What compression should be on a chainsaw

Chainsaw muffler – disassembly and cleaning

When the engine runs normally at idle and stalls as the load increases, the muffler may be the cause. The spark arrester becomes clogged with combustion products and prevents normal exhaust. In this case, the muffler must be removed and cleaned from carbon deposits with detergents, and then dried thoroughly. While the muffler is removed, plug the hole in the engine with a clean cloth. Excessive oil in the fuel mixture, poor quality or unsuitable oil for two-stroke engines can cause muffler clogging..

Chain lubrication system

In case of significant (small is considered normal) oil leakage, it is necessary to check the tightness of the connection of the pipes to the pump unions. They can subside or crack. Violation of the tightness of the oil line, except for drips, can lead to insufficient lubrication of the chain. How to measure the compression to check if it is possible where it is in the chainsaw? The pump begins to suck in air, which affects its performance. The violation of the tightness is eliminated by replacing the tubes or sealing them with a sealant.

The most serious failure of the lubrication system is cracks in the oil pump housing. In this case, it will have to be replaced..

Malfunctioning carburetor

If all the actions described above did not lead to success, perhaps the reason for the inoperability of the chainsaw lies in a malfunction of the carburetor.

In this case, it is checked and adjusted. However, if you have no experience in performing such work, then you should find through friends or by an ad of a specialist who knows how to do this. Otherwise, you risk being left without a saw. A carburetor is a device consisting of many parts of a very small size, which, if not skillfully disassembled, can simply fall out and get lost..

A video on how to repair a Stihl MS 250 chainsaw carburetor is presented below
(press triangle to play):

Question: “What is a breather? Where is he in the chainsaw? “

A breather valve, the task of which is to normalize the pressure in the gas tank as the fuel is consumed. Why is it necessary to normalize blood pressure? Since gasoline is produced, and the gas tank cap is sealed, a vacuum is formed in the tank, which means that the pressure on its walls increases significantly. The breather equalizes the pressure in the tank with atmospheric pressure. Located in the gas tank itself. A blockage can cause the chainsaw to start poorly or make it impossible. It will be cleaned very simply – with a needle.

Chainsaw design and general classification of faults

Correct diagnosis is essential for a successful repair. Therefore, in order to determine why the chainsaw failed and, most importantly, how to make it work again, you first need to know the features of its design.

It should be noted that it does not differ in particular complexity, since it includes:

  • engine (two-stroke gasoline);
  • functional part (saw bar and chain);
  • systems that ensure the joint operation of these two units (ignition, filtration, lubrication, etc.), that is, the tool as a whole.

In order to simplify the diagnosis of malfunctions, it is customary to distinguish them into two main categories:

  • engine malfunctions;
  • malfunctions of other parts of the chainsaw.

Small Engine Repair: Compression Test on a 2 Stroke Craftsman Chainsaw

Most often failures occur in the engine systems, however, the rest of the tool, due to intensive use or improper care, can fail at any time. How to determine exactly where the breakdown occurred? Simple logic will help us with this..

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If the problem is in the engine, it will:

  • do not start;
  • deafen;
  • work unstable;
  • overheat;
  • smoke;
  • develop insufficient power.

As for all other malfunctions, as a rule, they manifest themselves with a normally operating motor.

Troubleshooting the fuel system

Fuel cannot enter the cylinder for several reasons:

compression, chainsaw

Chainsaw troubleshooting

Unfortunately, chainsaws, like other units, can fail. Having a little idea of ​​how the saw works, you can fix most of the faults with your own hands. The complexity of the repair is not so much in the work itself, but in the correct determination of the cause of the breakdown.

For example, if the saw will not start, there may be several reasons, so you should not immediately start disassembling it. First, analyze the causes of failure, which are divided into 2 groups:

Troubleshooting Algorithm

If you are sure that you can assemble the chainsaw after disassembly, then the screwdriver is in your hands. To establish the true cause of the problem, it is necessary to analyze the condition and behavior of the saw.

DIY chainsaw repair

All chainsaw malfunctions can be divided into two main groups: engine problems with all its components and systems (ignition, fuel supply system, cylinder-piston group, exhaust system) and malfunctions of other components (chain brake, lubrication system, clutch, tire, etc. .).

Malfunctions of a Chinese chainsaw: an overview of popular ones and ways to solve them

The most popular malfunctions of Chinese chainsaws include:

  • Poor lubrication system performance. Clogged oil channels are cleaned, and if the oil pump fails, then replacement is necessary.
  • Starter rope breaks.
  • Breakage of the plastic starter shaft. To replace it, a regular metal bolt is suitable..

