Power Saw Decompression Valves – By: Andrew Brassard

There are a tremendous amount of myths and misconceptions that circle in the fire service about the decompression valve on rotary saws.  Some of these myths and misconceptions can lead to a lack of saw performance and or and inability to put the saw into action when it is needed most.

What is a Decompression Valve?

Basically, the way that a decompression valve works is that it is a small one way valve that is bored into the saws combustion chamber. When the starter cord is pulled the recoil turns causing the piston to raise causing compression to build inside the cylinder.  This compression can make the saw hard to start due to the resistance of the compression on the saws recoil. The valve allows a bit of the compression to escape, this in turn allows for the saw to start easier. The valve is semi-automatic meaning that it must be depressed by the saw operator before starting, after the saw fires the decompression valve automatically closes.

The Myths and Misconceptions

One of the biggest myths and misconceptions about the decompression valve is whether or not it needs to be depressed every time you pull the starter cord or not.  The answer is absolutely YES!  Now the reason for this is not necessarily to make it easier for the saw operator because lets be honest, if you can’t pull the cord without pushing in the decompression valve because it is too hard…. Than riding around on a fire truck is probably not for you!  There are two reasons that you want to ensure that the decompression valve has been depressed every time:

  1. The first reason is that starting the saw without the decompression valve engaged is tremendously hard on the saws starter recoil, this can lead to the starter cord breaking during start up or the cord starting to become loose and it will tend to hang out of the recoil.
  2. The second reason is that if the saw is constantly turned over it is generating tremendous amounts of compression inside the compression chamber, that compression is constantly pushed into the closed valve. What will start to happen is the valve will wear down and start to leak compression all the time. This leak in the compression can cause serious performance issues with your saw, if you find that your saw is bogging down a lot during roof or forcible entry operations then this could be an issue with the compression of your saw.

Another myth and misconception about decompression valves is that regular maintenance is not required.  The decompression valve just like any other part of the saw requires maintenance to run in tip top shape.  It tends to be a part of the regular service of the saws that is often overlooked, forgotten, or completely ignored. Much like a spark plug the end of the decompression valve is inside the combustion chamber of the engine.  Carbon, oil, and other unburned material tend to collect on the end of the valve.  This collection of unburned carbon can cause problems in the valve opening and closing or in the seal of the valve; this valve not closing properly can leave the pathway open for the compression to constantly leak.  Again a leak due to an improper seal or a malfunction in the valve will greatly effect the saws performance.

Maintenance

Maintenance on the decompression valve should be done annually or when ever the spark plug is being changed. The best practice for the cleaning of the valve is to first clean the dust and debris from the general area of the valve.  The reason for this is you don’t want any of dirt or debris getting into the cylinder. Once the area has been cleaned the valve can be removed with a wrench, and a clean rag can be placed into the hole to prevent any dust, dirt, or debris from getting into the cylinder.  After the valve has been removed it can be soaked in gasoline or WD-40 to remove the oils and unburned carbon. Once the valve has been cleaned in can be reinstalled.

During a fire ground operation, the decompression valve not being pressed won’t ruin your saw. It is extremely important to ensure that the valve is engaged during normal daily checks and training. This will build good habits that will no doubt carry over to the fire ground.

These are a couple of small tips that can make a ton of difference with your saws on the fire ground. Saws are a mechanical piece of equipment and they need to be cared for, inspected, and maintained if you want them to run in top form.

 

 

3 Comments

  • Donovan says:

    Good subject. I like to talk about tool maintenance about as much as I like to do it, which is a lot.
    I do have a small issue with one section but it's more of an administrative thing. Reason #2 for the advocation of using the valve, I think that is more a reason for maintenance than it is a reason to push it for start up. My reasoning is the few revs that happen during startup are miniscule compared to the thousands that happen during operation. On a 2 stroke saw engine at 8000ish RPM's it's theoretically hitting 4000 compression/power strokes/min, which sounds more likely to cause the stated build up and fouling.
    Thank you for your time.

  • Isabelle says:

    My fire dept recently purchased new 20 " Stihl chain saws for our ladder trucks. None of them have decompression valves. I have been on the job 17 years and never had a problem starting our previous saws but they had decompression valves. I am having trouble with these new saws hanging up, sometimes they pull pretty easily and other times every pull hangs up and I can't get them started……asking for help is embarassing. Do you have any suggestions for starting these saws consistently…..I have been playing around with body position. I am in very good physical condition, do triathlons, lots of upper body work (pullups with weight)…This is extremely frustrating. Any help or suggestions would be appreciated.

  • carroll price says:

    Some smaller chainsaws like the Stihl 025 are very hard to start because they are not equipped with a compression relief valve. Anyone who owns one of these saws will probably tell you that it was not as hard to start when new as it became later on. My theory, as to why this happens, is that after becoming fully broken in, the rings fully seat to the cylinder causing the compression to increase to the point where it becomes nearly impossible to start with the pull rope. As far as I am concerned, not including a compression relief valve on these size saws is a factory over-site. The saw I have, which is a Stihl 025 (Stihl 250) produces a whopping 150 psi of cylinder pressure when pulled by the starting rope. As a result, I am seriously considering modifying the cylinder head by installing a compression relief valve.

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