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How a Chainsaw Carburetor Works. Tips from Professionals

The carburetor is a crucial engine component in small engines that determines its performance. An ill-maintained carburetor results in a dozen of engine problems. Hence, this article shall elaborate on the operation of a chainsaw carburetor and also discuss appropriate methods to keep the carburetor in proper working order.

How does a chainsaw carburetor work?

In chainsaws, the carburetor consists of a venturi duct that helps draw fuel from the jets by forming a vacuum. The air/fuel mixture is then delivered to the engine and regulated by a throttle valve. A carburetor comes with adjustment screws that control the fuel flow in the jets. By adjusting these screws, the engine RPMs can be controlled. In addition to adjusting, a carburetor should be maintained well by properly checking and replacing its worn-out parts and timely cleaning of its jets.

In this blog post, I will further explain how a carburetor draws gas and how you can adjust a chainsaw carburetor. I will end with how you can fix a clogged or not properly working chainsaw carburetor.

How does a Chainsaw Draw Fuel?

The carburetor uses a venturi duct to draw fuel from a jet and mix it with air to form and deliver the mixture to the engine. The airflow in the venturi forms a vacuum that sucks the fuel out from the jet.

A carburetor is essentially an air tube lined with a few jets that supply fuel. This tube is known as a venturi. Air enters the venturi from the air filter. There is a narrow region in the venturi responsible for lowering the air pressure by accelerating the airflow. This lowered pressure draws the fuel from the main jet in the form of a spray. This fuel mixes with the air downstream in the venturi and is delivered to the engine.

Let’s have a look at some of the key components of a carburetor that supplement its working:

● Component 1: Throttle Plate:

The throttle plate is essentially a butterfly valve located in the venturi. When the throttle plate is closed, the engine is idling. Very little air flows along the venturi in this position, and no fuel gets drawn from the main jet. Only the idle jets provide the fuel sufficient to keep the engine running at idle. As the throttle is pressed, the valve opens, allowing more air to flow along the venturi.

● Component 2: Jets and Screws:

The fuel jets comprise mainly; idle jets and the main fuel jet. The idle jets are opened at all times; typically, they are three in number. The main jet delivers fuel only when the pressure in the venturi is low enough, and this occurs when the throttle plate is kept open. At the starting points of these jets, screws are present that control the amount of fuel flowing through them.

The screw on the main jet is the H screw, and on the idle jets, it is the L screw. The throttle plate also has a screw known as an idle screw (I). This screw controls the throttle plate position when the engine is idling.

● Component 3: Choke Plate:

A choke plate is present, which typically aids in starting a cold engine. The choke plate, similar to a throttle plate, is situated at the entrance of the venturi. When the choke lever is pressed, the plate closes the venturi. Hence as the throttle plate is opened, more fuel is drawn from the main jet while less air is available, and the mixture becomes rich.

A rich fuel mixture raises the engine’s temperature and hence causes it to start easily when cold. As the engine has started, the choke plate no longer needs to be activated. Therefore, pulling out the choke lever resumes the regular operation of a carburetor.

How do you Adjust the Carburetor on a Chainsaw?

To adjust your chainsaw’s carburetor, you must ensure that the low-speed (L) and the high-speed (H) screws are adjusted to obtain an optimal setting of the engine RPMs and performance.

A chainsaw carburetor has three screws; the low-speed screw (L), the idle screw (I), and the high-speed screw (H). Each of these screws controls the amount of fuel entering the engine at low-speed, idle, and high-speed settings. Consider adjusting these screws to optimize the engine RPMs when it is difficult to stay running or when it overrevs.

● Screw Adjustment Explained:

The idea behind adjusting the screws is to control the fuel flow to the engine. When a screw is loosened, it allows more fuel to mix with air, thus making the mixture rich. An engine with a rich mixture runs at comparatively lower RPMs. When the screw is tightened, it restricts the fuel flow and thus makes the mixture lean, revving the engine more. Hence, tightening the screw increases the RPMs, and loosening them causes them to fall.

Make sure you remember this before you begin adjusting your carburetor. I will discuss a step-by-step approach for adjusting the carburetor on a chainsaw below:

● Step 1: Pre-Adjustment Steps:

Ensure the air filter is clean and the spark arrestor is free of carbon deposits before tuning the carburetor. To achieve the best tuning results, always let your saw idle for a few minutes and keep the tank half-full.

  • Air filter: Before adjusting or tuning the carburetor, always inspect and clean the air filter of dust. When the air filter is cleaned after the carburetor setting, the air-fuel mix is disrupted, causing the saw to run lean.
  • Exhaust muffler: Also, inspect your exhaust muffler for carbon buildup and clean the spark arrestor screen.
  • Fuel level: During tuning, ensure the fuel tank is half-full and never empty. The engine could start running lean if the carburetor is adjusted on an empty tank.
  • 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.

