How to adjust the carburetor on a chainsaw - all the secrets of the correct settings

Stable, stable operation at any speed, developed power, efficiency, use of technical capabilities to the extent provided and laid down by the manufacturer, and even the service life of the chainsaw - all this depends on its carburetor. In addition to the fact that this unit must also be kept in proper condition and serviced in a timely manner, it must also be correctly adjusted. Adjusting the carburetor of a chainsaw is not an easy task, but with a leisurely, thoughtful approach and strict adherence to the instructions, almost anyone can do it, and it should be done at the first sign of the need for this, regardless of whether the chainsaw is in use new or well-worn.

In what cases is a chainsaw carburetor adjusted?

The need to adjust the fuel supply unit (hereinafter referred to as the FSU, that is, the carburetor) is indicated by the following reasons, symptoms and malfunctions:

• factory or own settings were lost due to loosening of the adjusting bolts and vibration, or during unsuccessful attempts to adjust the carburetor again;
• the UPT housing cover is damaged;
• the engine does not start or starts with difficulty, and then stalls almost immediately;
• the saw does not operate steadily at idle or does not hold it at all;
• excessive fuel consumption and too thick exhaust;
• spark plug electrodes are black (supply of an over-rich mixture to the engine) or with a white coating (lean);
• the chain saw does not develop maximum speed or power;
• the engine stalls while gaining speed;
• there is strong vibration of the chainsaw;
• due to malfunctions in the air purification system, debris particles got into the mechanism distributing the air-fuel mixture;
• the piston group has significant wear - in this case, adjustment of the UPT is used as a temporary measure to make the operation of the tool more stable.

Naturally, if dirt gets into the fuel system, it needs to be flushed - adjusting the carburetor cannot solve this problem. And a worn piston needs major repairs. However, in these cases, too, adjustments are made.

Adjusting the chainsaw carburetor

It may also be necessary to adjust the UPT even if there is a change in the brand or quality of fuel, atmospheric pressure, humidity and ambient temperature, altitude above sea level (when the place of operation changes from a plain to a mountainous area, then due to rarefaction of air, and vice versa).

Characteristics of the quality of the mixture at the carburetor outlet

A working mixture is formed directly in the carburetor, the composition of which can be adjusted within a sufficient range of operational adjustments. When the power unit operates on a rich mixture, the fuel does not burn completely, as a result of which the thrust drops and the engine burns fuel with less efficiency.

Combustion of a lean mixture is characterized by engine overheating, a drop in power, deterioration of the lubricating properties of the oil component of the fuel mixture, and the appearance of detonation phenomena that are unsafe for the design of the power unit.

Factory settings of fuel equipment are made on the basis of averaged data that does not take into account the influence of external factors, including air pressure and temperature, and the level of operational loads.

Periodic adjustment of the composition of the gasoline-air emulsion makes it possible to maintain the traction characteristics of the power drive at a given level and stabilize the economical consumption of fuel and motor oils.

Preparation for tuning, carburetor parts for its implementation and safety precautions

Before adjusting the UPT, you must make sure that both filters (air and fuel) are clean. In addition, there should be no air leakage into the engine from the side (through the seals, the gasket under the UPT or its accelerator pump, or anywhere else), otherwise it will not be possible to adjust the carburetor.

Preparing to adjust the carburetor

Almost any UPT is adjusted with 3 screws:

1. The main jet bolt, designated "H" and used to set the maximum speed of the saw motor.
2. The idle jet bolt, designated "L" and used to set low speed. That is, to regulate idle speed.
3. The idle adjustment bolt, designated in various cases, “LA” (Stihl), “T” (Partner, Husqvarna).

There are also carburetors with fewer adjustment screws. In this way, manufacturers protect their products from unprofessional intervention and at the same time try to simplify their setup.

Before you begin the adjustment, you should carefully study the operating instructions supplied with the chainsaw, especially the part where the angles of rotation of the adjusting bolts are given. This is an important condition that will allow you to configure it correctly without going beyond the operating parameters of the engine.

For safe regulation during its implementation, the following rules should be followed:

1. The tool must be firmly installed on a flat surface of a rigidly fixed object (workbench, rack, table).
2. The chain should be directed away from yourself and others present.
3. There must be a safe distance between the chain and any surrounding objects to ensure that they cannot come into contact.

