7 Symptoms of Bad Gas in a Motorcycle

Bad gas in motorcycle tank

Running the motorcycle on bad and stale fuel is harmful to the engine components.

Any indication that the gas has gone bad, you must replace it with fresh gas.

So, how do you know the gas has gone bad?

Here are the symptoms of bad gas in your motorcycle:

  • Pungent smell from the gas tank
  • Black smoke from the exhaust
  • Motorcycle starting problems
  • Abnormal speed changes
  • Low fuel economy
  • Damaged fuel filter
  • Gas color has turned dark

That’s the gist. Let’s deep dive into these symptoms in detail and preventive measures as well.

Symptoms of bad gas

Here are the symptoms of bad gas in a motorcycle:

#1. Pungent smell from the gas tank

Bad gas gives out a pungent fuel smell.

That’s one of the first and foremost symptoms of bad gas in your motorcycle.

Motorcycle gas tank - orange in color

The fuel smell gets much more noticeable near the gas tank.

And if you open the tank cap, the smell would be even stronger.

The only other cause for a pungent fuel smell is a clogged fuel filter.

Apart from that, whenever you smell bad from the tank, the gas has gone bad.

#2. Black smoke from the exhaust

Black smoke from the exhaust is another symptom when the gas is bad.

Black smoke coming from the motorcycle exhaust

This is a bit difficult to confirm.

The motorcycle running rich can also lead to black smoke coming out of the exhaust pipe.

But you can be sure that black smoke implies something is wrong with the fuel.

Either the engine is receiving too much fuel. Or a subpar quality fuel.

Coupled with the above symptom – pungent smell – you should be clearly able to tell the gas has gone bad.

#3. Motorcycle starting problems

This is obvious.

If the gas is bad or has degraded in quality significantly, the motorcycle won’t start at all.

Kickstarter on a Royal Enfield motorcycle

Of course, the gas might have gone bad but not to the extent that the bike won’t start (we will see that below).

Still, bad gas leads to motorcycle starting problems.

This is especially noticeable whenever you have left your motorcycle idle for long (3 to 6 months). And then try to start the motorcycle.

But because the gas has gone bad, the motorcycle won’t start easily.

#4. Abnormal speed changes

Another major symptom of running a motorcycle with bad gas is poor acceleration and abnormal speed changes.

The motorcycle won’t accelerate properly.

Motorcycle Speedometer and Fuel Gauge

And the speed goes haywire even if you haven’t done anything.

Feels like a bumpy road. Without the bumps.

This happens when the gas quality fluctuates a lot. The engine receives poor-quality gasoline intermittently.

In these cases, most likely the gas has water content in it.

The presence of water along with the gasoline, makes the engine sputter. And cause abnormal speed changes.

#5. Low fuel economy

With stale fuel entering the engine, the power output will be subpar.

The engine consumes too much fuel to generate the power required for the motorcycle to run and speed up.

Motorcycle odometer and fuel gauge - digital

No surprise. The fuel efficiency decreases.

The gas mileage goes down. Significantly so.

Old stale fuel doesn’t generate enough power. And that’s why the fuel economy of bad gas will be extremely poor.

#6. Damaged fuel filter

Bad gas starts to create problems in the components it flows.

The most affected part is, of course, the fuel filter.

Fuel filter on a motorcycle

The fuel filter basically screens out dirt, dust, and contaminants from the fuel.

So, when the gas is bad, the contaminants are so high – the fuel filter starts clogging up.

Most motorcyclists rarely check their fuel filters.

Heck, most don’t even remember the last time they changed it.

So, if there is bad gas in your motorcycle tank, check the fuel filter.

You might have to replace the fuel filter if it’s too clogged up along with changing the gas.

#7. Gas color has turned dark

Once you open the gas tank and look at the gasoline color, you should be able to confirm whether the gas has gone or not.

Gasoline will be clear in color with a yellow, red, or green tinge.


But bad gas will have an intensified red or brown color. It won’t be clear as well.

Bad gasoline

The gas color would have turned dark. Brown in color. Like a whiskey or a coke.

The bad gas will also have a frothy, non-clear texture.

In addition, there might be gums and varnish formations that can be clearly seen.

All these symptoms point for sure that the gas has gone bad.

And it’s time for you to fix the issue.

How to fix bad gas in a motorcycle

Once you confirm the gas has gone bad by checking its color and texture, the solution is to flush the old gas out.

How to do that?

Take a pan. Locate the fuel pipe. Turn the fuel petcock OFF.

A guy turning the fuel petcock

Next. Disconnect the fuel pipe from its carburetor end.

Motorcycle fuel pipe

Then turn the petcock ON. The gasoline flows out from the pipe. Collect it in the pan.

Drain the gas completely.

Bad gasoline collected in a pan

Once drained. Refill with fresh gasoline into the gas tank.

Filling motorcycle gas tank

That’s it.

You can even our a small fresh gasoline and drain it so that the gas tank is free of old gas.

How long does gas last in a motorcycle?

If the gas is sitting in a motorcycle gas tank, the gas goes bad in 6 months.

Although the gasoline itself in general lasts long. The gasoline in the motorcycle won’t.

Gasoline - new and old

This is because the gasoline we use for motorcycles has ethanol in it. Up to 10%.

The ethanol blend in gasoline absorbs moisture easily.

As a result, the water content increases the more the gas sits idle in the motorcycle.

That’s how the gas turns bad.

And doesn’t last long. 3 to 6 months is what I would give before the gas degrades.

Measures to prevent bad gas

Here are some of the preventive measures you can adopt to ensure the gas doesn’t turn bad:

  • If you are not going to use your motorcycle for a few days (more than a month), drain the gas from the tank.
  • You can use fuel stabilizers in the gasoline to prolong their shelf life. They slow the fuel degradation for some time. But not forever.
  • Never mix old fuel with new fuel. Flush the old stale fuel from than tank. Only then, refill the gas tank with the new fuel.
  • Ride your motorcycle regularly. Don’t let it sit for months at a time. The worst thing you can do for your motorcycle is let it sit for months.
  • You can try ethanol-free gasoline. Ethanol blend is the main culprit for fuel degradation. Ethanol-free fuel will be costly though.
  • Regularly service your motorcycle to keep it in good condition. Well-maintained parts in the motorcycle slow down fuel quality degradation.

Final words

The pungent fuel smell near the gas tank is the main symptom of bad gas.

That should make you check the gas in the fuel tank.

If the gas color has turned dark – dark red or brown in color – it’s confirmed the gas has gone bad.

Next. Drain out the bad stale fuel. Don’t mix fresh gas with the old gas.

Only once you have completely flushed out the bad gasoline, refill fresh gas.

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