What Does a Decompression Valve Do on a Chainsaw?

Chainsaws are powerful tools that allow us to cut through wood quickly and efficiently. However, getting a chainsaw started can be tricky and requires some technique. This is where a decompression valve comes in handy. 

A decompression valve is an important part of a chainsaw that makes starting the engine much easier. In this blog post, we’ll take a closer look at what a decompression valve does, how it works, the benefits it provides, and tips for proper use.

What does a decompression valve do on a chainsaw?

what does a decompression valve do on a chainsaw

A decompression valve, sometimes called a compression release valve, is a small mechanism found on most chainsaws. It is typically located right by the pull cord starter. The valve’s job is to release air pressure, or compression, from the combustion chamber when starting the chainsaw.

By releasing this pressure buildup, the decompression valve reduces the resistance that the starter cord encounters when pulling to turn the crankshaft. This makes the chainsaw much easier to start up by hand. Without a decompression valve, the operator would have to pull extremely hard on the cord to overcome the compression pressure just to get the engine to turn over.

So in summary, the decompression valve makes starting a chainsaw possible with a reasonable amount of pulling force on the starter cord. It allows the engine to turn over more freely so that ignition can occur and the saw can start up.

How a decompression valve works

The decompression valve is connected to the combustion chamber where fuel and air are compressed. During the starting procedure, the valve opens up temporarily to release this built up compression.

Here’s a quick overview of what happens:

  • As the starting cord is pulled, the crankshaft in the engine begins to turn. This motion starts compressing the fuel-air mixture in the combustion chamber, building up pressure.
  • When the crankshaft hits the decompression release point, the valve opens up, allowing the pressurized gases to escape. This release of pressure reduces the force needed to keep turning the crankshaft.
  • After the engine turns over a bit more, the valve closes. Compression starts building up again in preparation for ignition once the spark plug fires.
  • This process repeats each time the cord is pulled until the decompression valve does its job and the chainsaw starts up.
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The valve acts like a pressure release vent that briefly opens to make starting the saw easier on the operator.

Manual vs. Automatic decompression valves

There are two main types of decompression valves used on chainsaws:

Manual Decompression Valves

These valves require the operator to manually activate them before pulling the starter cord. There is typically a button, lever, or rubber bulb that must be pressed to open the valve. This decompresses the engine before starting.

Manual valves give the operator more control over the decompression process. However, they do require an extra step when starting the saw.

Automatic Decompression Valves

Automatic valves operate on their own during the pull start sequence without any extra input needed from the user. Once the cord is pulled, the valve opens and closes automatically at the right moments in the starting procedure.

Automatic decompression valves are more convenient since they don’t require any extra steps to activate. However, they can’t be manually controlled by the operator either.

Both valve types have their pros and cons. Manual valves allow more user control while automatic ones provide greater convenience.

Importance of decompression valves in chainsaw safety

Using a decompression valve on a chainsaw provides some key safety benefits:

  • Prevents kickback when starting – Without a valve to release pressure, ignition while pulling the cord can cause immediate and dangerous kickback of the saw. Decompression valves help prevent this by releasing compression.
  • Reduces starter cord wear – Pulling the cord on a saw without decompression requires huge amounts of starting force. This can quickly wear out the cord. Decompression valves allow reasonable pull forces, extending the life of the starter.
  • Avoids hand and arm injuries – The high starting forces needed on a saw without decompression can lead to hand, wrist, elbow and shoulder injuries over time. Decompression valves help operators avoid these issues.

Overall, decompression valves play an important safety role by making chainsaw starting smooth and safe. Their use helps prevent injuries and damage that can occur when trying to start a saw without proper pressure release.

Benefits of using a decompression valve

Beyond just making chainsaws easier to start up, decompression valves also provide some additional benefits:

Easier starting

The main benefit of a decompression valve is that it reduces the effort needed to start a chainsaw. Rather than needing to tug extremely hard on the pull cord, the valve lets the operator pull with a reasonable amount of force to turn over the engine.

This makes starting a saw much easier on the arms and hands. Almost anyone can start a decompression valve equipped saw, instead of needing large amounts of strength.

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Reduced wear and tear

Without a decompression valve, chainsaw starters experience a ton of strain each time the saw is started. The huge forces applied to turn over a compressed engine can quickly wear out starter parts.

By removing this strain, decompression valves greatly extend the life of the starter system. Starters last longer and don’t need replacement as frequently when equipped with a decomp valve.

Improved safety

We touched on safety already, but it’s worth reiterating that decompression valves provide huge safety improvements during starting. By releasing compression, they prevent kickbacks that could injure operators. They also reduce repetitive hand, wrist and arm strains.

