. Sometimes they do the trick! Then you don’t have to tinker with too many internal ATV parts, nor do you have to spend a lot of money.

If you’re not so much the DIY type, know that you don’t have to do any of this troubleshooting yourself. You can always take your ATV to a mechanic and let them do it 

Geoff Southworth

I am a California native and I enjoy all the outdoors has to offer. My latest adventures have been taking the family camping, hiking and surfing.

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