Design features

Despite the possible differences in design, chainsaw carburetors operate on the same principle, so the adjustment sequence for different models does not differ significantly.

Prestigious chainsaws from leading manufacturers are equipped with technically advanced carburetors with built-in compensators and automatic choke control. Such modifications contribute to operator comfort, but do not affect fuel economy and other operating parameters.

The list of additional equipment includes a booster pump designed to enrich the mixture before starting the power unit. Many models of professional-class chainsaws are equipped with carburetor heating devices, which are turned on when the tool is transferred to winter operation.

Adjustment in case of changes in operation or operating conditions of the chainsaw

In such cases, you can adjust the carburetor on a chainsaw as follows. First, we start the tool and warm it up for 10 minutes.

We begin adjusting the UPT by determining the position of the adjusting screw “L”, at which the idle speed will be the highest. To do this, very slowly and smoothly turn this bolt in one direction, then in the other. And so on several times until the desired position is found. After this, turn “L” 1/8–1/4 of a turn (the required value is indicated in the technical documentation for the tool) counterclockwise (hereinafter referred to as FC).

Then we continue adjusting the idle speed using the screw “S” (“LA”, “T”):

• If the chain is motionless after manipulating the bolt “L”, then you need to smoothly rotate “S” (“LA”, “T”) clockwise (hereinafter referred to as POCHS) until it starts to move. Then we tighten the “S” 1/8–1/4 turn back.
• If the chain moves, rotate “S” (“LA”, “T”) of the frequency control unit until it stops, and then twist “S” 1/8–1/4 turn in the same direction.

The engine should run smoothly. And we check it, how it accelerates. It is considered that the engine is working normally in acceleration mode if, when you smoothly press the accelerator, the speed increases to its maximum value in a short time and without “dips.” If the engine runs unevenly and/or accelerates poorly, then you need to slightly - a maximum of 1/8 of a whole turn - turn the “L” of the CV joint. After this, you must again make the above adjustment “S” (“LA”, “T”). We repeat these adjustments until the engine runs smoothly and accelerates well.

Checking the operation of the chainsaw

Now we check the setting of the adjusting bolt “H”. To do this, we accelerate the engine to maximum speed. You just can’t drive it for a long time without load, you have to take breaks. We look at the exhaust and listen to the operation of the engine. If too much bluish smoke comes out of the muffler, and the chain saw makes a lot of noise, it means the carburetor is making the mixture too rich. In addition, the maximum speed of the power unit is lower than it should be. And also carbon deposits will form in it. In this case, it is necessary to tuck the “H” slightly. Then we check the operation of the engine again and, if necessary, repeat the adjustment.

When the chainsaw makes a squealing sound, the engine gets very hot, the exhaust is accompanied by a crackling sound and/or misfires are heard, that is, detonation is occurring - the mixture is too lean, and the maximum engine speed is higher than it should be. This leads to overheating of the power unit, burnout of the piston, rapid wear of the cylinder and burnout of the spark plug. In this case, you need to turn “H” 1/4 turn.

To complete the adjustment, check the saw at idle speed. With a correctly adjusted UPT, it should work like this:

1. The chain is motionless.
2. The engine sounds almost like a four-stroke.
3. Pressing the accelerator leads to a rapid increase in maximum speed.

If these conditions are not fully met, you should use “S” (“LA”, “T”) as indicated above, or repeat the setting from the very beginning.

This is how an approximate adjustment is made. For accuracy you need a tachometer. We do the same thing, only we turn the adjusting bolts, focusing on the readings of this device. This will allow you to accurately and, most likely, adjust the carburetor the first time, achieving, depending on the model of the tool for working with wood, an idle speed setting of 2800–3300 rpm or higher and a maximum speed of 11000–15000 rpm. Which ones should be set should be looked at in the technical documentation (operating instructions) for the saw.

Operating principle of an internal combustion engine

The operating principle of a gasoline engine is to burn a compressed mixture of air and gasoline in a combustion chamber.

The main elements of any internal combustion engine (abbreviated as ICE): a cylinder in which the piston moves up and down under the pressure of the combustion products of the fuel mixture, and a crank mechanism that converts the reciprocating movement of the piston into the rotational movement of the crankshaft.

There are four cycles of engine operation:

• fuel mixture inlet;
• compression;
• working stroke;
• release of combustion products.

