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Chainsaw Idle Adjustment. Tips from a Professional

Your chainsaw is said to be idling when the engine is on and the throttle isn’t pressed. Most chainsaws either have too high or low RPMs at an idle state. When the RPMs are too high, it can be pretty dangerous as the chain can get engaged. In this blog post, I shall discuss everything you need to know to optimize your chainsaw’s idle condition.

Chainsaw idle adjustment. Tips from a professional.

The idler screw in the carburetor controls the idle RPMs of your chainsaw. The screw essentially controls the butterfly valve, which controls the amount of air-fuel mixture entering the engine for combustion at an idle state. Using the screwdriver, set the screw to an optimum point at which the chainsaw neither dies at idle, nor the RPMs are high enough to engage the chain.

In the article, I will explain this in more detail to help you tune and optimize your carburetor setting. I will also show how to adjust the H and L screws on a carburetor, how to do this without a tool, and explain if it is fine to let a chainsaw idle.

What do the L and H mean on a Chainsaw?

The L (low) and H (high) adjustment screws on a chainsaw govern the fuel flow at low and high RPMs, respectively.

A carburetor has three adjustment screws that are in charge of controlling the engine’s RPMs.

1. Idle Screw/Butterfly:

When the trigger is released, this butterfly valve controls the amount of air-fuel mixture that enters the engine. If the valve adjustment is too low, the valve inhibits the flow of air and fuel, causing the engine to shut down as soon as the throttle is withdrawn. On the other hand, the idle RPMs may increase if this adjustment is set too high.This is dangerous since the high RPMs will engage the clutch, forcing the chain to run even when the throttle is not held.

2. Low-Speed Adjustment Screw (L):

This screw governs the amount of fuel that reaches the carburetor at low speed. On the chainsaw, it is denoted by the letter L. When this screw is tightened, it prevents fuel from flowing to the carburetor. As a result, the air-fuel mixture becomes lean, and this can cause the engine RPMs to spike. Similarly, turning the screw out causes the mixture to become rich due to a high fuel supply. This can cause the engine’s RPMs to drop and die at low speed.

3. High-Speed Adjustment Screw (H):

On the chainsaw, it is denoted by the letter H. This setting controls how much fuel enters the carburetor when the throttle is held. When fully tightened, it inhibits fuel from entering the carb, resulting in a lean running engine. Please remember that a lean fuel mixture raises the maximum engine temperature by increasing the engine RPMs. An engine running on a rich fuel mixture, on the other hand, reduces its RPMs significantly. This is typically the case when the H adjustment screw is fully loosened.

Correct Way to Adjust or Tune the Carburetor on a Chainsaw?

Before tuning the carburetor, make sure the air filter is clean and the spark arrestor is unclogged from carbon deposits. Always run your saw idle for a few minutes and keep the tank half-filled to obtain optimum tuning results.

  • Air filter: Before adjusting or tuning the carburetor, you should always inspect and clean your air filter from any form of dust. When the air filter is cleaned after the carburetor setting, it will upset the air-fuel mix causing the saw to run lean.
  • Exhaust muffler: In addition, check your exhaust muffler and clean the spark arrestor screen from carbon buildups.
  • Fuel level: While tuning, make sure the fuel tank is half-filled and never empty. If the carburetor is adjusted on an empty tank, the engine could run lean when the tank gets filled.
  • Idle: Never tune your carburetor with the engine being cold. Doing this would make the carb setting rich when the engine achieves working temperature. It is best to let your saw idle for a few minutes before performing the adjustment.

How do you adjust H and L Screws on a Carburetor?

Using a flat head screwdriver, adjust each screw on the carburetor until a point is reached where the RPMs achieve an optimum value, which is neither too high nor too low.

● Low-Speed Screw Adjustment:

To perform the low-speed screw adjustment, follow the steps underneath:

  • Start the engine: Turn on your chainsaw and ensure the chain is tightened. Allow it to rest for a few seconds. Take note of if the engine runs smoothly or if it dies while idling.
  • Tighten the L screw: If the engine dies while idling, tighten the Low Speed (L) screw using a screwdriver to raise the idle RPM. As you tighten it further, you will reach a point where the RPMs begin to fall. Make a mental note of that point’s location. Loosen the screw from that point so that the RPMs climb again until they begin to decline. Create a mental note of the second point as well.
  • Find the ideal spot: Now, the perfect screw setting should be between these two positions. Turn the screw until you locate the perfect balance, and then leave it as such. At this time, the idle RPMs would be plenty to keep the engine running. Furthermore, the engine response will be much faster and lag-free as the trigger is pulled.

● Adjusting the Idle:

Following the low-speed adjustment, the chain may begin to rotate because the mixture is lean enough to engage the clutch. If your chain starts to rotate, loosen the idler screw until it stops. This is critical because a chain rotating at idle is extremely dangerous. If your chain does not rotate, skip this step and go to the next.

● High-Speed Adjustment:

The high-speed setting controls the maximum engine RPMs. An over-revving chainsaw is highly hazardous to your engine, as it can not only score the piston but also overheat its components, resulting in a total seizure.

It’s best to have a digital tachometer on hand to indicate the engine RPMs when adjusting your high-speed screw. If it is not available, consult your device’s manual to determine the best setting for the H screw. 

