DIY chainsaw carburetor adjustment

Recommendation points

• Chainsaw carburetor device
• Do you need disassembly
• Main adjusting screws
• Regulation rules
• Elimination of malfunctions

The internal combustion engine of a chainsaw requires special attention due to the harsh operating conditions, and in particular this applies to the carburetor. Without periodic maintenance, no motor system will last long. Today we will talk about the process of adjustment and elimination of various malfunctions..

Chainsaw carburetor device

To independently adjust the carburetor of a chainsaw, you do not need to be a professional motor technician. However, you still have to memorize some aspects of the carburetor’s operation regarding the process of preparing the fuel mixture..

The vast majority of chainsaws are equipped with Chinese-made Walbro carburetors. In their structure, they are simple, if not primitive. There is a monoblock body of the straight-through type with a through hole. A diffuser is located inside the hole – a narrowing of the passage with the fuel injection channel. There are dampers on both sides: the smaller one is the throttle one, it regulates the amount of the fuel mixture to be supplied to the cylinder. Larger damper – air, serves to limit the air flow during a cold start.

Walbro carburetor device: 1 – fuel inlet connection; 2 – impulse channel of the membrane pump; 3 – inlet valve; 4 – membrane of the fuel pump; 5 – exhaust valve; 6 – filter mesh; 7 – air damper; 8 – throttle valve; 9 – fuel channel; 10 – idle speed adjustment screw; 11 – needle; 12 – idle jets; 13 – control membrane; 14 – fuel chamber; 15 – main jet; 16 – diffuser; 17 – the main adjusting screw

All the magic of fuel preparation takes place in the covert channels and the diaphragm pump chamber. When the throttle valve is turned, the cross-section of the channel through which fuel is injected into the mixing chamber increases slightly and the fuel enters in a larger volume. In this case, the carburetor has two kind of valves: for feeding at low and high speeds. When changing the position of the throttle valve, the fuel flow changes proportionally between the two valves.

The need for such a device is that at idle and under load, the quality of the mixture should be different. At the same time, the channel through which the fuel is supplied to the mixer at idle speed does not completely overlap when the engine accelerates, and vice versa – the maximum speed channel is slightly undercut at idle speed.

Do you need disassembly

You can adjust the carburetor of the chainsaw without any disassembly at all; in rare exceptions, you just have to remove the air filter cover. There is a considerable amount of contradiction here: on the one hand, the chainsaw is designed for unhindered access to the adjustment mechanism, on the other, it is recommended to use it as little as possible..

True, experienced fellers and sawers always carry a flat screwdriver with a long blade and periodically adjust the carburetor. The explanation is simple: when making a chainsaw, its carburetor is roughly adjusted, designed for a certain composition of air and fuel. It is clear that the conditions at the place of work differ from the bench.

When changing the brand of gasoline or, for example, increasing air humidity, an experienced chainsaw user will always adjust the quality of the mixture for greater performance and less wear on the piston part. But this, we repeat, requires a lot of experience with a specific chainsaw and knowledge of the features of its behavior. In addition, the adjustment is incredibly fine – the screws are tightened by only 1/10 … 1/20 of a turn.

Another reason for adjustment is the repair or replacement of the carburetor. In any case, setting the quality of the mixture should be carried out only if the serviceability of the carburetor is reliably known (there is no debris, the gaskets are not poisoned), the air filter is clean, the plug, clutch and ignition system are in perfect order. Otherwise, there is a high probability that the malfunction will be eliminated at one point and the adjustment will suddenly become incorrect, and the engine will start to work for wear.

Main adjusting screws

There are only three bodies for adjusting the carburetor; these are screws shunted by springs for a flat screwdriver. At the bottom is a pair of screws, on the left is L, on the right is H. Just above is the T (or S) screw. So, this is what they are for:

1. Screw L – this is the same valve that regulates the cross-section of the fuel channel when operating at low speeds.
2. Screw H, by analogy with the previous one, is a channel flow regulator for high speed.
3. The T screw is the throttle control. It detects its position when the accelerator lever is fully released..

