How Chainsaw Carburetor Works? [Expert Guide]

How Chainsaw Carburetor Works

If you thought that a carburetor is confined to your car only, you’re totally wrong. Your chainsaw also comes with a carburetor. And it’s essential for the efficient functioning of this wood cutting machine.

The carburetor you’ll find on your chainsaw is pretty simple– as the carbs go- but not entirely uncomplicated. This carb is tasked with accurately metering the incredibly tiny amounts of chainsaw fuel you’re using and mixing it with the air that enters the engine. That simple task ensures that your engine runs correctly.

If it happens that there’s not enough fuel in your gas tank to mix with the air, your engine will “run lean.” And as a result, it will either fail to run or sustain serious damages.

And if there’s too much fuel to mix with the incoming air in the carburetor, your chainsaw engine will “run rich” and it will either fail to run (flood) or produce a lot of smoke when running, run poorly, and even waste your fuel.

Given that a crab puts into place measures to ensure the mixture right, it prevents the chainsaw operator from experiencing the above problems (as well as others).

So, let’s see how your chainsaw crab works to ensure your engine runs properly at all times…

How A Chainsaw Carburetor Works

Remember what we’ve just said above? A chainsaw carburetor is pretty simple that most carbs out there. This stems from the fact that it has only three conditions to work on:

  • It works when you’re trying to start a cold engine.
  • It works when the engine is idling.
  • It works when the engine is wide open.

No chainsaw operator is really interested in the gradations between idle and full throttle. Thus, incremental performance between the two extremes isn’t as much important. Such gradations are, however, necessary in cars- which explain why their carbs are a lot more complicated.

Main Parts of The Chainsaw Carburetor

  • The carburetor here is basically a tube.
  • Across the length of this tube, you’ll encounter a movable plate known as the throttle plate. Its primary job is to control the amount of air flowing into the tube (carburetor).
  • At some point in the tube, you’ll also encounter a narrowing- known as venture. It’s in this particular contraction that a vacuum (used to draw fuel from the gas tank) in created.
  • In the venture, there exists a hole (called a jet) that enables the vacuum to draw in fuel.

Under normal conditions (full throttle)

Now, the carb runs usually at full throttle. Under these normal conditions, the throttle plate is typically parallel to the tube length, which allows for the optimal flow of air into the carburetor.

The air flow produces a nice vacuum in the venture. The vacuum is then responsible for drawing in an accurate quantity of fuel via the jet (the narrowing).

You’re also likely to encounter a pair of screws at the top right part of your chainsaw carb. One of these screws is labeled Hi and lets you control the amount of fuel flowing into the venturing when the saw is at full throttle.

When the chainsaw is idling

When the chainsaw is running slowly (or idling if you like) the throttle plate will be nearly closed. And there’s not much air flowing through the venture to produce a vacuum.

However, note that there’s a lot of vacuum created at the back of the throttle plate since it is in a position that restricts airflow.

If you happen to drill a tiny hole through into the side of your carburetor tube, fuel can quickly be drawn into the cab using throttle vacuum.

The small hole is usually referred to as the small jet. The other screw- labeled Lo- comes into play here by controlling the amount of fuel flowing via the idle jet.

When starting a cold engine

In situations when the engine is cold, and you’re trying to start it using the pull-cord, the engine is operating at incredibly low rpm.

Because it’s also cold, it requires a super-rich air-fuel mixture to start properly. And this means using the choke plate. When the choke plate is activated, it entirely covers the venture.

With the venture fully covered and the throttle wide open, the engine vacuum can draw significant amounts of fuel via both the main and idle jets.

Usually, such a rich mixture allows the engine to fire once/twice, or to run slowly. On opening the choke plate, the engine starts running normally.

Adjusting Your Chainsaw Carburetor The Right Way

Now that you know how a chainsaw carburetor works let’s take a look at how you ought to adjust it for optimal performance.

Most carbs come with three adjusting screws, namely

  • Idle Speed/Throttle Stop.
  • Low-speed Fuel Adjustment (marked Lo), and
  • High-Speed Fuel Adjustment (marked Hi)
  • Idle Speed/Throttle Stop: Use this adjustment to set how much throttle valve (or butterfly) remains open when you release the throttle trigger. If you set the adjustment too low, the engine will die the moment you release the trigger. If you set it too high, the resulting high idle speed will make the centrifugal clutch engage (prompting the chain to run). This is quite dangerous and should be avoided by all means.
  • Low-speed Fuel Adjustment: this adjustment is used to control the amount of fuel present in the combustible air-fuel mixture at idle speed. If the adjustment is set too rich, it causes the engine to load up and die at low/idle speed. When the mixture is set too lean, it causes the engine to starve and surge or race. Extremely lean adjustment will cause the engine to die.
  • High-Speed Fuel Adjustment: this is the adjustment responsible for controlling the amount of fuel present in the combustible air-fuel mixture at full-throttle/cutting speed. This is the adjustment that determines how your saw runs when cutting objects. If you set the adjustment too rich, it will limit the saw from reaching the rpm level needed to produce maximum power. What’s more, the throttle response may become sluggish, and the engine would produce lots of smoke and perform poorly. A too lean mixture, on the other hand, allows the engine to obtain an rpm level cylinder seizure and bearing failure is likely to occur. It will also lack sufficient cutting power and become very hot.


A chainsaw carburetor is a critical part of the good overall performance of the saw. Learning how this part works is important for all chainsaw operators as they’ll become more familiar with it.

Because all the carburetor adjustments are necessary for the saw to output the best performance, learning how to properly tune them as described in our post above is also important.

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