If you own a carbureted motorcycle, there’s always the chance that gas will leak from your overflow tube at some point.
In most cases, gas leaking from the overflow tube will not damage anything, and it’s actually a good thing that the overflow system is working, instead of letting the fuel flow to the combustion chamber.
It’s a cheap fix, at least most of the time, and there are a few reasons why that happens. Furthermore, it will happen to most motorcycles at least once in their lifetime and it likely did to yours if you’ve stumbled upon this article.
But what causes gas to leak from the overflow tube? How do you diagnose and fix gas leaking from the overflow tube?
What Does an Overflow Tube Do?
In a nutshell, an overflow tube is a simple failsafe designed to protect the engine from hydro locking if the needle valve or the float fails and starts dumping fuel uncontrollably.
All motorcycle carburetors have a fuel bowl, which is used to store fuel. In most cases, the fuel flows into the bowl from the gas tank due to gravity.
There is a small float and a needle valve, which is used to keep the fuel level at a specified height.
In some very unfortunate cases, the float or the needle valve fails or gets stuck open – this makes the fuel from the gas tank flow into the fuel bowl uncontrollably and overfill it. Once overfilled, gasoline will make its way into the combustion chamber, flood it and cause your engine to hydro lock if you attempt to start it.
Now, to prevent that from happening, engineers designed a failsafe – the overflow tube.
The overflow tube extends a little bit higher above the normal fuel level inside the bowl, just in case the float or the needle valve fails and starts pouring fuel uncontrollably.
If something does go wrong with the float or the needle valve, the fuel level reaches the top of the overflow tube and then starts safely draining through it.
Since the fuel is now diverted through the overflow tube, which is specifically designed to evacuate overflowing fuel, your engine is saved from hydro locking.
Why Does Gas Leak From the Overflow Tube?
The overflow tube is a necessary evil that protects your engine from hydro-locking or fuel getting into the oil pan. Here are the 4 most common reasons why that happens.
1. Needle Valve Failure
One of the most common reasons for gas leaking from the overflow tube is the needle valve and the needle valve seat.

The needle valve is a small cone-shaped rubber valve that prevents the gas from the gas tank from flowing into the carburetor bowl.
Now, the needle valve is made from rubber, and therefore it’s supposed to be inspected for wear and damage every once in a while. The rubber part of the valve does wear out over time and makes the valve leak fuel.
Another way that a needle valve could fail is if there is corrosion or dirt trapped between the needle valve and the brass needle valve seat. The valve must seal tightly against the valve seat, but if there is some dirt, debris, or corrosion on the valve seat, then the valve won’t be able to close fully and this will cause fuel to leak and overflow the carburetor.
2. Stuck Float

Another very common reason for gas leaking through the overflow tube on a motorcycle is a stuck float.
Sometimes the float sticks open and no longer seals the needle valve against the needle seat, but floats can also get damaged and let fuel get inside them.
Once the float is leaking and is no longer air-tight, it will not actuate the needle valve and let the fuel overflow.
3. Leaking Overflow Tube
A leaky overflow tube is one of the most overlooked causes of this issue.

The brass overflow tube inside the carburetor is very small and it can be really difficult to spot if it is cracked or loose.
The overflow tube is most often damaged when disassembling the carburetor, but it can crack due to age in rare cases.
If you’ve recently disassembled your carburetor, then this might be the reason why you have gas leaking through the overflow tube.
4. Bad Float Height
The flat heigh is adjustable on all carburetors and if you’ve disassembled the carb recently or hit a huge bump, then chances are that the float height has changed and is now set too low, which allows for gas to overflow.
Luckily, setting the float height is easy and all the specs can be found in your motorcycle’s service manual. We’ll go through the steps of doing that later on in the article.
Can Overflowing Fuel Damage the Engine?
If the overflow tube system works as it should, then overflowing gas will not cause any damage to your motorcycle’s engine.
How to Diagnose & Fix Gas Leaking From Overflow Tube
When your motorcycle’s carburetor is leaking gas through the overflow tube, it’s most likely because the float got stock or because there’s some dirt lodged between the needle valve and the needle seat.
1. Gently Tap the Fuel Bowl With a Rubber Mallet
One of the best tricks I’ve learned from old-school mechanics is to tap the carburetor bowl with a rubber mallet or something similar. This usually shakes things up in there and sets the valve free.
However, if that doesn’t work, then the only way forward is to disassemble the carb.
2. Check The Float Height
The float height is the distance between the bottom of the float to the base of the float bowl. If it’s set incorrectly – you can definitely get fuel leaking through the overflow tube.
The exact float height is different for each make and model motorcycle, so make sure to consult your bike’s service manual for the exact number.

Measure the float height using Vernier calipers. If it’s out of spec, then you should bend the float accordingly to the specified height.
3. Inspect the Float and the Needle Valve
Look for anything abnormal with the float and the needle valve. Try to move the float with your hand – it should move extremely easy and you should feel no binding at all.

Keep an eye on the needle valve while checking the float. Make sure that the needle valve is connected to the float with a small hook and that it moves with the float.
If everything seems fine, then it’s time to remove the float and the needle valve.

With the valve and the float removed, it’s time to inspect the needle valve for wear. The rubber tip of the needle valve should be even and shouldn’t have any groves or wear at all. Clean the needle valve with carb cleaner afterward.

Next, check if the float is airtight by submerging it in gasoline or water. It should not leak air bubbles and should float to the top.

Lastly, check the needle seat for dirt, corrosion, and wear. Use carb cleaner to clean the needle seat.
If everything seems okay and has been cleaned thoroughly with carb cleaner, it’s time to reinstall the needle valve and the float. Remember to recheck the float height once again, in case you’ve accidentally bent it without realizing it.
Test the Overflow Tube for Cracks
Last but not least, you must check the overflow tube itself for leaks.

To do that, fill the float bowl with gasoline and watch for fuel dripping underneath the bowl.
If the brass overflow tube is leaking, then you should replace it with a brand new one, but if that’s not an option, you can add solder to seal the crack, as the overflow tube is made out of copper.
Conclusion
There you have it – that’s how you fix a motorcycle carburetor that’s leaking gas from the overflow tube.
Remember to always consult your bike’s service manual for the exact float height specifications as they differ from model to model.
Also, make sure to regularly check the overflow tube, the needle valve, and the needle seat for dirt, as that’s usually the main culprit behind fuel leaks.
Thanks for reading and I hope this article was helpful. Happy wrenching!

Nick Kowalski is a diesel mechanic by trade, but his passion lies in motorcycles.
He’s been riding them since he was just a wee lad, and he loves restoring old Japanese street bikes and ATVs in his spare time.
Nick is an adrenaline junkie at heart, and he loves nothing more than hitting the open road on his motorcycle.
I got a brand new carbie and it’s overflowing