Stihl 250 Does Not Pull Under Load

Stihl 250 Does Not Pull Under Load

A carburetor is an integral element of a chainsaw, the function of which is to mix the fuel mixture with air and further supply the finished fuel to the engine tool. Despite the high reliability of modern carburetors, they are often subjected to various breakdowns, which entails a complete stop of the chainsaw engine. Let’s look at the design of this important unit and the ways to independently resolve its malfunctions.

Situations in which adjustment is necessary

New chainsaws are supplied from the manufacturer with adjusted carburetors. This is called standard adjustment. But for the normal passage of the break-in process, it is necessary to lower the maximum engine speed, to adjust the carburetor. After running in, the chainsaw needs to be fine-tuned to achieve maximum engine efficiency.

Also, a signal for cleaning and adjustment is increased fuel consumption, which negatively affects the operation of the exhaust system. Simply put, the chainsaw begins to smoke very much and the muffler becomes clogged with carbon, which in turn will lead to a decrease in power. With a lack of fuel in the mixture, engine parts are poorly lubricated, and premature wear occurs. Also, a poor-quality mixture can cause the engine to jam due to overheating (this is one of the reasons why the chainsaw does not start).

Adjustment is necessary when the engine does not keep idle, stalls or does not develop full power.

The carburetor must be adjusted with clean air and fuel filters.

The first signs of a carb malfunction

Due to the untimely or unprofessional periodic maintenance of the chainsaw, as well as due to the heavy load, difficulties in its operation may arise from time to time. For professional workers, such breakdowns can reduce their work efficiency and ruin their mood for the whole day.

Carburetor malfunctions, in turn, have specific symptoms. From them, you can immediately determine the cause of the breakdown.

The following signs indicate incorrect carburetor settings:

  1. The tool starts up problematically, after which it immediately stalls.
  2. The chainsaw stalls when you press the gas.
  3. Gasoline is consumed in large quantities, and it still remains in the cylinders and does not burn out until the end, and the exhaust is excessively thick.
  4. The carburettor settings set at the manufacturer’s factory are lost due to improper use or self-adjustment.
  5. Debris has entered the air intake system. Because of this, she failed.
  6. Engine pistons used up. Setting the carburetor in this case only temporarily solves the problem.

In the event that dirt has got into the carburetor, one setting is indispensable. Here you just need to clean it. When the pistons are used up, an overhaul of the engine is required.

But after that, of course, you need to configure the carburetor jets, as well as blowing and cleaning them.

How is a chainsaw carburetor designed?

The main purpose of the carburetor is to mix fuel with air in the required proportions. If the latter are violated, this almost immediately affects the engine.

Chainsaw carburetor Stihl MS 181 C BE

Chainsaw carburetors may differ in design. However, their basic elements and the principle of operation remain approximately the same.

Carburetor device:

This is a wind tunnel, across which there is an air damper. With its help, the air supply intensity is regulated.

This part is where the base tube is contracted. Here the air flow rate rises, i.E. Air always flows into the carburetor under some pressure.

In this area, fuel is in contact with the air stream. Fuel enters the atomizer from the float chamber through the nozzle (dispenser).

  1. Float chamber

This is a container that allows you to keep the fuel mixture in a stable state.

The design of the carburetor chainsaw

A chainsaw carburetor works according to the following principle:

  • When the engine starts, the main damper opens and an air stream is supplied into the air duct at a certain speed;
  • The air flow rate, as well as the level of the float in the chamber are regulated by the position of the damper, namely due to the pressure difference in the float chamber and the air channel;
  • Fuel from the float chamber is sucked into the nozzle, and from there into the diffuser;
  • Passing through the diffuser, air captures the incoming fuel;
  • The resulting mixture is fed into the cavity of the cylinders through the inlet channels.

The principle of operation of the carburetor chainsaw

The pressure in the float chamber is approximately equal to atmospheric, but in the air channels of the carburetor when the engine starts the chainsaw, a vacuum occurs. Opening the damper increases the air supply. As a result, more fuel enters the carburetor, and the number of revolutions of the engine shaft increases.


Correct adjustment of the carburetor of the chainsaw should be carried out using a special tool. A tachometer. It allows you to most accurately determine the maximum number of engine revolutions and adjust them according to the requirements of the manufacturer. To adjust, you need to have a small slotted screwdriver. For some models of chainsaws, you will need a special adjustment key, without which it is impossible to carry out work.

The key for adjusting some carburetor models is available only in service centers authorized by the manufacturer for the repair of chainsaws. This measure is aimed at reducing damage due to improper settings by inexperienced users and those involved in the repair, without official permission to do so.

Setting up the chainsaw should, if possible, be carried out in official service centers that have a certificate from the manufacturer.

What is the primary adjustment for?

