Chainsaw Won’t Idle, What’s Wrong?

Here’s a common problem : Your chainsaw won’t idle and you have work to get done, what do you do? Most of us expect our tools to work all the time, whenever we need them to. In fact, we never wonder if our car will start, or if our oven will turn on, or if our television will work just as we’re about to turn it on to watch our favorite show. Chainsaws, and other mechanical tools are a little different. They do fail us from time to time and they require maintenance.

There are many reasons why a chainsaw won’t idle properly, and in this article we’re going to present a step-by-step process which will help you find the issue and rectify it before it causes any additional damage.

What do you Do if your Chainsaw Won’t Idle?

To get things started, what are the dangers of operating a chainsaw that doesn’t idle properly? Well, it depends on what type of idling issue we are talking about. Sometimes, your chainsaw will only idle for about 5 to 10 seconds after you let off the throttle and then it will shut down. On the other hand, there will be cases in which the chainsaw refuses to lower its rpm when you get off the throttle. This can be extremely dangerous, because the chain keeps spinning even when you aren’t holding on to the trigger and it can catch you when you least expect it, causing deep injuries.

A slightly less catastrophic version of idling issues manifests itself in the form of delayed or weak throttle response. This is when you hit the trigger hard right before plunging the bar into a log of wood, and instead of immediately revving up to high rpm like it should, the chainsaw instead takes its time. In some cases, the chainsaw will eventually get to its high rpm speed, in other cases it will not go beyond the low to mid rpm range no matter how hard you press the trigger.

In order to understand and correct these issues, we must take apart the chainsaw and inspect its powerhead (the section of a chainsaw which contains the motor, fuel, air filter, etc.). Note that this article is intended for gas powered chainsaws only, because idling is something that only internal combustion engines do. Electric motors operate in a completely different manner and can start/ stop instantly with immediate torque, without requiring them to remain in a standby state whenever you aren’t using the chainsaw.

VIDEO | Repairing a Chainsaw that wont’ idle

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How do gas-powered chainsaws work?

Chainsaws are powered by single cylinder 2-stroke engines which rely on an air-fuel mixture in order to generate power. The air- fuel mixture is created in the carburetor, which sucks in atmospheric air through an air filter and mixes it in a very specific ratio with fuel that it pulls from the gas tank of the chainsaw. The gas tank isn’t just an airtight container for fuel, it also packs a little fuel filter which separates all the sawdust and impurities from fuel before it is allowed to enter the fuel line and reach the carburetor.

Once the air-fuel mixture reaches inside the cylinder of the engine after passing through an intake boot (a rubber hose connecting the intake of the engine with the output of the carburetor), it is ignited by an electric spark. This spark happens when high voltage is generated between the insulated inner electrode of the spark plug, and the outer grounded terminal. Air fuel mixture is in aerosol form, and its properties change when subjected to the high electric potential difference of several thousand volts between the two electrodes.

Once the voltage exceeds dielectric strength of this gas, it ionizes the gas and makes it act like a conductor, which causes electricity to flow between the two terminals of the spark plug. This ignites the mixture, causing an explosive effect that pushes the piston down. The crankshaft takes this linear motion and converts it into rotational force which is transferred to the centrifugal clutch.

Centrifugal clutches are used in small 2- stroke engines and consist of 3 or more spring- loaded “clutch pawls” that engage once the centrifugal forces exceed the tension load of the coil springs holding them (happens when the engine speed goes beyond idling rpm levels). These pawls or shoes press against the inside of a clutch drum which is directly connected with a drive sprocket that turns the saw chain around the guide bar.

VIDEO | STIHL Chainsaw Won’t Idle

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Why won’t my chainsaw idle?

