As motorcycle riders, we stop at the gas station to spend our hard earned money and fill up the gas tank; in return, we expect to burn that gas and put it to good use. It just so happens that sometimes it does not go this way; when you approach the bike you can smell the gas and tell the grade of it, just like a sommelier does with the finest wine.

If you find yourself in this situation, turn off your machine, adjust any fuel line valves to the ‘off’ position and start investigating with the help of our list below.

Key point

Fuel leaks can be dangerous. So can doing maintenance in fuel systems if you’re lacking in experience. If you do not feel comfortable working on it by yourself, look for a licensed mechanic who will have the proper tools, space and the knowledge to execute the work in a safe manner.

1. Gas tank filler cap leak

If the only moment you can smell gas is when the gas tank is full or if you notice some damage to the gas tank paint, it could be that the gas tank cap is no longer sealing the filler neck. In this case, gas could potentially leak from the top of the tank while the motorcycle is moving until the fuel level goes down and gas can not reach the top of the tank anymore.

Solution:

Replace the gas tank cap with an original model bought from a dealership or from good aftermarket parts manufacturers. These components are not manufactured by the motorcycle manufacturer, so as long as you find a gas cap from a brand with good reviews, you should be safe. Make sure you are buying a proper one for your motorcycle model as they are not all the same size.

2. A brittle petcock valve seal

This is a classic one. If your motorcycle is from the full mechanical machine generations, there will be a petcock valve on the bottom of the gas tank; it could be on one or both sides of it, depending on how big the tank is. This valve allows the rider to control the gas usage by setting it in 3 different positions: on, off and reserve.

When riding, the valve should be set to the ‘on’ position; once the fuel level drops and reserve fuel is needed, the rider can swing the valve to ‘reserve’ – this can be done on the fly if you are an experienced rider and once executed you should start looking for a gas station. The off position is where the valve should be when the motorcycle is parked – especially if it will be there for a long time.

The petcock valve is attached directly to the gas tank and, in between them, there is a fuel resistant gasket whose mission is to prevent leaks. With time this gasket can become brittle and starts to allow fuel to pass by the gap.

Solution:

The gasket has to be replaced. First step will be to drain any fuel left over inside the gas tank; with the petcock valve on the ‘off’ position, disconnect the rubber line that comes out of it and empty the tank in a safe container (if a proper gasoline can is used for this, you can return the fuel back to the gas tank after you are done fixing the leak).

Proceed to separate the petcock from the gas tank by either removing the screws that keep it in place or rotating it in case the valve and tank are threaded together. Replace the seal with an OEM version (these will last longer) and put it all back together. The new seal should put an end on the leak.

3. Rusty gas tank welded seams

Back in the days, almost every bike used a steel gas tank to hold fuel and some still do nowadays. With time, it is possible for rust to build up inside of the tank and sometimes it can perforate the material (gasoline has some strong solvents in its composition). One of the spots where this happens the most is right at the seams, where the tank panels are welded together.

Solution:

There are two ways to go from here: one is to find a replacement gas tank, which can be difficult sometimes considering the age of the motorcycle; the second is to repair the original tank.

Rust can be removed from inside with help from some rust remover product, then the seam weld can be repaired followed by the application of a coat of rust inhibitor on the inside and proper painting on the outside if necessary.

This can be done at home, but requires many tools that not everyone has access to, so looking for help from a motorcycle mechanic/rebuilder can be easier.

4. Worn out petcock ball valve

If the motorcycle has higher mileage and the petcock valve was used a lot or if the owner leaves the valve on the ‘on’ or ‘reserve’ positions even when the bike is parked for longer periods of time, it can happen that the valve itself will wear out. This will open space for gas to leak through and any rider should be able to identify this by just looking at the valve and checking if it is wet on the outside.

Solution:

The way here is to replace the petcock valve. Luckily, there are many options from OEM to aftermarket which drives their price down. To replace this valve, you can follow the same steps as if you were fixing a leaking petcock gasket.

Key point:

Some petcock valves have vacuum systems integrated to better draw the gas from the tank. If this is the case, it might be worth rebuilding it as the seal kit is generally cheaper than the whole valve. Here is a video showing how to replace one of the seals in a vacuum operated petcock and further information.