● Step 2: Adjusting the L and H Screws on a Carburetor:

While adjusting the screws, use a flat head screwdriver to turn the screws and carefully note the RPM changes upon turning each screw.

– Screw 1: Low-Speed Screw Adjustment:

Follow the procedures below to conduct the low-speed screw adjustment:

  • Step 1: Turn on the engine:  Crank your chainsaw by pulling the chord. Allow for a few seconds of idle run. Keep track of whether the engine runs smoothly or if it dies while idling.
  • Step 2: Tighten the L screw: If the engine dies while idling, use a screwdriver to tighten the Low Speed (L) screw to raise the idle RPM. As you tighten it further, you will see that the RPMs begin to fall. Make a mental note of where that point is. Loosen the screw from there, allowing the RPMs to rise again until they begin to fall. Also, make a mental note of the second point from where the RPMs begin to fall.
  • Step 3.: Find the ideal spot: The ideal screw setting should now be anywhere between these two points. Turn the screw until you find the optimal balance, then leave it as it is. The idle RPMs would be sufficient to keep the engine running. Moreover, when you pull the trigger, the engine response will be much faster and lag-free.

– Screw 2: Idle Adjustment:

After the L adjustment, the mixture might be lean enough to engage the clutch, and the chain may begin to rotate. If your chain begins to revolve, loosen the idler screw until it comes to a halt. This is crucial since a rotating chain at idle can be highly harmful. If your chain does not rotate, skip to the next step.

– Screw 3: High-Speed Adjustment:

The high-speed screw governs the maximum engine RPMs. When the screw is too tight, it can cause the chainsaw to overrev as the throttle is pressed. An over-revving chainsaw is highly detrimental to your engine because it can not only score the cylinder walls but also damage the piston, increasing the risk of a total engine seizure.

When adjusting your high-speed screw, it’s helpful to have a digital tachometer around to note the RPMs. If unavailable, consult your device’s manual to determine the best H screw setting.

  • Step 1: Start-Up: Start your chainsaw and pull the trigger to raise the RPMs.
  • Step 2: Tighten the screw: Tighten the H screw with the throttle wide open. This will cause the saw to run at its maximum RPMs. Note the tachometer reading, which now shows the maximum engine RPMs.
  • Step 3: Adjust the screw setting: Loosen the screw by half a turn to reduce the maximum RPMs at WOT by 1000-2000. That is the most appropriate setting for your engine. Keep your engine at a few thousand RPMs lower than its maximum prescribed RPMs.

After completing this step, you have properly adjusted your chainsaw to operate at an optimum speed with no lag.

How to Fix a Chainsaw Carburetor?

Chainsaw carburetors tend to get clogged, possibly due to old fuel residues. Hence, cleaning these jets is a crucial aspect of chainsaw maintenance.

Besides keeping it set correctly, carburetors must be kept clean and free from debris buildup to perform efficiently. The fuel being used nowadays is usually blended with ethanol. Therefore, it tends to develop sticky white deposits when kept for a long time. These deposits can clog the jets in the carburetor and lower its efficiency.

Moreover, certain components, such as rubber seals and the diaphragm, can also get worn out and need replacement. Hence, let’s discuss a step-by-step method to fix your carburetor.

● Step 1. Remove your Carburetor:

To remove the carburetor, open the top cover of your chainsaw near its handle. The carburetor would lie underneath the air filter. Remove the air filter entirely by loosening its screws. Now, you need to clamp off the fuel lines so that no fuel leaks out of them when removed. Unscrew the carburetor and remove the fuel lines. Now, take the carburetor out for inspection.

● Step 2. Remove the diaphragm:

Remove the screws at the top of the carburetor. Remove the diaphragm as well if it appears faulty. After removal, the fuel jets and passages should now be visible.

● Step 3. Use a Cleaner Liquid:

A carburetor cleaner liquid is easily available at any local store. Spray the carb’s interior and all the jets with this cleaner liquid. It helps to dislodge clumps of debris that might have plugged some of the jets. Please ensure that you don’t use hard metallic tools on your carburetor, as these jets are quite delicate and may get damaged if not handled with care.

● Step 4. Perform Replacements:

After you’re finished cleaning, replace the parts which appear worn out, such as the diaphragm or the springs. It is better to purchase a carburetor repair kit. It is pretty inexpensive and contains a variety of replacement parts. Make sure you purchase the one recommended by your manufacturer.