Bad gas in a lawnmower, how to fix it?


Owning a house with a lawn requires you to take care of it. You have to mow your grass every once in a while to let it stay healthy and visually pleasing. To make the mowing job easy, you can use various types of lawnmowers. But what if your lawnmower sputters or does not start after you have not used it for some time? Before panicking or calling a mechanic, do a check-up of your lawnmower. One of the more prevalent problems is bad gas. This article will cover all topics related to bad gas and will provide a clear insight into what one should do if one’s lawnmower’s gas goes bad.

Bad gas in a lawnmower, how to fix it:

If your lawnmower is sputtering and not starting after you have not used it for some time, there is a big chance that the gas has gone bad. After confirming that the fuel’s problem, you have to drain the old or remaining gas from the fuel tank. The easiest way is to do it is by using a siphon hose. If the mower still does not work properly, check the carburetor and clean all of its parts. The mower should work after filling the tank with new gas.  

Now that the basic procedure of fixing bad gas in a lawnmower has been explained, let’s get into the details of this topic.

How to tell if your lawnmower’s gas has gone bad?

If your mower makes thumping noises or has difficulty starting, you should check if the gas in your lawnmower is still ok. There are various methods for how you can do this:

● Smell Test:

You can tell if the gas has gone bad simply by smelling it. Move your mower to somewhere where there are no cars, bikes, or machines using gas so that your nose does not confuse the smells. Get the tank’s lid off and smell the gas inside. Gasoline has a very particular smell. If the smell is sour, rotten, and strong, the gas has gone bad. Otherwise, your lawnmower has some other problem that is hindering the mower’s start-up process.

● Appearance Test:

Bad gas has a very different appearance from fresh gas. Take out some gasoline from the tank and pour it into a transparent container. Keep some fresh gas with you for comparison too. If the tank’s gas has a darker color than the fresh gas, it has most likely gone bad. Comparison with fresh gas makes the assessment easier, but even if you do not have fresh gas with you, bad gas can easily be recognized because of its darker color.  

Why does gas go bad?

The first cause depends on gas’ volatility. The volatility of gas allows it to burn. The lightest chemicals evaporate first, leaving the incombustible gas, which turns into a gummy, varnish like substance with time.  This substance can clog up fuel lines.

Keeping old gas in the tank for a long time can make the gas go bad. The old gas has hydrocarbons that react with oxygen to produce new compounds, resulting in oxidization. This produces a gum-like substance that can clog gas lines, fuel filter, and carburetor. This is most harmful to carburetors because fixing them can be expensive, and they would not run properly until their insides are cleaned. This happens for empty tanks, too, because some residue gas remains in the tank. This is why it is important to empty the tank before filling new gas properly.

Sometimes the fluctuating temperature can cause condensation of gas, and when the resulting water finds its way into fuel tanks, it causes problems. 

Effects of bad gas on a lawnmower:

As mentioned earlier, bad gas can clog up the fuel lines, resulting in the engine not running properly. But other than that, bad fuel is also said to be killing your engine. Water in the engine caused by the condensation of old gas often contributes to rust formation and corrosion. Rust can make the engine stop working unless the mower is taken to a mechanic and properly cleaned. In extreme cases, you have to get the engine replaced too. Water in the engine also hinders the engine’s lubrication. So, it is best to avoid bad gas and if there is some in your engine, get it fixed as soon as possible.

Fixing bad gas in a lawnmower:

You have to drain out the previous gas from the tank and replace it with a fresh one for the mower to start running.

– Equipment needed:

  • Gloves
  • Safety Glasses
  • Siphon hose
  • Carburetor cleaner
  • Rags or towels
  • Plastic container
  • Tarp
  • Crescent Wrench
  • Lawnmower’s user manual

– How to fix bad gas in your lawnmower step by step:

● Step 1. Park the lawnmower straight:

Set the lawnmower at a place from where its underside is easily accessible. Make sure that the mower does not tip. Tipping can cause the fuel to move from the fuel tank to other parts of the motor, creating many new problems. If you have to tip it at all costs, then do it backward instead of the side to avoid problems.

● Step 2. Use a tarp:

Spread a tarp under the lawnmower to avoid getting the ground dirty with oil spills. Also, make sure to follow the safety precautions like wearing safety glasses and gloves.

● Step 3. Cleaning:

First of all, clean the surrounding areas of the gas tank. When the outer portion is cleaned, remove the fuel tank lid and wipe out the filter’s insides using rags or towels or other cloth you might have.

Step 4. Container and siphon hose:

Place a container on the ground to catch the gas. Use a siphon hose to drain the gas from the tank to the container. Make the siphon hose create a vacuum to drain the gas from the tank by squeezing the bulb a certain amount of times. Try to drain all the gas from the tank.

Step 5. Check if all gas is drained:

Wipe the residue with a towel or a rag to make sure it is empty now and no bad gas remains in it.

Step 6. Refill and check if the lawnmower runs well:

Refill the gas tank and start the lawnmower. If the mower works after that, you are good to go, but if it runs for a while and dies, it means the carburetor is clogged and needs to be cleaned.

● Step 7. Find the Carburetor drainage bolt:

Then, place the container beneath the carburetor and look for a drainage bolt on the float tank’s bottom.

● Step 8. Remove the drainage bolt:

Remove the drainage bolt and let the fuel run out of the float tank into the container under it. The drainage bolt can be easily removed by using a crescent wrench.

● Step 9. Remove the Carburetor:

Remove the whole carburetor and spray all carburetor parts with carburetor cleaner and reattach it to the engine.

● Step 10. Reassemble all the parts:

Put together all parts again and fill the fuel tank with fresh gas. Spray some amount of carburetor cleaner into the filter, too, for easy start-up of the system. Start the lawnmower and allow it to sputter for some minutes before it starts running smoothly.

● Step 11. Change the oil:

This is an extra step for those who own 4-stroke lawnmowers. In that case, change the oil too for the mower to work properly. Consult your lawnmower’s user manual to see what type of lawnmower you have.

Precautions for future use:

Most of the bad gas issues stem from old gas. Try not to store gas in tanks for more than a month or two. If, for some reason, gas cannot be replaced and has to be stored, make sure to use a fuel stabilizer for it because it prevents oxidation of gas, which results in gas going bad. Also, whenever you change your lawnmower’s fuel, make sure to empty the tank. Wipe out the residue with towels or rags or whatever cloth you have so the tank is completely free of any old gas. That way, there would be no gas to get oxidized and clog the fuel lines. To avoid the condensation of gas, try to keep the mower where a steady temperature is maintained. If you keep it outside in the open, where it can be exposed to any temperature change, there are high chances that it will become a cause for bad gas.

Final Remarks:

Bad gas is one of the primary reasons for a lawnmower’s defective function. It is usually the first thing you should check when a mower is not working correctly. It is usually caused by old gas, which gets oxidized over time and damages the whole system. It can hinder fuel flow because it clogs up fuel lines. It is better to get it immediately fixed to prevent it from causing further problems. You have to drain out the bad gas entirely from the engine and wipe it clean. Only then should you add fresh gas and start it again. It is essential to change your fuel at least once in two months, so the fuel does not grow old and start causing problems. It is also necessary to keep your lawnmower where it does not get exposed to significant temperature changes as temperature changes can cause many problems.

Recent Posts