In case of the slightest suspicion of improper operation of the chainsaw, you should immediately make a complete diagnosis of the tool. Only with proper operation and timely maintenance can you count on a long service life of the chainsaw.

How to check the compression on a chainsaw

With a compression level below 8 atmospheres, the engine will not have enough power, which means that the chainsaw will not be able to fully function.

To measure compression you need:

  • Remove the protective cover and unscrew the spark plug.
  • Insert the tip of the compressor into the hole where the candle should be located.
  • Using the starter cord, rotate the piston and memorize the maximum readings of the measuring device.

Engine malfunctions

  • the engine will not start;
  • starts up, but stalls;
  • works unstable;
  • does not develop power, i.e. works normally at idle, but stalls under load.

When repairing a chainsaw with your own hands, it is necessary to act in an exclusionary way – to inspect alternately all possible prerequisites for a malfunction, starting with those whose verification and elimination takes the least time (about the device and operation of a two-stroke motor, see here).

Ignition system. Almost all types of engine problems listed above can be caused by faults in the ignition system. Therefore, when they occur, it makes sense to start troubleshooting with a candle. This is all the more justified since the state of the latter can provide useful information about the operation of the fuel system. So, if a problem occurs, you need to unscrew the candle and inspect it..

The wire is disconnected and the candle is unscrewed with a special key.

If the plug is dry, this means that the problem is most likely not in the ignition system, but in the fuel mixture entering the cylinder. You can wrap the spark plug back and start the fuel system. If the spark plug is heavily splashed with a fuel mixture, then there is an excess of it. This can occur from a misalignment of the carburetor or a violation of the starting rules. The spark plug is wiped and dried, the cylinder is drained by shutting off the fuel supply and operating with the starter to ventilate the combustion chamber and remove excess fuel. Then put the candle in place and repeat the start.

The presence of a large amount of black carbon also indicates a malfunction in the fuel system..

The ratio between oil and gasoline in the mixture may be disturbed, the carburetor is not adjusted or poor-quality oil is used. The candle is washed in gasoline, cleaned with a needle or an awl from carbon deposits, the electrodes are cleaned with a fine sandpaper and installed in place.

In all these cases, it will not be superfluous to check the gap between the spark plug electrodes. Depending on its brand, it should be in the range of 0.5-0.65 mm. It is also necessary to check the spark plug gasket. If it is worn out or damaged, the tightness of the combustion chamber may not be ensured, which leads to a decrease in compression in the cylinder and problems in engine operation..

Read more:  How to Wear a Chain on a Chainsaw

Even if, when checking the spark plug, there is good reason to believe that the problem is in the fuel system, you need to check the spark plug for the presence of a spark. If only to be sure that everything is in order with this. For this:

  • put the ignition cable on the spark plug;
  • using pliers with insulated handles, apply the thread or nut of the candle to the cylinder;
  • pull the starter handle and see if there is a spark or not.

If there is no spark, you just need to replace the spark plug. in general, share the compression – the pressure created in the figure should not be. If the new spark plug does not have a spark either, you need to check the condition of the high-voltage wire for an open circuit and contact with the spark plug..

I periodically monitor the queries of search engines for which visitors come to the site from search engines. For these requests, as well as for the questions asked in the comments, I choose the most frequent ones and answer them in the “Chainsaw: Questions and Answers” section. The previous parts can be found here:
Chainsaw: questions and answers”
“Chainsaw Husqvarna 137e: Questions and Answers.”

So the most popular queries of the last few months:

How to put the piston on a chainsaw correctly

The work on replacing the piston for most models of modern chainsaws is carried out according to the following algorithm:

  • Top and side covers are removed.
  • Unscrew the candle.
  • The handle is removed from the shock absorbers and the handle is detached from the body.
  • The leading sprocket and starter are removed.
  • A piston stopper is installed in the spark plug hole and the nut that fixes the flywheel and clutch is unscrewed.
  • The flywheel, the clutch, the worm drive of the oil pump, located immediately behind the clutch mechanism, is removed.
  • The screws securing the carburetor and air filter are loosened, after which these units are removed together with the engine control lever.
  • The ignition coil is removed, as well as the muffler.
  • The saw is turned upside down and, by unscrewing the screws fastening to the body, the engine is disconnected.
  • The pallet is unrolled and the piston is removed.
  • Since the piston is removed only together with the crankshaft, it is necessary to remove the circlip to disconnect it..
  • The old piston is replaced with a new one, but as carefully as possible. This is due to the high fragility of the compression rings..
  • The crankcase is put in place, and a sealant is used instead of a gasket..