The design of the first two adjusting screws is extremely simple: at their end there are needles inserted into variable cross-section spools (cones). They all have right-hand threads, so turning the screw clockwise will drive the needle deeper and reduce the flow. When rotated counterclockwise, the opposite happens.

Thus, the quality of the mixture is regulated in different modes of operation. If the fuel content increases, gasoline does not have time to burn completely and the engine develops less speed. On the other hand, when the mixture is leaner, the speed increases, and at the same time the engine temperature rises. It should be noted that even when the screws are tightened all the way, the duct of both channels does not completely overlap, unless there is a blockage.

Regulation rules

The carburetor setting diagram and the recommended position of the adjusting screws are indicated in the instructions for each specific tool. When getting to know a new saw, it is recommended to tighten the screws all the way, counting the number of revolutions, so that later you can restore the factory settings. The general principles of adjustment are as follows:

1. Before adjusting, the engine must be warmed up at idle speed for 10 minutes.
2. Turning the screw L it is necessary to achieve such a quality of the mixture that at idle speed the engine runs uniformly at 1500–2000 rpm. In this case, the acceleration (acceleration) when pressing the accelerator lever should be fast, but uniform. A dip in the set of revolutions indicates a too lean mixture, the screw must be unscrewed until this phenomenon disappears.
3. After adjusting the mixture for low rpm, screw T is tightened until the chain starts to rotate, and then unscrewed 1/2 … 1/3 turn. It is important to monitor the serviceability of the clutch mechanism..
4. Screw H in artisanal conditions is not adjustable.

The danger of self-adjusting screw H is that too lean mixture at high speeds, and especially under load, leads to rapid piston wear and malfunctions in the ignition system. It is possible to crawl to the adjustment of high speeds only if three conditions are met:

1. The chainsaw obviously does not produce the required power when sawing.
2. The person performing the tuning has a general knowledge of the structure of carburetor engines.
3. A means of measuring engine speed is available.

The latter can be either a tachometer or a multimeter with a built-in oscilloscope. To connect a multimeter, wind 3-4 turns of thin copper wire around the spark plug wire.

The main task of the H screw is to compensate for the excess or underestimation of speed due to a change in fuel type, oil concentration or air composition. Insufficient precision of the low speed screw adjustment can also knock down the “upper” setting. The adjustment scheme is simple – the screw is tightened or unscrewed until the saw motor at maximum gas produces 14.5-15 thousand rpm or the value specified in the operating instructions.

If you are using an oscilloscope, its reading (spark frequency) should be in the region of 230-250 Hz. Take into account the accuracy class of the measuring instruments and the fact that under load the speed sags by 10-15%. It is better if the engine runs at a lower speed than at a higher speed, even if only slightly.

Elimination of malfunctions

As a conclusion, we describe typical malfunctions associated with improper carburetor adjustment and how to fix them:

1. The engine will not start or will start and stall after a few seconds. Obviously, the reason is the improper adjustment of the mixture quality at low speeds, it is too poor. With a working carburetor, you need to slightly enrich the mixture by unscrewing the screw L by 1/2 turn, and then complete the adjustment with the screw T.
2. The engine loses power noticeably under load. In this case, the carburetor produces too rich mixture at high speeds. Screw H needs to be tightened 1/8 of a turn, if the engine load is still not sufficiently supported, but progress is noticeable – tighten a little more.
3. The engine “sings” at maximum acceleration without load. The mixture at high rpm is too lean, the rpm is too high.
4. Engine stalls if gas is applied after idling. Too poor or rich mixture, or rather, by the state of the candle.

Please note that high vibration can cause the carburetor settings to become confused. For this reason, a well-chosen adjustment must be remembered by counting the turns and the position of each adjusting screw. If you have determined the best type of gasoline and oil ratio, save a small portion of this mixture so that in case of malfunction, you can check if the malfunctions are related to the quality of fuel and improper adjustment, or if the cause is a malfunction of the chainsaw..