Tuning a chainsaw carburettor: Adjusting different models (special features)
On its way from the fuel tank to the combustion chamber, the fuel mixture bypasses a very important element of the system. The carburetor, which provides a regular supply of the engine “nutrient” liquid, has a special influence in the process of chain saw operation. Even a small malfunction in the miniature mechanism of this device causes marked deviations in the performance of the entire tool, reducing efficiency and increasing fuel consumption. In this topic, you will learn how the adjusting screws affect the system and how to achieve their optimal position.
The more complete the understanding of the principle of operation of the device, the easier it is to operate, maintain or repair. The carburetor mechanism is quite multifaceted and interacts with several adjacent assemblies (crankcase, gas pedal and tank). Driven by air jolts, wiggling the diaphragms, but more about that in due course. For now, let’s look at the device in general.
As it should be the “heart” of the fuel system, the carburetor design is not simple, but can be understood with a smooth and consistent disassembly. Let’s start with the general list and go straight to the interaction of components. Depending on the model, the mechanism may include about 20 parts, the location of which is clearly illustrated by the following diagram of a chainsaw carburetor of the modern sample (actual from 90s).
- Fuel nozzle. Connects to the hose coming from the gas tank.
- Impulse channel nozzle. Connects to the engine crankcase through which air is forced out and sucked in, affecting the diaphragm.
- Fuel pump diaphragm. Performs the function of pumping the fuel into the chamber and its further flow into the fuel duct.
- Inlet valve. Controls movement of the mixture, allowing it to enter the diaphragm chamber and limiting movement back.
- Exhaust valve. Functionally identical to the previous item, but provides control of overflow from the diaphragm chamber to the fuel channel.
- Filter element. Final stage in the cleaning of fuel from fine particles. Manufactured as a fine-meshed screen.
- Fuel line. Provides mixture to the bottom of the carburetor.
- Needle. Blocks the fuel channel, preventing the mixture from flowing back to the top of the mechanism. Functionally similar to valves, but of a more complex design.
- Control diaphragm. Acts on the needle through a spring-loaded lever mechanism, changing its position depending on the amount of fuel in the chamber.
- The high speed adjustment screw (H). Connects to a needle that limits the flow of fuel to the stroke nozzle and atomiser.
- Throttle opening. operating throttle. Provides metered flow of mixture to the atomizer.
- Idle speed adjustment screw (L). Identical in structure and function to the previous screw, but its needle limits the flow capacity of the channel to the other 2 outlets.
- Middle (left) and idle jets (right). Technological orifices that regularly inject the mixture into the air mixing chamber.
- Air damper. Auxiliary component required to start a cold engine. Is always in the open position when operating.
- Throttle choke. Adjusts the volume of fuel mixture intake into the mixing chamber. Activates only 1 idle speed throttle in closed position; when open, alternately activates 2 others.
- Diffuser. The narrowing part of the carburetor’s main channel, where the fuel and air mix. A peculiar shape, increases the velocity of the airflow.
- Fuel chamber. Fuel mixture accumulation point, where it is fed by the diaphragm pump and from where it is distributed, passing through the nozzles into the mixing chamber.
The mixture starts its high-speed route through the fuel system of the vehicle from the tank, where it is sucked in through the filter and through the hose, through the nozzle, goes to the fuel pump chamber, passing the inlet valve. When the engine piston rises, the air in the sealed crankcase is compressed, pressing the diaphragm against the top cover, to which it is connected through an impulse hose (this is how gasoline is pumped). As the piston descends, crankcase pressure builds up and deflects the diaphragm in the opposite direction, causing the accumulated air in the chamber to flow along the only accessible route, through the bleed valve into the fuel passage.
Before entering the fuel chamber, passing through a channel of the same name, the fuel flows through the cone-shaped needle tip, pressed to the passage opening by a spring. The position of the needle is controlled by a control diaphragm, which retracts into the carburettor when a vacuum is created, pushing the tip back and allowing the mixture to enter the chamber. On filling (even partial) the vacuum decreases and the spring presses the needle again, closing the channel.
From the fuel chamber, the mixture is directed to the nozzles, passing through technical openings, the width of which is regulated by screws (L and H). At the outlet of the nozzles, the fuel is picked up by the air stream and atomized into small particles. With the throttle closed, fuel is drawn only from the last hole (idle) closest to the engine. When the throttle trigger is pulled gently, the choke opens, gradually increasing the activity of the adjacent nozzle (mid-stroke). In the maximal position, the outermost atomizer (high rpm) is engaged, increasing the amount of fuel and piston speed, speeding up the whole system.
Design and principle of operation
A carburetor is a device in which the mixture is made in specified proportions of fuel (gasoline) and air, with the subsequent supply of the prepared mixture into the engine cylinder. Almost all models of chainsaws have the same type of carburetors, so the device and principle of operation are the same.
The main elements of the carburetor design are:
- housing, which is cast out of aluminum to reduce the weight. Chainsaws constantly have to be moved from place to place, they are held by hands during work, so reducing the weight of the tool is the first priority of the manufacturers;
- Diffuser with an air damper, which are located at the inlet;
- nozzles, which are valves for metered fuel supply;
- atomizer, which provides petrol injection into the chamber (in the schemes, it is designated as the main nozzle);
- fuel (float chamber).
The principle of operation of the carburetor can be described as a cycle consisting of the following steps:
- When the engine is started, the choke opens.
- The piston stroke creates a vacuum in the air duct and causes air to flow into the diffuser.
- Gasoline flows from the gasoline tank to the float chamber. The rate at which it flows in is regulated by the jets.
- Gasoline and air mix in the diffuser, resulting in the formation of a fuel-air mixture, which then enters the engine cylinder. The more mixtures flow into the combustion chamber of the cylinder, the higher power and RPM will be.
