Chainsaw Over-Revving: Tips from a Professional


A chainsaw optimal cutting speed can ease up the cutting performance and save time. But as they say, “excess is bad,” an over-revving chainsaw can be dangerous to operate and is likely to cause an engine seizure. If you are also encountering a similar problem with your chainsaw, this blog post can help you fine-tune your engine and prevent it from over-revving.

Chainsaw over-revving:

  • If your chainsaw is over-revving, it indicates a lean air-fuel mixture is supplied to the engine.
  • The carburetor might need tuning, or there might be an air leak behind this.
  • Clean the air filter and adjust your carburetor’s screws that control the amount of fuel in the fuel mix.
  • Check for possible air leaks at the carb-engine attachment.

I will look at a chainsaw over-revving into more detail and show how to tune it step by step. Investigate what causes a chainsaw to surge. And what to do if it is constantly revving and has a reluctance to turn off.

Chainsaw Carburetor: Adjustment and Tuning:

In most gas-powered chainsaws, the carburetor is responsible for formulating and delivering the air-fuel mixture to the engine. The adjustment controls on a carburetor determine the engine RPMs at throttle and idling conditions. Hence, it becomes mandatory to perform tuning and adjusting your carburetor after some time to ensure optimal performance.

Before I move towards controlling the engine’s speed, it’s important to understand the dynamics involved in fuel flow. There are three adjustment screws on a carburetor that is responsible for controlling the engine’s RPMs.

● 1. Idle/Butterfly Screw:

This butterfly valve controls the amount of air-fuel mixture reaching the engine when the trigger is released. If the valve adjustment is set too low, the valve restricts the air-fuel flow, and the engine would die out as soon as the throttle is released. On the other hand, if this adjustment is set too high, it could cause the idling RPMs to shoot up. This condition is quite dangerous as the high RPMs will engage the clutch causing the chain to run even without the trigger pressed.

● 2. Low-Speed Adjustment Screw (L):

This adjustment controls the amount of fuel reaching the carburetor at idle speed (or low speed). It is marked by L on the chainsaw. If this screw is tightened, it inhibits the flow of fuel to the carburetor. As a result, the air-fuel mixture becomes lean. This could cause the engine to surge at idling speed. Similarly, if the screw is turned out, the mixture becomes rich due to a high fuel supply. This can reduce the engine’s RPMs and cause it to die out at idle.

● 3. High-Speed Adjustment Screw (H):

It is marked by H on the chainsaw. This adjustment determines how much fuel enters the carburetor as the throttle is pressed. When fully tightened, it prevents fuel from entering the carb. Thus, causing the engine to run lean. Please note that a lean fuel mixture causes the engine RPMs to go high, raising the maximum engine temperature. In comparison, an engine that is running at a rich fuel mixture drops its RPMs considerably. This is usually the case when the H adjustment screw is loosened to its full.

With the carburetor adjustment explained, we shall now perform the tuning to adjust an over-revving chainsaw.

How to Tune an Over-Revving Chainsaw?

Chainsaws need to be tuned to optimize the engine’s performance. This means that the engine RPMs at load or no load are adjusted by altering the composition of the fuel mix to better suit the application. The adjustment controls can be found on the carburetor.

A chainsaw tune-up is quite easy to perform, and only takes around 5 minutes. Hence, I would advise you to perform this adjustment procedure yourself instead of a mechanic’s help. If you’re skilled in working with chainsaws, you are better equipped to tackle your future problems or even help your peers with theirs. Let’s dive into this procedure step-by-step.

● Step 1. Clean the Air Filter:

I recommend that you begin your tune-up by cleaning the air filter. It is usually located at the side of your chainsaw. Remove any screws that secure it. After removing, wipe off any dust or debris accumulated. Then soak it up in soap water for some minutes and let it dry.

If you clean your air filter after tuning, your engine will run leaner than its setting. Hence, running an engine with a dirty air filter already makes it run rich (more fuel content than air).

● Step 2. Clean the Muffler:

Like the air filter, the muffler of your device also needs to be serviced to ensure that your tuning becomes effective. Remove your muffler by unscrewing its bolts through an Allen key. After that, inspect the spark arrestor screen. If there’s a considerable carbon buildup on it, you should clean it using a wire brush. A torch can also help dislodge the debris. Once cleaned, let’s proceed to adjust the carburetor controls.