  • Pull the trigger: To increase the RPMs of your chainsaw, start it and push the trigger.
  • Tighten the screw: With the throttle wide open, tighten the H screw all the way. Take note of the reading from the tachometer if it is available.
  • Adjust the screw setting: The idea is to loosen the screw by half a turn so that its maximum RPMs at WOT drop by 1000-2000. That is the best setting for your engine. Remember to keep your engine at a couple of thousand RPMs lower than its maximum prescribed RPMs.

With this step accomplished, you have effectively set your chainsaw to function at an optimum speed with no lag.

How do you Adjust a Carburetor Without a Tool?

To adjust your carburetor screws without a dedicated adjusting tool, use a hacksaw to saw away the adjusting screws and create a rectangular slot. Now, these screws can be adjusted simply by using a flathead screwdriver.

Some carburetors need a specialized tool to adjust their throttle control screws. E.g. depending on the screw type, you may need a Single-d or a Double-d adjusting tool. Most chainsaw users may not have this tool at hand. In that case, there is a rather simpler technique for making the screw adjustment.

You will need a hacksaw to cut away a slot at the screw heads. To do this, detach your carburetor from the chainsaw and fix it on a bench vise. Use the hacksaw to cut a flat rectangular slot on the top of the screws. Afterward, you can use a readily available flathead screwdriver to adjust the screws as you want.

Is it OK to Let a Chainsaw Idle?

It is recommended to let your chainsaw idle for a minute or so just after starting the engine and before shutting it off after a run. An idle run ensures that the engine temperature gets normalized.

● Idling After Start-Up:

I would always recommend running a chainsaw at idle for some time before going full throttle. The piston and the cylinder get warmed up properly as a result. Running at full throttle right away causes the pistons to expand thermally. The piston usually expands more than the cylinder walls, and as a result, the oil film between them thins down.

This could cause your saw to overheat and is likely to damage your piston rings or seals. Idling your chainsaw for a minute or so is quite ideal. In colder temperatures, you can go a bit higher than a minute but avoid making it a regular practice as it may consume fuel unnecessarily.

● Idling Before Shut Down:

Apart from idling at startup, it is also good to let your chainsaw idle after an extended operation. This form of idle helps to cool down an engine before shut down. Since chainsaws are air-cooled, idle runs after full-throttle provide sufficient time for the high engine temperatures to fall.

Shutting them off without idling removes the air-cooling ability and may thermally overload the system, and this may cause damage to certain components in the chainsaw over time.

How Long should a Chainsaw Idle?

A chainsaw should only idle for a few minutes to warm up the engine and the cutting chain. Idling it for longer durations wears the clutch bearing and causes carbon buildup at the exhaust.

A few minutes of idle time helps regulate the engine temperatures so that your chainsaw gets ready to operate at WOT (wide open throttle). However, avoid letting it rev for longer durations (such as 5 to 10 minutes or higher).

Idling of small engines essentially results in incomplete combustion of the fuel. As a result, the exhaust gases are high in carbon content which can clog the arrestor screens in the muffler. This can further lead to engine stall issues. Furthermore, extended idling intervals lead to increased fuel consumption and bar and chain oil. Not to mention, your clutch bearing wears out at an accelerated rate.

Hence, idling a chainsaw for too long does more harm than good. I recommend idling it for some minutes to help regulate the engine temperatures and, after that, crank up the RPMs by pressing the throttle.

Should a Chainsaw Spin at Idle?

A chainsaw chain should never spin when idle. This can be quite dangerous as the chain might damage anything in its vicinity immediately after the cord is pulled.

The chain should only spin when the throttle is pressed. Otherwise, it can further aggravate the dangers of using a chainsaw. If a chain spins at idle, it can be due to the following underlying factors:

  • Damaged Clutch: The clutch assembly keeps the chain from rotating until the throttle is engaged. A broken clutch spring or a worn-out clutch plate could be the reason behind the problem. To investigate, remove the bar and chain and try to examine the assembly.
  • Idle Adjustment: In most cases, adjusting the idle screw solves this problem. If the idle screw is loose (lean setting), it can raise the idle RPMs beyond the limits, and hence the clutch fails to keep the chain from rotating.
  • Air Leak: If the idle adjustment doesn’t fix the problem, the chances are that air is leaking into the engine via the carburetor or the engine seals. Investigate the problem by performing a vacuum test which confirms the presence of a leak and also reveals its location.

Why does my Chainsaw only Runs on Choke?

When a chainsaw only runs on a choke, the fault is likely in the fuel delivery system, including the fuel filter, fuel lines, and carburetor.

The choke switch aids the engine starting by increasing the amount of fuel entering the engine, or ‘enriching’ the air-fuel mixture. The choke is normally switched off after the engine runs. However, when some chainsaw engines are starved of fuel and run leaner than usual, they can only be kept running by keeping the choke on.

Following are some possible causes of this issue:

  • Poorly mixed oil and fuel: Fuel and engine oil are mixed at a 50:1 ratio for most two-stroke engines. When chainsaws get old, most users prefer to run them lean, with lower mixing ratios like 30:1 or 25:1. This may cause starting problems in some chainsaws with malfunctioning filters and old carburetors, causing them to operate only when the choke is turned on.
  • Faulty fuel filter: A clogged fuel filter deprives the engine of adequate fuel, forcing it to operate leaner than normal and only when the choke is used.
  • Carburetor problems: When carburetors aren’t serviced regularly, they, too, can make the air-fuel mix lean by restricting the fuel jets, causing the engine to struggle while starting up.