Why a Chainsaw Spontaneously Gains Speed

Chainsaw owners sometimes have to deal with the fact that the saw, for no apparent reason, stops working normally. To understand why the chainsaw stalls under various operating modes, and also describe how to diagnose a malfunction, we will try in our article.

Why chainsaw stalls when heated

The situation in which a chainsaw starts up in the cold, and during heating can stall, is familiar to many who deal with them. The main reasons for this saw behavior are as follows.

  • Faulty ignition module (coil). You can determine the malfunction by checking the spark first on a cold engine, and then on a hot one. If there is no spark on a hot engine, then you should try to fix the problem by replacing the ignition coil.
    Why a Chainsaw Spontaneously Gains Speed
  • The chainsaw carburetor has the wrong setting (pours a candle on a hot one). You can identify the problem by the way the saw starts up in the cold. If it is not necessary to close the air damper for a cold start, and the engine starts without a preliminary flash, then the fuel mixture is not formed correctly and after heating the engine, the fuel fills the candle and makes starting impossible. The problem is fixed by adjusting the carburetor.
  • Defective spark plug. In such a situation, similarly to a faulty ignition coil, a spark disappears when the engine is heated. It is worth trying to replace the spark plug with a new one, and if this does not help, then you need to check the ignition coil and the gap between it and the flywheel.
    Why a Chainsaw Spontaneously Gains Speed

You can check the ignition coil with a multimeter or by replacing it with a known working one. The multimeter checks the primary and secondary windings in the resistance measurement mode. The resistance index of the primary winding, the working coil, is 1 kOhm. secondary 4 kOhm.

Why the chainsaw stalls under load

In some cases, the chainsaw under load is unable to work normally and simply stalls. A common reason for this behavior is incorrect carburetor adjustment, or air leaks through the crankshaft oil seals or carburetor gasket. And also this behavior can occur due to leaking carburetor.

Diagnose the cause and eliminate the fact that the chainsaw starts and stalls under load, it is best in the service center, because a special tool is needed to check the tightness of the carburetor and crankcase.

The saw, unable to work under load, will also hesitate to maintain idle speed. They will "swim." Diagnostics begins with a leak test of the crankcase and carburetor. If air leakage is detected through the crankshaft oil seals, they must be replaced. If air leakage is detected in the carburetor, the cause can be eliminated by installing a repair gasket kit.

Why a Chainsaw Spontaneously Gains Speed

Only after the above actions have failed and the engine crankcase with the carburetor are tight, you can proceed to adjust the carburetor.

The reason for this algorithm is as follows: if the air intake from the crankcase has not been eliminated, and the carburetor has simply been adjusted before the engine is operating normally, the fuel mixture will contain more fuel than is required for a sealed crankcase, which in turn will significantly increase its consumption. Also, most likely, carbon deposits will appear in the engine, which will also adversely affect the further operation of the chainsaw.

In some models of chainsaws, to ensure the operation of the fuel pump, special hoses are installed that transmit air impulse from the crankcase to the carburetor fuel pump. If this hose is frayed, the saw will idle and will not work under load at all.

Video: Why a Chainsaw Spontaneously Gains Speed


Owners of household chainsaws HUSQVARNA 137 and HUSQVARNA 142 for any abnormal engine operation should first check the impulse hose, since this is considered the most common cause of failure of these chainsaws.

A video showing how to replace a defective impulse hose with your own hands on a HUSQVARNA 142 chainsaw can be seen below. It steps through the process of disassembling the saw, shows the impulse hose and stipulates the consequences of its malfunctioning HUSQVARNA 142.

Why the chainsaw is not gaining momentum and stalls

If the chainsaw, one minute after start-up, is not capable of gaining speed or stalls when the trigger is pulled, the carburetor or engine crankcase is most likely leaky. As in the previous case, the problem can be solved by checking for leaks and adjusting the carburetor.

If the chainsaw does not gain momentum and stalls after it has worked normally for about 5 minutes, the cause may be a defective fuel tank breather. The vacuum created in the fuel tank during the operation of the tool does not allow the engine to receive the necessary amount of fuel for its normal operation, and for this reason it will be unable to gain speed or stall. At the same time, if you stop the tool, open and close the fuel tank cap and start the chainsaw again, the tool will again work normally for about five minutes, after which it will stop gaining speed or stall.

The reason that the chainsaw is not gaining momentum may be excessive soot in the muffler, which makes it difficult to exhaust the exhaust gas, thereby reducing engine power and speed.

Why a Chainsaw Spontaneously Gains Speed

If you wish, you need to check the tightness of the crankcase, but the lack of a special tool, you can use the following test procedure.

Crankcase leak check algorithm

  • Remove the side cover and tire from the chainsaw.
  • Unscrew the spark plug and lay the cord with the knots in the cylinder through the spark plug hole, thereby blocking the movement of the piston and crankshaft.
  • As a lace with knots, you can use the old cord from the starter, having tied knots on it after 3-5 cm.
  • Unscrew the drive sprocket for the chainsaw.
  • Remove the oil pump cover and the pump itself.
  • Remove the starter from the chainsaw.
  • Unscrew and remove the flywheel.
  • Remove the carburetor.
  • Using a syringe, pour gasoline into the crankcase through the hole for the carburetor in the cylinder, while observing the crankshaft oil seals. If gasoline flows through them, then the oil seals have become unusable and must be replaced with new ones.
  • It is better to entrust the installation of oil seals to professionals.

An indirect sign of crankcase leaks are oil smudges in the oil seal area.

Why a Chainsaw Spontaneously Gains Speed

Why the chainsaw starts and immediately stalls

The chainsaw starts and stalls immediately, the reason for this behavior may be hidden in the carburetor. A malfunction of the fuel pump or a malfunction of the main fuel jet can cause this behavior of the saw. The inability to operate the chainsaw after startup requires a complete diagnosis at the service center.

All of the above options for improper engine operation have similar causes, which, depending on their neglect, can have different manifestations. For example, with a slight suction of air into the engine crankcase, its operation will not change much, the engine power will drop slightly, its maximum speed will increase, and it will be able to warm up a bit more. In the middle stages of air leakage, the engine will stop idling and will become very hot during operation. A strong suction of air will make the saw impossible to work, it will start and immediately stall.

In parallel with the visible manifestations of malfunctions, for example, the inoperability of the chainsaw after starting, inside the engine at the time of its malfunction, there will be processes that irreversibly lead the saw to the most serious consequences, for example, fusion of the piston, jamming of the crankshaft.

Failures leading to malfunction of the chainsaw

There are a number of malfunctions that must be taken into account in the diagnosis, because they can lead to interruptions in the saw.

  1. Worn crankshaft bearings. With worn crankshaft bearings, a beating on the main shaft where the flywheel is mounted is inevitable. In such a situation, it is impossible to adjust the gap between the flywheel and the ignition coil, which entails interruptions in spark formation, and unstable operation of the chainsaw.
  2. Impulse channel pollution. The pulse channel serves to ensure the operation of the fuel pump; accordingly, its pollution leads to improper operation of the pump, which will make the saw impossible to work (it will start and immediately stall).
  3. CPG wear. As a rule, with increased wear of the CPG, a decrease in compression in the chainsaw engine is observed, which will necessarily affect its ability to develop speed.
  4. Pollution of fuel and air filters.

To summarize

When the first signs of malfunction of the chainsaw appear, it is worthwhile to conduct a complete diagnosis. In order to avoid damage to the saw due to improper maintenance, it is better to entrust the work to identify and troubleshoot engine malfunctions to professionals. And most importantly, you do not need to use a chainsaw with minor defects, because this will lead to even greater problems.

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