During the intake stroke, the piston moves down, creating a vacuum in the cylinder, and, like a pump, sucks the fuel mixture inside.

During the compression stroke, the piston moves upward, compressing the intake mixture.

When the piston reaches the top point, the mixture is compressed to its maximum. At this moment it is ignited by an electric spark. The compressed mixture burns very quickly, almost instantly. Something like an explosion of the fuel mixture occurs. This explosion pushes the piston down and the engine begins to operate.

During the working stroke, the piston moves from the top to the bottom, pushing the crankshaft and doing useful work.

During the exhaust stroke, the piston moves upward, pushing the spent combustion products out of the cylinder into the atmosphere. Exhaust occurs.

Then everything repeats all over again.

Adjusting a partially or completely misaligned carburetor

After maintenance, such as disassembly and washing, or previous unsuccessful attempts to adjust, this unit may become completely out of alignment. So much so that the chain saw will not start at all. In this case, it is necessary to configure the engine power system in 2 stages, conventionally called:

• basic – with the engine not running;
• final - performed on a warm engine.

At the first stage, slowly screw in the adjusting screws “L” and “H” until they stop. In this case, you do not need to make a lot of effort and try to tighten them. Then we turn both screws back - for many models of saws by 1 turn. And for some - by 1.5 turns. There may be other settings. This must be clarified in the technical documentation for the chainsaw. For a number of models, the hint is printed directly on the cover of their case near the adjusting screws.

This will perform the recommended factory base carburetor setting. In this case, we do not touch the screw “S” (“LA”, “T”) under any circumstances, unless, of course, it has already been turned. When it is already twisted, you will have to work with it. Slowly screw in the “S” of the PRChS until it stops.

Let's move on to the final stage. We are trying to start the engine. If there were no manipulations with the “S” (“LA”, “T”) adjustment and “L” and “H” were set exactly as recommended, then it should start.

If “S” had to be turned on for the reasons stated above, then the engine most likely will not start right away. After each attempt, it will be necessary to tighten this screw 1/4 turn and try to start the engine again. It has to make money eventually.

Setting up the carburetor

Then warm up the engine for 10 minutes. If, after starting it or during the warming up process, the motor operates unstably or the chain begins to move, then you need to slowly and carefully rotate “S” (“LA”, “T”) of the frequency control unit. As soon as the engine starts running steadily and/or the chain stops, we immediately stop adjusting. If the engine stalls, then tighten this screw 1/8–1/4 (look in the technical documentation) of the POCHS turn.

During the warming up process, the idle speed may begin to increase, then, again, it will be necessary to “S” (“LA”, “T”) to begin to slowly turn up the frequency control until they become normal by ear. With the engine already warm, we continue adjusting the power system unit, as described in the previous chapter.

Adjusting the carburetor after running in the chain saw

After running in the saws, only fine tuning of the UPT is performed. To do this, you will definitely need a tachometer and technical documentation for the sawing tool used. The fact is that almost optimal settings are already installed at the manufacturer's factory. Just to break in the engine and grind in other moving parts of the chain saw, the carburetor is configured to prepare a slightly richer air-fuel mixture than the calculated optimal one. Due to this, the maximum speed is slightly lower than that specified by the manufacturer and, in addition, the motor is better lubricated. Thanks to this, running-in of the engine and running-in of other moving parts are carried out in a gentle manner.

Therefore, if there is no tachometer and/or reliable information (passport data) about the maximum speed at idle, then you should not change the factory settings after running in, since it will not be possible to do this correctly. And in general, fine adjustments in this case are not necessary - the chainsaw will work just fine with the existing basic settings. And if you’re going to do it, it’s better to do it at a service center. It is recommended to entrust the fine tuning of the carburetor to a completely new instrument to specialists.

Fine adjustment

Before making fine adjustments yourself, it is strongly recommended that you mark the previous position of the adjustment bolts as accurately as possible. This will allow, in case of an unsuccessful attempt, to return them to their place - to the basic factory settings. We begin the adjustment with the “H” bolt. After setting the maximum speed, check the idle. If they differ from normal, we set them first using “L” and then “S” (“LA”, “T”). In doing so, we are guided by the rules and principles set out in the second chapter of this article. But we turn the adjusting bolts, focusing on the tachometer readings. Then we continue to adjust the carburetor as indicated in the same chapter. We turn the screws, watching the tachometer readings.