In the first hours of operation, the 2-stroke engine of the chainsaw requires a break-in, on which the quality of its further work depends. During the break-in process, the carburetor is initially adjusted. The optimal parameters for preparing and supplying the fuel mixture are established.

Primary break-in of the chainsaw: work with thin material

Most modern chainsaws go on sale regulated. Nevertheless, the quality of the factory settings does not stop to check. In the end, it is up to you to work with the tool.

At the first start, the chainsaw should work in a gentle mode. Therefore, for several hours after the start, it is recommended not to overload the engine and cut only small branches and trunks up to 10 cm thick.

Simple rules will help to insure against failures in the operation of the carburetor of the chainsaw:

    Always mix gasoline and oil according to the figures given by the manufacturer.

The proportions of the fuel mixture for the chainsaw engine

  • Do not prepare the fuel mixture in advance. The properties of gasoline and oil in a mixed state are lost over time. The exception is fuel for an intensively used tool, however, even here you should not exceed about a weekly rate.
  • Use oil designed for the engine of your chainsaw. A fuel mixture with a non-normative composition will disable it seriously and for a long time.
  • When preparing the fuel mixture, consider the impact on its components of climatic conditions.
  • A chainsaw should start correctly, provide a stable sprocket rotation speed and a smooth increase / decrease in power. When the engine is running, there should be no extraneous knocks, pops or noises.

    Professional hand chainsaw at work

    The main thing during the initial run-in of the tool is to ensure that the fuel consumption matches the power used, i.E. In practice, the saw should not go out, “sneeze”, smoke and jerk.

    Chainsaw carburetor adjustment. When is a procedure required?

    Often owners of chainsaws think about why a chainsaw shoots at a carburetor, or why gasoline does not flow into a carburetor of a chainsaw? In both cases, the cause of the malfunction lies in the failure of the factory settings of the unit, and to eliminate the breakdowns, the carburetor of the chainsaw needs to be adjusted. To understand that the chainsaw carburetor needs to be adjusted, a number of other signs will help:

    • The engine does not start at all, or stalls immediately after it starts up. The reason for this may be a shortage of fuel paired with an excess of air;
    • Increased fuel consumption. In this case, the amount of exhaust gases increases rapidly. The reason in this situation should be sought in increasing the amount of fuel and reducing the amount of air;

    There are also a number of mechanical reasons that make it necessary to adjust the carburetor. These include:

    • Strong vibration. It leads to damage to the protective cap, resulting in a loss of fixation of all three adjusting screws;
    • Deterioration of the piston part of the motor. In such a situation, adjusting the carburetor will only extend the tool operation for a while. To completely eliminate the malfunction, it is necessary to completely replace the piston group;
    • Clogging. They appear as a result of using low-quality gasoline, damage to the filter or scale. In such cases, it will be necessary to dismantle, disassemble and clean the carburetor.

    Stihl 250 Does Not Pull Under Load

    In any of these cases, it is better not to delay the adjustment or flushing procedure of the assembly. Otherwise, it will break other, no less important elements of the chainsaw.

    How to adjust the carburetor

    The algorithm for setting up the chainsaw is quite simple and consists of the following steps:

    1. Adjustment of the engine for smooth operation at low speeds.
    2. Setting the carburetor to the correct flow of the fuel mixture at maximum speed.
    3. Fine carburetor adjustment for idling.
    4. Checking the chainsaw in all modes.

    The carburetor must be adjusted after the engine has run a little and warmed up.

    Now we will consider how the correct adjustment of the carburetor of the chainsaw goes in more detail. Almost all carburetor models have three adjusting screws. The screw (T) is responsible for fine tuning in idle mode. Screw (H) setting for maximum speed. The screw (L) controls the formation of the mixture at low speeds. By tightening the screws (L) and (H), the fuel supply and the mixture are leaner, respectively, revolutions increase.

    Carb settings

    You should be aware that a carburetor can only be properly adjusted if:

    • Purification filters (gasoline and air) are not dirty;
    • The jets and their channels are clean;
    • Membranes are not damaged;
    • The needle valve is in good condition and the required volume of the fuel mixture enters the float chamber.

    To adjust the carburetor of imported chainsaws, you must adhere to the following rules.

    1. If possible, before adjusting, warm up the engine at idle or at minimum speed for about 10-15 minutes.
    2. By turning the “L” knob, the engine is so driven that it idles one and a half to two thousand revolutions per minute. But you should pay attention to how the engine is gaining momentum. When you press the accelerator lever, acceleration should go quickly and evenly. If the “failure” of the turns is detected, the screw should be slightly unscrewed until this phenomenon is eliminated, since it is caused by insufficient enrichment of the fuel mixture.
    3. When the mixture has been adjusted at low revs, you should begin to tighten the “T” screw until you notice the rotation of the saw chain. After that, the screw should be unscrewed by half or a third of a revolution, observing the behavior of the clutch mechanism. Normally, at idle, the saw should not rotate.

