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Gas Leaking From Overflow Tube (Motorcycle) – How To Diagnose & Fix

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If you’ve owned a carburated motorcycle for long enough, chances are you will eventually encounter gas leaking from the overflow tube. It’s a quick and cheap fix, at least most of the time, but most importantly – if you see gas leaking from the overflow then engine damage is highly unlikely. 

Now, there are a handful of reasons why that happens to pretty much all motorcycles eventually, and if that piqued your interest – this article is for you. 

What’s an Overflow Tube and What Does It Do?

The carburetor has a dedicated bowl where it stores a bunch of fuel. The fuel runs into the bowl thanks to gravity and stops when the float reaches a certain height, because it actuates a needle valve that closes the fuel inlet from the tank, pretty much identically to how the toilet in your house works.

Sometimes the float gets stuck and no longer stops fuel from filling inside the carburetor bowl. At that point, the overflowing fuel will continue flooding the carburetor and will eventually leak all the fuel inside the fuel tank into the combustion chamber if it’s not stopped. 

To prevent that from happening and hydro-locking the engine, carburetors are fitted with a copper overflow tube that extends above the float. If the float fails to shut off fuel, then the overflowing fuel level gets above the top of the overflow tube and flows through it and onto the ground instead of leaking inside the combustion chamber.  

So to put it simply, the overflow tube acts as a failsafe if the float and the needle valve fail to shut off fuel flow. 

Why Does Gas Sometimes Leak From the Overflow Tube?

Gasoline leaking from the overflow tube is definitely not something that you’d want to see, but at least you know that the overflow tube is doing its job and likely protecting your engine from hydro-locking or fuel getting into the oil pan. 

There are a handful of reasons for that: 

Stuck Float

A stuck float is probably the most common reason for overflowing fuel. What usually happens is pieces of dirt travel with the fuel and get lodged between the open needle valve and the needle seat, thus prevent the needle valve from closing.

Worn Needle/Dirty Needle Seat 

This is the second most common culprit to gas leaking through the overflow tube. The needle valve has a cone-shaped rubber tip which has to make a perfect seal against the needle seat. Now, because it’s made out of rubber, the cone tip does not last forever and it will wear out eventually. 

The needle valve should be nice and even, there cannot be any ridges of wear on the rubber cone. 

While the needle valve is considered a wear item and it’s made out of rubber, the brass needle seat can also get worn or dirty even though it’s made from a tougher material. In most cases, if the needle valve is not worn, then it’s very likely that the needle seat isn’t worn as well. 

Lastly, the brass needle seat could accumulate grime and corrosion over time, which could get in between it and the needle valve, thus preventing a proper seal and allowing fuel to overflow. 

Cracked/Leaky Copper Overflow Tube

Another reason for an overflowing carburetor might be a bad copper overflow tube inside the bowl. These are quite common on certain Honda models. 

The copper overflow tube can go bad by either developing small cracks or no longer sealing against the bottom of the bowl. 

Incorrect Float Height

If you’ve recently disassembled your motorcycle’s carburetor, then you could have accidentally bent the float a little bit without noticing, but enough to make the fuel level inside the bowl above the overflow tube. 

Now, the height of the float can change over time without even touching the carburetor, but that’s pretty rare. 

How to Fix Gas Leaking From the Overflow Tube

Now that we’ve discussed the different ways that gas could leak from the overflow tube, it’s time to fix it and get you back on the road.

Most of the time, if gas is leaking through the overflow tube, then your carburetors float is probably stuck open. A quick way to fix a stuck float is just to gently tap the carburetor bowl with a rubber mallet or something similar to shake the contaminants and free the needle valve. But if that doesn’t work, you’ll need to take the carburetor off the motorcycle and remove the bowl. 

Unscrewing the carb bowl

Make sure that you use the right screwdriver for your specific screw, otherwise, you’ll risk rounding them out. 

Carburetor with the bowl removed

With the bowl removed, visually inspect the float and the needle valve for anything that could stand out. Look for cracks in the brass overflow tube that’s in the bowl.

Testing the Carburetor Overflow Tube

To test the brass overflow tube, fill the bowl with gasoline and look for gas leaking down the bottom. If the brass tube is leaking then you can either replace it with a new one (recommended) or you can solder it with a basic solder gun that you use for wiring. 

Float and Needle Movement Test

Next, try to move the float up and down. It should move freely and with absolutely no binding. Keep an eye on the needle valve while you’re moving the float. The float should be connected to the needle valve via a small hook. Make sure that the valve moves with the float.