Important! It should not be assumed that the operation of the carburetor is cyclic, as the description might suggest. All processes are permanently and uninterrupted.
The carburetor’s function, design and principle of operation.
The carburetor prepares the fuel-air consistency for powering the engine. If the proportions in this consistency are incorrect, engine operation is disturbed. If the proportion is not enough fuel, the mixture is called oversaturated, but if the fuel is not enough. poor. As with any carburetor on a chainsaw, you can see if the carburetor is adjusted correctly. Neither is best for the engine.
In different models of chainsaws carburetors differ in design, but on the principle of action they are identical. The air stream moves with the highest velocity through the air duct. The flap that closes the channel makes it possible to change the speed of the air jet. Fuel is fed through the atomizer into the channel, where it mixes with the incoming air to form an air-fuel mixture. The acquired mixture flows into the engine.
Pressure in the float chamber, where there is fuel before mixing, is actually equal to atmospheric pressure. But a vacuum is created in the air duct, so the fuel is drawn into the air flow. The more the flap opens, the sharper the air moves, and the amount of fuel also increases. the speed of the motor rpm increases. How to adjust the carburetor on a chainsaw with your own hands. This is a simplified representation of how a carburetor works.
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For efficient, smooth operation of the tool, the greatest impact and with all that economical use of fuel requires adjustment of the carburetor chainsaw. Check the fuel and air filters before adjustment. they must be spotless. Most chain saws adjust with 3 screws: idle, high rpm, low rpm. Proper carburetor adjustment is evidenced by these signs:
- The chain will not move when idling.
- The engine accelerates smoothly and quickly.
- The sound of the engine is smooth, similar to the sound of a four-stroke engine.
Does it need to be disassembled?
You can adjust the chainsaw carburetor without any disassembly at all, in rare exceptions you will only have to remove the air filter cover. This is where things go awry: On the one hand the chainsaw is designed for easy access to the adjustment mechanism and on the other hand you should not have to use it as often as possible.
It’s true that experienced woodcutters and sawyers always carry a flat blade screwdriver with a long sting and periodically tune the carburetor. The reason for this is simple: when a chainsaw is built, the carburetor is fine-tuned for a specific air and fuel composition. it is clear that the conditions at the place of work are different from the test bench.
If you change the brand of gasoline or, for example, if the air humidity increases, an experienced chainsaw user will always adjust the quality of the mixture for better performance and less wear on the piston part. But this, we repeat, requires considerable experience with a particular chainsaw and knowledge of the peculiarities of its behavior. In addition the adjustment is incredibly fine. the screws are turned only 1/10 1/20 of a turn.
Another reason for adjustment is to repair or replace the carburetor. In any case, the adjustment of the mixture quality should be carried out only if the carburetor serviceability is reliably known (no debris, gaskets do not bleed), the air filter is clean, the spark plug, the clutch and the ignition system are in perfect order. Otherwise, there is a high probability that the malfunction will be eliminated in one moment and the adjustment will abruptly become incorrect, and the engine will begin to work on wear and tear.
Adjusting the maximum rpm
This is the purpose of the adjusting screw marked “H”. Turning it clockwise increases the RPM and counterclockwise reduces it. It is necessary to set the maximum speed in the range 11500. 15000 rpm. The excess threatens the fact that the engine will work “off” and there will be violations in the ignition process. Turning the “H” screw should be guided by how the ignition occurs. When a malfunction occurs, the clockwise rotation should be stopped and the counterclockwise screw should be returned slightly.
The most optimal designs of carburetors for chainsaws developed by the world’s leading manufacturers. German STIHL, Japanese Walbro and others. These finest designs are hard to beat. Therefore, most Chinese manufacturers copy the best models.
Today on sale a large supply of Chinese carburetors for chainsaws.
Some are nameless, from an unknown manufacturer and do not even have markings. They are available simply for a specific chainsaw model. For example, the carburetor for the STIHL MS 361 chain saw.
Others have brand names of famous brands.
Thirds have their own catchy names. in Russian transcription King-Cong, Technopark, Reset, and others.
Basic Tuning Husqvarna’s 236
Your Husqvarna chainsaw needs to be tuned regularly to ensure good performance and prevent damage caused by an irregular engine. Three screws on the Husqvarna 236, 361 carburetor are used to fine-tune the engine to ensure smooth operation.
Basic tuning requirements:
The air filter must be clean. A carburetor with a dirty air filter will result in the saw producing too lean a fuel mixture. This can cause serious engine damage.
Self-adjustment is not all about saving money
Of course, each chain saw manufacturer wants its own service center to do the necessary work: repair, replace, and warranty. But if the warranty period has expired or if the tool has failed due to the fault of the owner, the services of professionals will be quite expensive. Other, after finding out the cost of repair, decides that it is not much more expensive to buy a new tool.
But if a person got used to this particular chainsaw in a year or two of work with it, it is a pity to part with it, even when it malfunctions. The solution: to learn how to take care of the unit, lubricate, clean, timely change filters and, of course, adjust the carburetor. After two or three successful adjustments the owner will not need a tachometer. he will by ear determine the approximate number of revolutions.
Adjusting the carburetor of a Chinese chainsaw, for example, Carver, is done in much the same way as the factory models. Here are the same three basic adjustment screws L, H, T. Sometimes these models have an option to adjust the no-load. But as a rule, the workers of specialized services and owners of such saws claim that it is not always possible to tune the Chinese chainsaw qualitatively at once.
For the classic Chinese models, the L screw should be 1.5 turns off the maximum. Screw H should be released one turn from maximum. Adjust idle speed in the same way.
Thus, independent adjustment of the carburetor chainsaw is not a very complicated process and can in most cases be done by a self-taught person.