    The “H” screw compensates for understated or overspeed due to a change in fuel type, change in oil concentration or ambient humidity.

    Also, the “upper” setting can fly off if the low speed is not adjusted correctly with the corresponding screw.

    Screw “H”, not having enough experience, it is better not to adjust. If the carburetor of the chainsaw is set up by an incompetent person, there is a danger that too high a mixture will enter the combustion chamber at high speeds, especially under load. Such engine operation will inevitably lead to rapid wear of its piston system and to a malfunction in the ignition system.

    You can start adjusting high revs if the person who will perform the adjustment has the knowledge and skills to adjust carburetor engines. The master tuner must have at its disposal a special device. A tachometer or multimeter with an oscilloscope function.

    The carburetor of the chainsaw, namely, high engine speeds, is set up according to a simple scheme.

    1. It is necessary to tighten or unscrew the screw “H” until the engine develops about 15 thousand revolutions per minute (by tachometer) at maximum gas or the value that is indicated in the manual for this unit.
    2. When using an oscilloscope, the frequency of the spark discharge should be in the range from 230 to 250 Hz. Keep in mind that under load, the frequency can decrease by about 10-15%.

    We can conclude that for safety it is necessary to tune the ICE to slightly lower revs.

    The carburetor of the Chinese chainsaw should be set up in the same way as described above. On its body there are holes in which to adjust the engine speed, you need to insert a flat screwdriver. Each hole is signed with any one of the three letters. As a rule, the letter “T” can be seen near the top hole, and the letters “L” and “H” near the bottom (shown by arrows).

    The same arrangement of screws has a Chinese chainsaw Carver (CARVER). The following figure shows what the carburetor looks like from a Chinese chainsaw outside the unit body.

    The main malfunctions of the carburetor on a chainsaw

    It should be understood that the dimensions of all holes are very small. Even the smallest pollution can block the flow of air or fuel, so filter elements are installed on the inlets. Unfortunately, they do not always cope with the task. The following symptoms indicate malfunctions of the carburetor of the chainsaw:

    • High gas mileage and heavy smoke from the exhaust;
    • The engine does not pull;
    • Floating revolutions;
    • The motor starts, but stalls immediately or after a few seconds;
    • The motor does not start at all.
    • Chainsaw shoots at carburetor

    All these malfunctions are a sign of a violation of the carburetor adjustment on the chainsaw or it is dirty. Hence the need for adjustment, cleaning or repair. How to do this. Consider below.

    How to adjust the carburetor on a chainsaw yourself

    The factory settings of the carburetor of the chainsaw provide for the optimal parameters of suction of fuel and air. But in the process, they can go astray due to vibration. Even a shift of ¼ revolution in any of the screws can lead to a serious change in the preparation of the air-fuel mixture and it is necessary to adjust the chainsaw carburetor.

    The chainsaw carburetor is adjusted using three screws marked H (High), L (Low) and S. The last screw, depending on the brand and manufacturer, may have other names, for example T or LA.

    • H: responsible for the operation of the main jet and adjusts the maximum speed.
    • L: adjusts the position of the idle nozzle and allows you to adjust low revs.
    • S / T / LA: is intended to regulate single turns of the engine.

    To know how to properly set up a carburetor on a chainsaw, a technical passport is required. It contains information about the angle of rotation of the adjusting screws. After that, go directly to the adjustment. By the way, here you will need a screwdriver to adjust the carburetor of the chainsaw or a regular flat screwdriver:

    1. Install the chainsaw on a flat surface, preferably a table or something similar.Important: The chainsaw must be positioned so that the tire is pointing in the opposite direction. The engine must be turned off.
    2. Using a wrench or a screwdriver, unscrew the fasteners of the housing cover and remove it to get to the carburetor.
    3. Remove the air filter that covers the carburetor for the chainsaw. The manufacturer can install a shock-absorbing foam insert on top of the carburetor. It must also be removed.
    4. Now starts setting up the carburetor chainsaw. Screws L and H must be fully tightened. Twist them slowly and carefully so as not to break.
    5. Loosen both screws approximately 1.5 turns.
    6. Start the engine and let it run for about 7-10 minutes so that it warms up well. It will not work to adjust the chainsaw carburetor on a cold engine.
    7. After the engine warms up, set its operation at medium speed.
    8. Slowly start anti-clockwise tightening the idle screw. Spin until the motor runs smoothly. In case the engine starts to stall, it is necessary to continue adjusting the carburetor of the chainsaw and gradually return the screw clockwise.