As we explained earlier, there could be various reasons. Maybe the air filter is clogged, so the engine is running an air fuel mixture that is too rich which is resulting in loss of power and dull throttle response. It could also be a jammed up spark arrestor which happens when you put too much oil in the gas- oil fuel mix, this results in a whole lot of unburned material since oil combusts at a much higher temperature than gas and this soot clogs up the spark arrestor preventing exhaust gasses for escaping freely. Spark plugs with corroded electrodes might also be the culprit, and we all know that you can’t have a fire without a spark. Or maybe you have a spark, but it is too weak because of carbon buildup around the electrodes, which results in incomplete combustion of the fuel- air mix. Most major chainsaw manufacturers like Stihl recommend that you replace the spark plug every 100 service hours (that’s twenty 5- hour workdays).

You might want to check the fuel line and fuel filter within the gas tank. Sometimes, the ethanol in cheaper gas will corrode the soft rubber tubing that carries fuel from the tank to the carburetor. You can flash a light into the tank after emptying it, and if you see holes or cracks in the fuel line, it is time for a change. Might as well change out the fuel filter while you’re at it, since those are pretty cheap anyways. A bad fuel line will cause the chainsaw to run lean (too little fuel or too much air in the mixture), and this means it will only run at idle for 5 to 10 seconds before it shuts off completely.

Running a lean mixture for too long will cause irreversible damage to the internals of the engine. An air leak around the intake boot or head gaskets will result in lower compression pressures and your chainsaw won’t idle properly. You can tug the starter rope to get a feel for the compression levels, or you can take the chainsaw to a shop where they will do a compression test.

Finally, you definitely want to check the adjustment screws on the carburetor since these screws control little needle valves which determine how rich/ lean the air fuel mixture is at various rpm levels (idle, low, and high). Every modern chainsaw should come with at least 2 adjustment screws, one for low and the other for high rpms. It is recommended that you never tamper with the high adjustment screw because it is very easy to blow your chainsaw engine if you set it too aggressively. Which is why, manufacturers often install limiter caps on the screws which prevent you from screwing up the carburetor and engine tune beyond repair.

✓ If your chainsaw won’t start, not even an idle, then read : Why Won’t My Chainsaw Start?

VIDEO | Chainsaw Maintenance from Husqvarna

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Tuning the carburetor

When you want to tune the carburetor, we highly recommend referring to your chainsaw owner’s manual since different companies use different labels for these adjustments. Stihl uses “H”, “L” and “LA” which stand for HIGH, LOW, and IDLE respectively. Husqvarna uses “L”, “H”, and “T” which means LOW, HIGH, AND IDLE.

By default, the needle valves are set from the factory such that the chainsaw delivers optimum power output with good fuel efficiency. You will find that in some owner’s manuals they recommend that you run the chainsaw with a richer mix than usual for the first few hours after you’ve purchased it. Then you can take it to an authorized dealer or chainsaw shop for precision carb tuning where they will adjust the HIGH rpm fuel mix for you. We recommend purchasing a handheld digital tachometer if you want to reliably tune your chainsaw’s idle and high rpm speeds. Just point it at the spark plug and it will tell you the rpm of the engine.

Generally speaking, driving the T- screw in on a carburetor means the engine will get a leaner air- fuel mix which allows for higher rpm. Turning the screw counter- clockwise will pull it outwards which delivers a richer air- fuel mix to the engine, resulting in lower rpm. You want to find a sweet spot, not too low nor too rich. Adjust the low and idle screws first before you start tinkering with the high rpm settings. And make sure that the air filter is clean, and that the spark plugs are functioning properly before you tune the carburetor.

VIDEO | How to Tune Chainsaw Carburetor

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Faulty centrifugal clutch

One of the most common reasons a chainsaw chain creeps forward or keeps spinning even when the engine rpm is at idle, is a faulty clutch. It could be an issue with the return springs in the clutch. One of them may be broken, which means the clutch shoes don’t return to their resting position when the engine slows down, causing the drive sprocket to keep spinning. Another possible issue is a dirty clutch drum with dried up, caked residue inside that shortens the amount of distance the clutch pads have to travel before they engage with the drum. Take some brake cleaner and wipe the inside of the clutch drum clean if that is the case.