How does a Primer Bulb Work on a Chainsaw? A Guide

Knowledge Based

Ever heard the annoying sound your chainsaw engine makes when it takes longer than usual to start? It happens when an engine sits unused for long with almost evaporated fuel in the carburetor.

That’s where the carburetor primer bulb comes in.

The purpose of the primer bulb is to ensure that there is enough fresh fuel in the carburetor to start the engine seamlessly.

But, the working of a primer bulb ain’t just as simple as you may think.

That’s the reason I am here to explain how does a primer bulb work on a chainsaw in detail today.

Have a look, without any further ado:

working of primer bulb in chainsaw

How does a Prime Bulb Prime the Engine in a Chainsaw?

A primer bulb is a small rubber bulb near the carburetor, which one can press to “prime” the engine before starting—it works by creating a vacuum that draws fresh fuel into the intake fuel line. The fuel is then deposited into the carburetor, preparing it to make the perfect fuel-air mixture.

Once you’ve primed the engine properly, it will work smoothly.

Components of Primer Bulb

Here are the components of the carburetor primer bulb that play along to make the magic happen;


When the bulb is squeezed, a vacuum draws fuel from the fuel tank through the fuel lines and into the bulb. As the bulb expands, it pushes the fuel through the check valve and into the carburetor.

You need to squeeze the bulb with your hand manually. However, some modern chainsaws may have automatic primer bulbs that do not require manual squeezing.

Check Valves

Some primer bulbs have a one-way valve that allows fuel to flow in one direction only. However, modern chainsaws have two primer check valves on the base.

  • Inlet Valve
  • Outlet Valve

They work in conjunction with the check valve to create a one-way flow of fuel through the primer bulb. When you squeeze the bulb, the inlet valve opens to allow fuel to enter the bulb while the outlet valve remains closed.

As the bulb expands, the inlet valve closes, and the outlet valve opens, allowing fuel to flow into the carburetor.

Fuel Pipes/Tubes

Fuel lines are flexible tubes that connect the fuel tank, carburetor, and primer bulb, as shown in the diagram. They allow fuel to flow from the tank to the engine.

Fuel pipes are typically made of rubber or other flexible materials that can withstand exposure to gasoline and other fuels.

Chainsaw Primer Bulb Diagram

Types of Primer Bulb Used in a Chainsaw

So, if you’ll step into the market, you’ll find two types of primer bulbs being used in chainsaws.

  • Snap-in primer bulb
  • Screw-in primer bulb

Snap-in Primer Bulb

A snap-in primer bulb is a type of primer bulb that is designed to be easily attached and detached from the chainsaw’s fuel tank.

The “snap” part of the name comes from the way the primer bulb attaches to the fuel tank. It’s like snapping together two puzzle pieces!

When you push the primer bulb into the hole in the fuel tank, it makes a snapping sound that tells you it’s locked into place.

The snap-in design is quick and easy to use, allowing you to easily remove the primer bulb if you need to replace it.

However, if installed improperly, they can allow air to enter the fuel tank, which can cause the engine to run poorly or not start at all.

Screw-in Primer Bulb

As the name suggests, it screws into the carburetor’s body or the fuel line between the tank and the carburetor. Screw-in primer bulb functions in a similar way to the snap-in primer bulb but has a different attachment mechanism.

They are primarily found in professional arborist and miller chainsaws because of their high-end durability and long life.

Purge Bulb vs Primer Bulb

A purge bulb is a button near the carburetor that you press to get rid of air bubbles in the fuel line.

You see, sometimes air gets into the fuel line of a Craftsman chainsaw and makes it hard for the engine to start. So you press the purge bulb to force the air out and get the fuel flowing properly.

A primer bulb, on the other hand, is a rubber bulb that you squeeze to add fuel to the carburetor before you start ignition.

Both help the engine start more easily, especially if it has been sitting for a while or it is cold outside.

So the main difference between purge and primer bulbs is what they do. The purge bulb helps get rid of air bubbles in the fuel line, while the primer bulb helps add fuel to the engine to make it easier to start.

Is a Primer bulb Necessary?

A primer bulb is not absolutely necessary for a chainsaw to function—but here are two reasons to convince you to have it for smooth engine functioning;

Perfect Fuel Delivery

A chainsaw primer bulb helps to deliver fuel to the carburetor efficiently. Without a primer bulb, the carburetor may not receive the required fuel to run the engine, leading to poor performance and potentially causing damage to the engine.

Furthermore, a primer bulb can also help prevent engine flooding. The engine may flood when too much fuel is present in the carburetor. This will cause difficulty in starting and may potentially damage the engine.

The primer bulb helps to add the right amount of fuel, reducing the risk of flooding.

Instant Engine Start

For arborists, time is money, and a chainsaw that doesn’t start quickly or makes excessive noise can be frustrating and slow down their work.

A primer bulb vanishes this trouble from your life!

It makes starting the engine much more accessible, especially in cold temperatures or after the chainsaw has been unused for a while.

How do I know If my Primer Bulb is Working?

So, there are three easy ways to know if your chainsaw’s primer bulb is working;

  • See if it is still hard to squeeze. When you press it, you should be able to see it coming back to its original shape. And that’s only possible when it is firm and hard. If it’s not, it means there’s not enough pressure being created to draw fuel into the carburetor.
  • There shouldn’t be any cracks, holes, or breaks in the line pipes or primer bulb. When there are leaks, fuel can escape from the system, making it hard for the engine to start and run properly.
  • Start the engine! And see how it runs. If the engine starts up quickly and runs smoothly, the primer bulb is probably working just fine! However, if the engine has a hard time starting or it doesn’t run very well, it could mean there’s a problem with the primer bulb.

By the way, you can also check if the primer bulb is working properly by removing the fuel line from the carburetor and pressing the primer bulb. If you feel resistance and the bulb doesn’t collapse easily, it indicates that the bulb is working properly and creating pressure in the fuel system.


How does a primer bulb work?

A primer bulb is a small rubber bulb near the carburetor that can be pressed to “prime” the engine before starting it. It works by creating a vacuum that draws fresh fuel into the intake fuel line. The fuel is then deposited into the carburetor to produce the ideal fuel-air mixture.

Is the primer bulb supposed to fill with gas?

When you press the primer bulb, it creates a vacuum that draws fuel from the fuel tank through the fuel line and into the carburetor.

As a result, you may notice that the primer bulb fills with gas when you press it. This is entirely normal and shows that the primer bulb is functioning properly and drawing fuel into the carburetor as intended.

Do all primer bulbs have a hole?

Primer bulbs basically have a small check valve rather than a hole. The check valve allows fuel to be pulled into the carburetor when needed. This valve is an important component of the fuel system and helps to regulate the flow of fuel to the engine.

Does the primer bulb go before or after the filter?

The primer bulb goes after the filter for a reason. Since the filter is meant to remove debris from fuel that will be sanctioned by the primer bulb into the carburetor. So, in short, it is installed between the filter and the carburetor.

What causes the primer bulb, not to prime?

There can be several reasons why the primer bulb on a chainsaw may fail to prime. The most common cause is a problem with the fuel system, such as a clogged fuel filter or a blocked fuel line.

When the fuel system is restricted, the primer bulb may not be able to draw fuel into the carburetor, causing it to fail to prime.


And here you go, buddy!

I have explained everything from the working of the primer bulbs to all the other questions that may knock your mind.

The primer bulb is a long-lasting part of the chainsaw. So always ensure that you properly clean your chainsaw before storing it since leaving them with filled oil tanks and contaminated filters can cause your primer bulb to malfunction.

Patrick McMann