5. Brittle or loose fuel lines

Motorcycles have their fuel lines made out of special rubber in order to withstand the corrosive power of gasoline. As time passes though, these rubber lines can become brittle due to weather exposure as well as the clamps which hold the lines against the fittings can come loose; in both presented scenarios, fuel leakage is a possibility.

Solution:

Replace the fuel lines as well as any clamps that no longer hold the lines against the fittings as well as the fuel filter if your bike has one (if it does not, it can be a good idea to add one).

There are two important things to know while doing this:

1. You have to find proper fuel lines – if you use any kind of rubber not meant for gasoline, the rubber will get perforated by the solvents and gas leak will happen.
2. Replace one line at a time, this way you will not connect the new fuel lines to the wrong fittings.

6. Old carburetor float bowl/housing seal

If your motorcycle still uses a carburetor to feed fuel to the engine – and many motorcycles still use it -, then you might have to look at the seals in this component.

Very commonly, the seal that sits in between the float bowl shells has to be replaced. It can dry out with time and no longer offer good sealing. The symptom for that will be that your carburetor can look wet when the engine is cold (when the engine is hot gasoline will evaporate fairly quickly).

Solution:

If you will be taking your carburetor off the motorcycle, I would recommend buying a full rebuild kit for it and replacing all the components that might fail sooner than later. One of the elements on the rebuild kit is the float housing seal. This replacement should be easy as normally the housing is kept in place by two to four screws.

You might need a pick tool to remove the seal from the groove where it sits. Once the old one is out, add the new one in, close the housing again and voila, the bike should be ready to go. Remember to do all that in a safe environment and plenty of flowing air. Gasoline fumes evaporate quickly and it is flammable.

7. Worn out carburetor float needle

Like mentioned before, the float needle is part of the rebuild kit. Its main function is to seal the float when this lifts; the float will lift because the housing is full and no longer needs gasoline from the feeding line. If this needle is worn out, gasoline will leak through it and overflow the housing creating a fuel leak.

Solution:

Rebuild the carburetor and replace the needle. After removing the float housing you will have access to the float itself. You can then remove the float and unscrew the float needle. Screw the new one back in, reinstall the float, close the housing and it is all done.

Once you open the feeding lines again, give a minute or two for the bowl to fill up again before trying to start the motorcycle. This will preserve your battery from spending energy trying to start an engine that has no fuel in.

8. Worn out fuel injector seals

If your motorcycle belongs to the modern pack, it most likely will have fuel injector instead of the previously mentioned carburetor. This does not mean your life will be trouble free. Yes, fuel injectors require less maintenance and are more precise in feeding the engine with the proper air-fuel mixture, but the fuel injectors can also cause some problems.

All injectors have rubber seals to seal them against the housing to which they are attached. If these seals dry out they can become brittle and no longer offer proper sealing; they can also wear out due to excessive vibration and some other reasons. This will allow for gasoline to leak from the injectors.

Remember that fuel injectors require a fuel pump in order to operate properly, which means they operate with fuel under higher pressures, so you do not need that big of a gap for pressurized fuel to leak through.

Solution:

If your motorcycle is still under warranty and you suspect that there is a fuel leak, take it to the nearest dealer and they should execute the work for free. This also gives the manufacturer a chance to understand if there is a design problem with their product, especially if it is a new model. If you have a higher mileage bike, no longer under warranty, these seals are easy to find and cheap to buy.

When working on it, remember there will be fuel under pressure in between the closed injector and the fuel pump meaning if you just pull the injector out of the feeding line, fuel will spill; look for a pressure relief valve that some systems offer and remember to disconnect your battery to avoid any fuel pump problems.

Depending on the bike and the space you have, you might not even need to disconnect the injector from the feeding line to replace its o-ring, saving you a lot of time by only having to remove the injector from the engine housing where it sits.

Conclusion

Gasoline has many solvents in its composition and it is common for rubber seals to fail overtime. Also, many components are exposed to weather and wear out much quicker than we expect.

Fuel leaks can be dangerous and require some experience to be dealt with. If you have the technique, proper tools and a safe environment, I am sure you can tackle the issues by yourself. Otherwise, look for professional help and enjoy the ride.