All subsequent actions are carried out according to the reverse algorithm of disassembly.

Gasoline does not flow into the cylinder of the chainsaw

There are several reasons why the fuel mixture stopped flowing into the cylinder:

compression, chainsaw

how to do a COMPRESSION test on a chainsaw or 2 cycle engine

  • Clogged air filter.
  • Carburetor settings went off.
  • The carburetor membrane is no longer integral.
  • The channels through which gasoline is transported are clogged.

How to check and replace the fuel filter

To check the current state of the fuel filter, you must perform the following steps:

  • Remove the fuel hose from the carburetor fitting and direct it to a working container.
  • Press several times on the so-called “swap”.

If the gasoline comes unevenly and jerkily, then it’s time to put in a new filter element. This is done quite simply:

  • The fuel tank cap is unscrewed.
  • Using tweezers or any other device, the filter itself is removed.
  • Having disconnected the old filter, you must put a new one and lower it into the fuel tank.


“Rich” or “lean” fuel mixture will sooner or later cause a breakdown. Perhaps even costly. To prevent this from happening, it will be useful to regularly check the filter for scuffs, as well as for a breakthrough of the filter cloth itself..

Malfunction of undefined category

In some cases, the owners of chainsaws have to solve more complex problems in order to fully restore the functionality of their cutting tool..

compression, chainsaw

When measuring compression with an open damper, the picture is different. A large amount of intake air and an increase in cylinder pressure, of course, contribute to an increase in leakage. However, they are obviously smaller than the air supply. As a result, the compression does not drop so significantly (to approximately 0.8-0.9). Therefore, measuring compression with an open damper is better suited for determining more “gross” engine defects, such as: piston breakage, ring coking, valve burnout, scuffing of the cylinder surface.

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In both measurement methods, it is desirable to take into account the dynamics of the pressure rise – this will help to establish the true nature of the malfunction with a greater probability. So, if at the first stroke the pressure value measured by the compressor is low (0.3-0.4), and during subsequent strokes it rises sharply, this indirectly indicates the wear of the piston rings. In this case, pouring a small amount of oil into the cylinder will immediately increase not only the pressure on the first stroke, but also the compression..

Compression is measured with both open and closed throttle valves. Moreover, each of the methods gives its own results and allows you to determine your defects. So, when the damper is closed, obviously little air will enter the cylinders, so the compression will be low and will be about 0.6-0.8 MPa. Air leaks in this case are comparable to its entry into the cylinder. As a result, compression becomes especially sensitive to leaks – even with small leaks, its value drops several times..

Repair yourself or send to a service center

It should be understood that not every problem can be solved on your own. Not every owner of a chainsaw has a compressor at home or a special device that allows you to create a vacuum and the right pressure, for example, to check the crankcase for leaks. In this case, you simply cannot do without contacting specialized specialists..


In most cases, the following problems are encountered in engine operation:

  • the engine stopped starting;
  • the engine starts, but after a while it stalls;
  • the engine is not able to develop the required power;
  • the engine runs extremely unstable.

What compression should be in a Husqvarna chainsaw

Compression in STIHL MS180 and POULAN 2250 chainsaws.

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Today we will measure the compression. For this purpose, we need – an electric drill with a reverse function and a maximum speed of about 2500-3000 rpm, an automobile compressor, a flexible shaft for transferring rotation from a drill to a chainsaw crankshaft and, in fact, the experimental models of Stihl MS180 and Poulan 2250 chainsaws..

It is worth mentioning that Stihl is absolutely a working saw (it was handed over for repair to replace the starter cord), but Poulan with a deliberately ditched piston.

For accuracy of measurement, carburetors were dismantled on both chainsaws (intake resistance should be as low as possible), the starter covers were removed (in order to be able to put the flexible shaft with the head on “13” on the nuts securing the flywheels), the spark plugs were unscrewed and the shutdown keys in the off position

So. Our “gentleman’s set” # 128578;

We insert the compressor with its shank into the spark plug hole, put the drill with a flexible shaft and head on the flywheel nut. We turn on the email. drill with counterclockwise direction of rotation. 3-4 seconds and the measurement is made.

Result – 0.5 MPa or 5 atm.

Now it’s the turn for the Stihl MS 180. The procedure is the same.

Result – 0.95 MPa or 9.5 atm.

This issue of the mailing list turned out to be neither educational nor explanatory. I would say that in it we stated a fact: at 5 it does not work, at 9.5 it does. Conclusions, as they say, do it yourself!