7 Reasons Why Your Motorcycle Smells Like Gas and How to Fix It


Have you ever noticed the smell of gasoline while walking past your motorcycle in the garage? A properly maintained motorcycle should not smell like gas when it is parked and it is important that you find the cause.

A motorcycle that smells like gas could be as a result of fuel vapor escaping as the gas expands above 60 degrees Fahrenheit. Over-filling a motorcycle gas tank, a gas leak in the fuel line, or a too rich air-fuel mixture will also make a bike smell like gas.

It is not normal for a motorcycle to smell like gas and it is important to find the cause of the fuel smell. It poses a fire risk if any naked flame were to find its way close to the bike.

Honda XR650L stock gas tank
Park your bike in a well ventilated area if you smell gas

Here are seven reasons why your motorcycle is smelling like gas, as well as how to fix it:

#1: Gas Vapor on a Hot Day

Gasoline vaporizes as it heats up. At 60 degrees Fahrenheit around 50% of the liquid gas turns to combustible fuel vapor. In the U.S. the gas volume is considered normal at 60 degrees and as the temperature increases above 60 the volume will expand and more liquid gas will vaporize.

Your motorcycle’s gas tank is vented to atmosphere so that the pressure inside the tank can equalize. As it gets hotter, the gas vapor will escape through the tank vent and the smell may become noticeable. This could be as a result of the bike standing in direct sunlight coming through a garage window.

What can you do? On very hot days, make sure you bike is not in direct sunlight. Leave a garage window open to ventilate the building. If the smell is really strong and none of the other causes below are to blame, consider parking your motorcycle outside if it is safe.

WARNING: Whatever you do, don’t bring any open flames near a bike that smells like gas.

#2: Over-Filled Gas Tank

If you park your bike soon after filling the gas tank to the brim, it may overflow, especially if it is hot outside (see #1 above). As the gas expands above 60 degrees, liquid fuel may be pushed out the vent hole instead of just gas vapor. You may notice a wet spot where the gas dripped from the vent pipe.

What can you do? Don’t don’t fill up your gas tank if you know you are going to park the bike overnight soon after. If I know I am on my way home, I don’t fill the tank right to the brim. And if the tank was overfilled, I usually ride a few miles to lower the gas level a bit.

If I’m camping on the bike, I don’t worry too much as the bike will stand outside in the open air and I don’t want to waste fuel that I’ll need to get to the next town the next day. Just don’t park next to your campfire!

Another tip is to park your bike upright or in a position that ensures the air bubble in the gas tank is under vent, and not the liquid gas.

#3: The Gas Line Has a Leak Somewhere

Often, when there is a strong smell of gas around a motorcycle, it is due to a fuel leak somewhere. On carbureted motorcycles, the gas tank petcock seal can perish and rubber fuel hoses develop cracks as they get old. A gas hose can also get chafed through, especially on dirt bikes and dual sport motorcycles with aftermarket gas tanks. On fuel injected bikes, the O-rings on the injectors can get brittle and start to leak.

Honda XR650L gas line
Check rubber fuel hoses for leaks and replace any cracked hoses

What can you do? Check for wet spots or drops of fuel around the gas tank petcock. Follow the fuel lines all the way to the carburetor or injector rail (on fuel injected bikes). Look for loose clamps and replace any cracked hoses. It is a good idea to close the petcock on the tank if you are not going to ride for a few days.

If you can’t find any leaks, remove the gas tank and check for chafed lines below it. On fuel injected bikes, look for wet spots around the fuel injectors and replace leaky O-rings. Carburetors can also leak around the float bowl if it is not tightened properly.

Honda XR650L carburetor
Make sure your carburetor is not leaking from the float bowl

#4: Fuel Mixture is Too Rich

On a motorcycle with a carburetor, a too rich air-fuel mixture can result in the bike smelling like gas. If the mixture is too rich, some of the fuel may not burn completely or burn in the exhaust, instead of the combustion chamber. Remove a spark plug and check the tip. If it is black and sooty it means your bike is running rich.

What can you do? Have the bike carb tuned by someone who knows how carburetors work. Or if you are not afraid, YouTube it and adjust the air screw on your carb to lean it out. Use quarter turns at a time to keep track of the adjustments you make and re-check the plugs after riding a few miles. You don’t want to go too lean, as that could cause more damage in the long-run.

If you live at high altitude, your bike may run too rich and will need to be re-jetted (usually to a small pilot and main jet) to correct for the lower air pressure. When we traveled through Ethiopia on 200 cc Chinese delivery bikes, I adjusted the air-fuel mixture daily as we reached altitudes of 10 000 feet and higher.

#5: The Carburetor is Flooded

A flooded carburetor can also cause a strong gas smell. If the motorcycle’s engine turns over but is hard to start, the carb could be flooded. There are many possible causes, including a worn needle and seat (the valve that regulates gas to the float bowl), a hole in the float, a too rich air-fuel mixture, a choke that is stuck in the closed position, or a weak spark.

What can you do? A worn out needle and seat may not be enough to prevent gravity from pushing fuel past the valve, thereby flooding the float bowl. Close the petcock when the bike is parked to prevent this. You can inspect the float inside the carb float bowl for damage. If it has a hole in it, it will sink to the bottom and therefor not cut off the fuel entering the float bowl.

Honda XR650L gas tank petcock
Close the gas tank petcock to prevent your carb from flooding

Make sure the ignition is in good tune and that the air-fuel mix is correct. Ensure that the choke is fully open and that the choke linkages are clean and can move freely.

#6: Worn-out or Damaged Gas Cap

If the spring inside your gas tank cap is worn out it won’t be strong enough to overcome the pressure in the tank and it will allow vapor to escape. Open the cap and depress the spring inside with you fingers. If there is very little resistance, the spring may need replacing.

Honda XR650L stock gas tank
Some bikes have springs in the gas tank cap which can become worn

What can I do? Replace the spring inside the gas cap or replace the whole cap if you can’t find the spring separately. Some riders have reported that simply bending (by pulling) the spring out helped get rid of the gas vapor smell.

#7: Retarded Timing

If the ignition timing is retarded (too slow), the spark plug will fire after the cylinder is at the top of the power stroke and not all of the fuel will fully burn. Some of the unburned gas can evaporate through the airbox causing the smell of gas. This is only a problem on some classic bikes without capacitor discharge ignition (CDI) that regulates the ignition timing automatically.

What can I do? Advance the timing until the engine runs smoothly. Do not overdo it though, as advancing the timing too far may cause knocking. This is when the spark fires before the piston reaches the top of the cylinder and could cause serious damage to the engine.

Conclusion

It is not normal for a motorcycle to smell like gas and it should not be ignored. Apart from being bad for the environment and your health, it poses a fire risk. Take the time to figure out why your bike smells like gas or take it to a mechanic to help you diagnose the cause.

This is another reason why I always try to do maintenance and repairs myself. The more you get to know your machine, the quicker you will pick up if there is something that requires your attention.

Happy riding!

Related Questions

Can a motorcycle gas tank explode on its own or when it gets too hot? Read this post to find out more.

Francois Steyn

I've been riding motorcycles since I was in school and have traveled thousands of miles on various bikes through more than 10 countries. For more info, check out my about page: https://adventurebiketroop.com/about-us/

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