Measuring Float Height

If everything looks and feels fine, check the float height by inverting the carburetor and measuring the distance between the bottom of the float to the base of the float bowl gasket. Make sure that the float height is according to factory specifications, which can be found in your motorcycle’s service manual.

If the float height is off, then you’ll need to adjust it to factory spec.

Once you’re confident that the float height is correct, then it’s time to remove the float to clean and inspect the needle valve and the needle seat.

Illustration of a new and a worn needle valve

Remove the needle valve and inspect for wear. If the needle valve is worn then you’ll have to replace it. It’s always a good idea to replace the needle seat as well. If the needle valve looks fine, gently clean it with gasoline and a towel.

Inspecting the float seat surface

Next, check the needle seat for corrosion or grime. Spray it with carb cleaner and clean it up with a q tip. 

Once you’ve done all of that, then it’s time to reinstall everything back, check the float height again, change the bowl gasket, and put the carburetor back on the motorcycle to check if it no longer leaks fuel.

Can Overflowing Gas Cause Engine Damage to My Motorcycle?

The overflow tube is specifically designed to prevent fuel from overflowing the carburetor and getting into the engine, and if you see fuel dripping from the tube, then it’s working as it should. 

With that said, sometimes either because of a restriction in the overflow tube or due to fuel rushing in too fast, the fuel can still overflow the carburetor and leak into the combustion chamber.

Usually, if you can see fuel dripping from the airbox then it’s very likely that fuel got inside the combustion chamber. 

Another way to tell if fuel got inside the combustion chamber is to check the oil level. If the oil level seems higher than before, it could mean that gasoline got into the oil pan and thus increased the oil level. If that’s the case, starting the engine could be disastrous and should be avoided. 

If you want to be on the safe side and would rather not risk hydro-locking your engine, you can remove the spark plugs and turn the engine over to push fuel out of the combustion chamber through the spark plug holes. And if you’ve noticed that the oil level has risen, it’s a good idea to drain it and replace it with fresh oil. 

New Carburetor Leaking Gas From Overflow – What’s Wrong?

When it comes to aftermarket parts for motorcycles (especially old ones) – it’s a hit or miss. Sometimes you get a great part that fits perfectly and works great. Other times – not so much.

We occasionally get readers writing in who have installed a new carburetor on their bike, only to find that it’s now leaking gas from the overflow tube. So what’s the deal?

Cheap aftermarket carburetors are oftentimes set up incorrectly from the factory which means the float level is off. As a result – fuel leaks from the overflow tube on your brand new carburetor.

The good news is that this is an easy fix. You simply need to adjust the float level to factory specifications. It’s just a 10-minute job, and while you’re at it, you’ll be able to look through everything and make sure that everything else was installed correctly as well.

In extremely rare cases, the needle itself can be defective and so can the needle seat. They must fit together precisely in order to make a proper seal. If you find that the float level is correct but you’re still leaking fuel, then this is where you should probably look next.

Once you’ve sorted out the float level, give the carburetor a good once-over to make sure that everything else was installed correctly and there are no other issues.

Final Thoughts

A carburetor is a relatively simple device, but it’s also very sensitive. Though it’s rare, carburetors can occasionally leak fuel through the overflow tube.

In this article, we’ve gone over the most common reasons why this happens and how to fix it. If you’re still having issues after adjusting the float level, be sure to check the needle and seat for any damage or debris. Cleaning or replacing these parts should solve the problem.

As always, if you have any questions or would like to share any experiences you’ve had with carburetor leaks, feel free to leave a comment below!

6 thoughts on “Gas Leaking From Overflow Tube (Motorcycle) – How To Diagnose & Fix”

  1. Hi Jake. Im pretty sure my needle seat are trashed and old old needles bent. Got rebuild kit and both tries still overflow. Considering shop rebuild. I just don’t think I can get it done. Old CBs ad Kzs no problem for me. But this Vulcan 500 is a pain in th butt.

  2. Hi Jake,
    Thanks for your great article breaking down the faults and consequences.
    I have been experiencing leak down into the cylinders and engine oil and subsequently have removed my carburettors for inspecting and cleaning.

    Since carburettor re-assembly I decided to preliminary test them (off bike) for leakage using a funnel fed standing quantity of fuel. I still see leakage from the carb inlet areas and the overflow outlet when topping up the funnel feed.
    Should this leakage be possible, even on say, a known good working carburettor?

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