● Step 3. Adjust the Low-Speed Setting:

The low-speed screw controls the engine RPMs at idle. To perform the adjustment, please follow the steps below:

  • Start the engine: Start your chainsaw, making sure that the chain is tightened. Allow it to idle for some seconds. Notice whether the engine runs well or dies out while idling.
  • Adjust the L screw: If the engine dies out while idling, then tighten the Low Speed (L) screw using a screwdriver such that its idle RPM increase. As you continue to tighten it, a point will come from where the RPMs would begin to decrease. Make a mental note about that point. Loosen the screw from that point so that the RPMs increase again to a point when they would begin to decrease. Make a mental note about that second point.
  • Find the ideal spot: Now, the ideal screw setting should lie between the abovementioned points. Keep turning the screw until you find that sweet spot and leave it there as it is. At this point, the idle RPMs would be enough to prevent them from dying out. Also, the engine response would be much quicker and lag-free as the trigger is pulled.

● Step 4: Adjust the Idle:

After the low-speed adjustment, the chain might begin to rotate as the mixture would be lean enough to engage the clutch. If your chain rotates, loosen the idler screw until it stops rotating. Ensure that the chain is not rotating at idle, else it would be very dangerous. If your chain doesn’t rotate, skip this step and proceed to the next.

● Step 5: Adjust the High-Speed Setting:

The high-speed setting controls the maximum engine RPMs, which is the subject of this blog post. An over-revving chainsaw is extremely dangerous for your engine as this can not only score the piston but overheat its components, thus leading to a total seizure.

To adjust your high-speed screw, it’s better to have a digital tachometer at hand to get an idea of the engine RPMs. If not available, you can consult your owner’s manual to locate the optimum setting for the H screw. You can also find this information online.

  • Step 1. Press the trigger: Start your chainsaw and press the trigger to raise its RPMs.
  • Step 2. Tighten the screw: With the throttle pressed wide open, tighten the H screw to its fullest. If a tachometer is available, note the reading it gives.
  • Step 3. Adjust the screw setting: Your goal is to loosen the screw by about half a turn so that its maximum RPMs at WOT drop by about 1000-2000. That is the ideal setting to operate your engine. Remember not to operate your engine at its maximum prescribed RPMs. Rather keep it a couple of thousand RPMs less.

With this step completed, you have successfully tuned your chainsaw such that it now operates at an optimum speed without any lag.

Why does my Chainsaw Keep Revving?

If your chainsaw’s chain keeps rotating even when it’s not engaged, the throttle valve is adjusted too high, and it delivers the air-fuel mix to the engine at a large rate.

If the air-fuel flow is increased to the engine, the idle RPMs increase to a point when the clutch is engaged, and the chain begins to rotate even without the trigger pressed. This can be an alarming situation from a safety perspective.

To fix this problem, I would advise you to loosen the idle screw, which closes the throttle valve and limits the air-fuel delivery while idling. Loosen the screw until the chain stops revving without the trigger pressed with the engine on.

What causes a chainsaw to surge?

If you ask an expert, the number one factor behind the chainsaw surge is the engine running lean. This may be due to an air leak or possibly a clog in the tank vent or the carburetor.

Surge is caused by erratic air leakage into the engine or the fuel unable to flow to form the air-fuel mixture. The air turns the air-fuel mix lean, thus further increasing the engine RPMs and its temperature. It’s not safe to run your chainsaw if it surges, as this may score the piston cylinder.

I have examined various chainsaws undergoing surges and constant revving. In most cases, the tank vent gets plugged due to deposits which create a vacuum lock and reduce the fuel flow to the carburetor. This produces a lean fuel mixture. It would be best if you cleared out the tank vents with a needle or a metal wire. Also, I would advise you to check the carburetor’s fuel lines for debris and clogs that might restrict fuel flow.

Lastly, I have also seen air leaks causing chainsaw surges. You should inspect the carburetor’s attachment with the engine and check if the O-rings are intact or not. A damaged O-ring or seal could result in air leakage that could cause the engine to surge.

Chainsaw’s Constant Revving and Reluctance to Turn off, what to do?

If your chainsaw has a hard time turning off, air leakage into the carb-engine attachment can be the cause. Check the O-rings and gaskets to see if they are worn out or not.

Air leakage turns the mixture lean, which causes it to rev higher. Even when you try to turn it off, there’s sufficient air and fuel to keep it running. I recommend you perform a pressure testing of your chainsaw and examine any possible sources of the leak. Before doing that, make sure all fuel is drained, and there’s no chance for the chainsaw to start.

Most commonly, the leaks originate at the gaskets and O-rings. The cylinder’s screws might be another leakage source that could become loose over time. Once you identify the leak, it would be easier to perform the repair.

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