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Generator Backfires and Won’t Start: 7 Causes To Check

Generator Backfires and Won’t Start: 7 Causes To Check

Generators are used in a variety of situations. They provide electricity where it may not be otherwise available, from work-sites and wood shops to tailgating parties and RV users. But what happens if it backfires and won’t start?

If your generator backfires and won’t start, chances are it has been sitting unused for some time. In this case, you might need to replace older fluids, broken fuel lines, or even faulty spark plugs. Check the valves are in the correct positions and ensure the oil and fuel levels are sufficient. 

This article will explore the most common causes of backfiring and reasons your generator might not be starting. We will also discuss how to fix these issues and how to prevent them in the future.

Common Issues With Generators

Whether your generator has been sitting for a week, a month, or a year, you may find that it will not start up right away. Luckily, there are a few common causes behind backfires and starter issues that you can fix with ease. 

If your generator is brand new and not starting right from the get-go, it is a good idea to take some time and double-check that everything has been set up according to the manual.

Low Oil

When you first set up your generator, you should have filled it with good quality oil and allowed it to run through the system. Over time and after being used, the oil level will have depleted, and it could be that there is not enough to start the appliance properly.

Check the oil level by pulling the dipstick. Keep in mind that readings (and those sensors in the unit) could be off if your generator is on an uneven surface. Refer to your owner’s manual for the correct amount of oil required.

In the video below, you can see how to fill the oil reserve:

How-To: Add Oil To Your Generator

Low or Old Fuel

Much like with the oil level, your generator may be simply low or completely out of gas. Every generator will have some kind of fuel gauge that will display the level in the tank. Before you try to start the unit, you should always check this level and top it up when necessary.

However, in some cases, the fuel gauge may be displaying sufficient fuel and still not starting. As fuel sits unused, it begins to degrade. You may see that it has become cloudy or even has begun to separate. If you think the fuel is more than a few months old, you should siphon the old fuel from the generator and refill it with fresh fuel. 

The Choke Is Open

The choke valve is used to restrict airflow when starting an internal combustion engine and should be closed when you want to start your generator. By limiting airflow, the engine is flooded with a higher fuel quantity and should start far easier. Once the engine is going, the choke should be opened to allow for optimal functioning.  

If your engine is backfiring or not starting, it could be that the choke has been left open, and the engine is not getting enough fuel to ignite correctly.

Closed Fuel Valve

In contrast to the need for a closed choke valve, your generator requires the fuel valve to be open to starting. In some cases, people often close the valve when storing the generator between uses. If the valve is still closed as you try to start the engine, the fuel will not flow through the proper channels. 

Clogged or Damaged Fuel Line

The fuel line is used to allow the fuel to flow from the tank into the car carburetor. Issues with the fuel line can be caused by:

  • Cracks caused by old plastic
  • Pinching from the tube being compressed
  • Clogs caused by debris in the air and fuel

If the fuel cannot flow freely, you will likely experience backfiring, and the engine will not start. You can check the line by carefully removing the carburetor’s fuel line (have a bucket ready) and see if any fuel flows. 

If you notice the fuel not flowing or flowing inconsistently, you can replace the line to fix the problem. When not in use, be sure that the line is not pinched or bent when storing away to prevent cracks. 

Clogged Carburetor

If your generator has been sat without having been started, or if you did not drain the remaining fuel after the last use, chances are the carburetor is clogged with old fuel. As mentioned above, fuel will degrade over time and can become cloudy and separate. If left undisturbed in the carburetor, it can clog up and prevent the newer fuel from igniting. 

With the unit off, locate the drain and allow any older fuel to drain into a bucket underneath. You may need to remove the entire bowl and clean any residue out in some more extreme cases. Once the older fuel is out, you can try to start the generator again with fresh fuel.

In the future, if storing the generator for long periods, you can prevent this issue by either draining the fuel after the last use or periodically starting and running the unit to allow the fresh fuel to flow. 

Bad Spark Plug

A spark plug is necessary for a combustion engine to create the spark that ignites the compressed fuel and starts the engine. As with the fuel line, it is not uncommon to see debris build up on the plug, which can inhibit its function. A good cleaning can often fix the issue, though sometimes it will need to be replaced. 

To remove, clean, and potentially replace the spark plug, follow the steps below: 

  1. Ensure that the unit is powered down, with the switch set to “off.”
  2. Remove any cables that may be connected.
  3. Locate the spark plug and remove the cable connection.
  4. Using a socket wrench, loosen and remove the spark plug.

At this point, you can inspect and clean the plug. If the unit looks to be in working order once clean, you can replace and try to start the generator up again. However, if you see damage or the generator still will not start, it should be replaced. 

In the video below, you can see how to remove a spark plug from a generator for replacement:

Changing the Spark Plug - Generac Portable Generator

How to Set up Generator

If your new generator is not working correctly, chances are it hasn’t been set upright. Similarly, a properly set up generator can prevent many future issues down the line. 

To set up your new generator:

  1. Read through the manual to check you have all the parts required, including the oil and fuel needed.
  2. Ensure you are set up outside, away from any dwellings or vehicles.
  3. Remove the spark plug and pour a small amount of oil into the chamber.
  4. Once lubricated, slowly pull the cord a few times to encourage proper lubrication.
  5. Check your manual and add in the indicated amount of oil.
  6. Add fuel to the tank.
  7. Startup the generator (it may take a few pulls for the first start).

Once your generator is set up according to the manual, it is a good idea to allow it to run through, letting the oil and fuel work through the system. This is known as “breaking in” your generator. Not only will this properly lubricate the entire engine, but it will also allow for any debris or small metal parts that may have become loose to collect and be removed. 

To break in your generator at the very start of its lifecycle:

  1. After setting up and starting for the first time, allow the generator to run for an hour on the lowest setting with nothing attached.
  2. Drain the oil – to help remove any debris that may have been shaken loose – and refill with fresh oil.
  3. Run-on low for another hour.
  4. Drain and replace the oil once more. 
  5. Run for another hour with a small load of no more than 500 watts. 


For a generator that is backfiring and not starting, there is often a simple solution. Low or degraded oil and fuel will prevent the engine from starting correctly and will need to be drained and replaced. Likewise, damaged or faulty fuel lines or spark plugs will also inhibit proper fuel flow and will need to be either clean or replaced.

Before attempting to start your generator, check the fuel and oil levels, along with the position of the choke and fuel valves. The choke should be closed, and the fuel valve needs to be open for the unit to start.

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