Primer Bulb Fuel Line Routing (For 2-Cycle Engines)

Understanding the fuel line routing can be confusing for someone who’s replacing it for the first time. But in truth, the fuel routing is rather simple. Even though fuel lines don’t require frequent replacements, it’s better to be prepared. 

In two-cycle engines, one fuel line is connected to the carburetor that starts from the fuel tank. Another fuel line travels from the carburetor to the primer bulb. There’s another line that connects the primer bulb to the fuel tank. 

If you want to know why the primer bulb fuel routing is this way, make sure to read the whole article. 

Primer Bulb Fuel Line Routing

The diagram below will paint a picture of the fuel line routing. Let’s take a look.

primer bulb fuel line routing diagram

As you can see from the diagram, there are three fuel lines in this entire system. One line is connected to the fuel filter and the carburetor.

The function of this particular line is to deliver fuel from the tank to the carburetor continuously when the engine is running. 

The next line connects the carburetor to the primer bulb. The function of this line is to suck unused fuel back from the carburetor into the primer bulb.

Another line connects the primer bulb to the fuel tank. This line returns the unused fuel back to the fuel tank, where it gets filtered through the fuel filter and goes back to the carburetor. 

Understanding this is essential because you may have to replace any of the components involved in this system. If you replace the carburetor, you will have to remove the fuel lines that are connected to the carburetor and attach the lines to the new carburetor. 

Sometimes the fuel lines can crack and require replacements. The fuel filter can also be at fault. Damages to these components will either cause fuel leaks or other relevant issues.

So, replacing the damaged components as soon as possible is mandatory. You can learn about other parts of your chainsaw from my article on chainsaw diagram

How to Replace Fuel Lines on 2-Cycle Engines?

Three different fuel lines can make things confusing. Which line is for sucking the fuel out of the carburetor and which one is for delivering the fuel to the tank is tricky. Luckily, there is a simple way to solve this. 

First of all, you will have to take some two-cycle fuel in a small container. The top of the container should be open and visible. You can cut a bottle or use a small bowl for this. 

Now, connect two fuel lines on the primer bulb and dip the lines into the bowl. Push the bulb and look at which fuel line is causing bubbles.

The bubble-generating line indicates that this is the outgoing line. This is the return line and is supposed to be connected to the fuel tank.

primer bulb fuel line

There are two ports on the carburetor for connecting fuel lines too. So, you will have to determine the carburetor path as well. Take the carburetor and connect the line from the primer bulb that wasn’t creating bubbles. 

Take another fuel line and connect it to the carburetor on the other port. Now, place the open fuel line ends on the fuel bowl and press the primer bulb. If you notice bubbles or fuels coming out of the return line, the connections are correct.

carburetor for connecting fuel lines

However, if you see no bubbles or the primer bulb gets stuck, it means that you need to switch the lines connected to the carburetor.

primer bulb gets stuck

 If your carburetor and prime bulb diagram doesn’t look like the one explained here, it must be connected on top of the carburetor. E-replacement Parts explain how to replace those primer bulb fuel lines. 

Replacement Parts

If you are looking for replacement parts for your two-cycle chainsaw, here are some recommendations for you to look into. Make sure to match the replacement parts with your chainsaw model before buying them. 

Products  Features
HIPA Fuel Line Hose + Fuel Filter Diverse compatibility.

Reasonable price.

Includes fuel lines and fuel filters.

QIUYE Fuel Lines, Fuel Filter, and Primer Bulb Replacement Set High and low temperature resistant.

Easy to install.

Includes fuel line, fuel filter, and primer bulb.

Some Things To Remember

  • When checking the fuel line routing, the lines don’t make bubbles even after replacing them with new ones. This could mean that the carburetor is damaged. 
  • Before replacing any component of your chainsaw, make sure to match the model with the spare parts. Different chainsaws require different models of spare parts, and one won’t fit into the other. 
  • Follow the guidelines from the manufacturer when replacing the components of a chainsaw. Following the user manual is an excellent start. 
  • Check your chainsaw frequently for damaged components. Faulty parts can affect the engine and cause further damage.
  • Always use the correct oil and fuel recommended by the manufacturer. Also, use the proper ratio to ensure that the engine doesn’t get damaged. 
  • The replacement parts should be high-quality enough so that they don’t affect your chainsaw negatively. 

Safety Tips

  • Never use a chainsaw with a faulty fuel line. If you think there’s a fuel leak or the lines are cracked, check and replace them as needed.
  • If you are not certain about how to replace the damaged components of your chainsaw, seek professional help. 
  • Always keep any source of ignition away from your workshop, especially when you are working with exposed fuel. 
  • Use gloves when replacing any component and earmuffs when you’re running the chainsaw. 
  • Drain your chainsaw fuel tank if you’re going to store it for more than a month. Stored fuel can damage the chainsaw. 
  • Never ignore the safety measures suggested by the manufacturer for your particular chainsaw model. 


Where is the primer bulb on a carburetor of 2-cycle engines?

You will usually find two carburetor designs in 2-cycle engines. In some cases, the primer bulb will be placed on top of the carburetor. They are called local primer carburetors. In other cases, the primer will be separately connected with a fuel line. They’re called remote primer carburetors. 

What is the fuel ratio for chainsaws?

The required fuel ratio can be different depending on the brand and the model you have. Some chainsaws require a 40:1 fuel-to-oil ratio, whereas some require a 50:1 fuel-to-oil ratio. The best way to be certain is to check the user manual. 

What are the signs of a bad fuel line?

The fuel line ensures that the fuel flow remains uninterrupted, and the chainsaw runs without any issues. So, if your chainsaw is struggling to turn on, stay on, or misfire, it can be an indicator that the fuel lines are damaged. However, you can only be certain by examining the fuel lines. 

Wrap Up

Understanding the primer bulb fuel line routing helps you replace the fuel lines, fuel filter, and primer bulb when necessary. It’s an easy task to do. So, there’s no need to spend money on visiting a mechanic. 

Get the replacement parts, and change them yourself. Before getting any replacement part, make sure to thoroughly check the chainsaw to be certain about which part is at fault.

Jim Boslice

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