    After adjusting the carburetor of a Chinese-made chainsaw or any other, it is necessary to “drive” the engine at different speeds.

    Process description

    If the carburetor of the chainsaw is not adjusted, and it is not possible to start the chainsaw, then the setting is started by setting the screws (L) and (H) to the standard setting. This is 1/5 turn of the screws until fully tightened. After which, it is necessary to start the engine, let it run for a bit and warm up. The next step is setting the low speed mode. Optimum engine operation at the “bottom” can be achieved by adjusting the screw (L) and idle speed (T). To do this, first screw (L) is tightened until the maximum engine speed is obtained, then it is released by 1/4, and the screw idle is normalized by screw (T).

    After adjusting the work on the “bottoms” is the adjustment of traction and maximum engine speed. To do this, slowly tighten the screw (H) and check the speed using the electronic tachometer.

    Exceeding the maximum engine speed declared by the manufacturer is unacceptable.

    An important step is to check the saw in all operating modes after adjusting it, for this you need to allow the chainsaw to cool, and then start according to the cold tool start algorithm. If the saw starts up normally and is idling smoothly, you need to check how it saws. If during the verification process deficiencies in the power of the chainsaw are found, it is necessary to repeat the adjustment of the screw (H) by unscrewing it and checking the traction in operation.

    Loss of traction at maximum speeds recommended by the manufacturer is an indicator of CPG wear or poor fuel quality.

    Saws Stihl and Partner

    In the Stihl 250, 361 chainsaws, as well as the Partner 351, 350, the carburetor is adjusted using the N and L screws by setting the proportions of gasoline and air. If you let them go, then a large part of the fuel will be added and therefore the speed will be high, otherwise, on the contrary, they will fall.

    Idling revolutions in partner chainsaws are regulated using a screw with the letter T, and in Shtil, as a rule, you need to look for the marking of the aircraft. The standard setting of the chainsaw is carried out during the final tests. Basically, after such settings, the percentage of fuel in the working mixture is more than air, which does not correspond to the optimal parameters. But they must be preserved in the first hours of operation during the break-in of the device. After that, the saw can be adjusted independently, but it will work well at the factory settings.

    But if it is decided to regulate independently, then actions must be performed in the following sequence:

    • Screw adjustment L;
    • Then set the jet N;
    • Then proceed to idle adjustment using the T screw (if the Stihl chainsaw is LA).

    First screw L must be twisted in both directions in order to find the maximum engine speed. If that point is found, then turn the same screw by ¼, thereby slowing down a bit. If at the same time the circuit continues to idle, it is necessary to immediately turn the screw T (LA) until the chain stops completely.

    After setting the maximum speed with the screw H, it is necessary to check the frequency with a tachometer. In the event that it does not match those indicated in the service book, the setting must be repeated. When a lot of smoke comes out at the maximum speed from the exhaust pipe, the mixture is too rich in fuel, if you hear noises and the engine heats up very much, on the contrary it is poor. Then you need to adjust the idle speed. With proper tuning, the engine should sound at low revs and the chain should not move.

    Husqvarna chainsaw adjustment

    During the break-in period of the Husqvarna 142, 240, 236, 137 chainsaw and other similar models, the carburetor settings are set in such a way that the engine speed is 600-700 less than ordinary workers. In the case when it is not possible to adjust the operating frequency by the tachometer, it is not recommended to reset the factory settings with the screw H.

    The manufacturer categorically prohibits exceeding independently the working number of turns. When, after setting the carburetor, the chain of the chainsaw moves at idle, it is necessary to stop it by turning the screw T counterclockwise.

    In order to fine-tune the saw, it must be installed on a flat surface with the tire forward. You also need to warm up the engine for at least 12-15 minutes. First, determine the highest idle frequency. To do this, in the chainsaw screw L must be turned clockwise, and then in the opposite direction. After determining the desired frequency, the same screw rotates counterclockwise from 12 to 25% of its full revolution.

    Idling in the models of this manufacturer is configured using a jet with the letter T. The setting is made by turning it clockwise until the chain moves. After that, turn easily counterclockwise until the chain stops moving.

    The screw H of the chainsaw is adjusted correctly only when the saw does not make a lot of noise when the gas is pressed to the end, and at the same time it does not weigh down with any load.

    In the event that the saw is too noisy during operation and smokes, then this nozzle must be turned from left to right to adjust.

    The setting of the carburetor of the saw can be studied in the below. In this, the wizard shows how the step-by-step adjustment of a Chinese-made chainsaw is performed without using a tachometer. Carries out after adjustment check in various modes.

    Adjusting the carburetor of the chainsaw, although it seems a fairly simple process, but requires a very careful and responsible approach. The article describes the process in sufficient detail, however, if you have never regulated a chainsaw, you should turn to specialists, this will save time, nerves and money.