Starting a chainsaw equipped with a decomp valve is far safer compared to starting one without any pressure release mechanism.

Potential issues with decompression valves

While decompression valves provide major benefits, they aren’t without some potential downsides:

Leakage concerns

A worn out or damaged decomp valve that doesn’t seal properly can potentially leak compression during chainsaw operation. This air leakage can reduce engine power and performance.

Most decompression valve issues arise from failing to do proper maintenance and inspection. Routinely checking the valve can help avoid major problems down the road.

Maintenance and repair

Like any component, decompression valves need proper maintenance and repair over time. The valves operate in dirty, dusty environments and are under a lot of pressure. This can lead to wear and tear or debris causing them to stick partially open.

Stuck, leaking or damaged valves should be repaired as soon as possible to avoid creating bigger issues. Thankfully, valves are easily replaced on most chainsaw models.

How to use a decompression valve correctly

Using a decompression valve properly is important to maximize starting ease and safety. Here’s a step-by-step guide to using a decomp valve when starting your chainsaw:

Step-by-step guide

  1. Move the Master Control Lever to the full choke position if the engine is cold. This provides extra fuel enrichment for starting.
  2. Push in the decompression valve button or move the lever to the open position before pulling the cord. This decompresses the cylinder.
  3. Firmly grip the front handle and place your right foot into the rear handle opening for leverage.
  4. Pull the starter cord several times until the first ignition pops are heard.
  5. Move the Master Control Lever to the half choke position and pull the cord several more times to start the engine.
  6. Allow the engine to warm up for 15-30 seconds before using full throttle.

Tips for proper use

  • Always activate the decompression valve before EVERY starting attempt. Just get in the habit of pushing the button or lever before pulling the cord.
  • Make sure the decomp valve closes fully when releasing it. Listen for the audible click and confirm visually that it has fully retracted to the closed position.
  • If the saw doesn’t start up after 10 pulls with the valve open, close the valve and try another 10 pulls to prime the engine.
  • When storing the saw, leave the decomp valve in the closed position to prevent leakage.
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Using these best practices will ensure easy, quick starting and long decomp valve life.

Frequently Asked Questions

How do I know if my chainsaw has a decompression valve?

Check near the starter cord area for any buttons, levers or rubber bulbs marked “decomp”. If equipped, the decompression valve is always located right in the starting mechanism area for easy access.

Can I add a decompression valve to my chainsaw if it doesn’t have one?

Most chainsaws built in the last 20-30 years will come standard with a decomp valve from the factory. But if you have an older saw without one, retrofit valve kits are available for many models. Installation is straightforward for most do-it-yourselfers.

How often should I check and maintain my decompression valve?

Inspect the decomp valve regularly when cleaning the rest of the saw. Check that it fully opens and closes, and closes tightly. Look for debris buildup that could impede operation. Overall, valves are durable but may need replacement after years of heavy use.

Are decompression valves necessary for all chainsaw models?

Almost all gasoline engine chainsaws will benefit greatly from having a decompression valve for easier starting. They are standard equipment on the vast majority of chainsaws now. Only the smallest 20cc-25cc saws may lack a valve as their low compression allows reasonable starting effort without one.

Can a decompression valve affect my chainsaw’s power output?

When operating correctly, a decomp valve should not impact chainsaw power or performance – it only engages during starting. However, as mentioned earlier, a valve that leaks due to debris or damage can reduce proper compression and therefore reduce power. This is why regular inspection and maintenance is important.

What should I do if my decompression valve is leaking?

Try cleaning around the valve with compressed air and carburetor cleaner spray to remove any built up debris causing the leak. If cleaning doesn’t solve the issue, the valve likely needs to be rebuilt or replaced. Thankfully replacement decomp valves are easy to install yourself on most chainsaw models.

How can I tell if my decompression valve is working properly?

Start by pressing the valve and feeling that it fully depresses and springs back fully when released. Next, try slowly pulling the starting cord – the force required should reduce significantly as the valve opens during the pull. If the valve isn’t fully opening, the saw will be difficult to turn over. This likely indicates a stuck or damaged valve in need of repair.


Decompression valves play an essential role in making starting a chainsaw possible without extreme physical effort. By temporarily releasing cylinder pressure, they allow easy and safe engine turnover. Maintaining your chainsaw’s decomp valve ensures smooth starting every time you use your saw. While valves may on rare occasions need repair, overall they are durable, hassle-free components that make chainsaw operation much more user-friendly.

So next time you use your saw, remember to thank the handy decompression valve for saving your arms and making your equipment much easier and safer to operate! Proper valve use and care will provide reliable operation over years of hard work.

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