How to Test Compression On a Chainsaw

How to Test Compression On a Chainsaw

As you know, the compression ratio in the engine allows you to get some information about the state of the motor without disassembling and troubleshooting the power unit. Compression measurement is a diagnostic operation, which conditionally allows you to evaluate the "tightness" of the cylinder and the compression efficiency of the mixture.

Next, we will talk about what methods allow measuring compression by cylinders, what malfunctions are indicated by a decrease in compression, and also how to determine compression in an engine without a device, that is, perform the necessary diagnostics in the field if necessary.

Engine compression: what it is and what deviations from the norm lead to

As for the compression itself, without going into details, this is the pressure in the cylinder. The indicated pressure directly depends on the degree of compression in a particular motor. To measure compression, the maximum air pressure at the end of the compression stroke is measured.

If we talk about the problems of internal combustion engines, a decrease in compression leads to the fact that the fuel-air mixture in the cylinder will not be compressed enough for normal combustion of the charge. As a result, the power unit can start up with great difficulty, work unstable, triple (one cylinder or several do not work or work intermittently). There is also a decrease in power, increased fuel consumption, etc.

Compression measurement: without a compressometer or using special equipment

So, in practice, only two methods are actively used that allow you to make compression measurements:

Naturally, the availability of the device allows you to get the most accurate readings, however, in emergency cases or to obtain approximate estimated results, the decision to measure the compression manually is also practiced.

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Immediately, we note that to measure compression without instruments, certain skills and knowledge will be required, while the operation does not pose any great difficulties, even for beginner motorists.

  1. First you need to unscrew the spark plugs, leaving only the spark plug in the first cylinder.
  2. Then the crankshaft must be turned so that the piston in the first cylinder is at the end of the compression stroke.
  3. To accurately determine that the piston is in the correct position, you need to check the marks.
  4. Next, you should remember approximately how hard the crankshaft rotated by hand.
  5. After that, the candle is screwed into the second cylinder and so on.

Note that although the method is quite primitive, however, in some cases even it allows you to get a general idea and identify a problem in a particular engine cylinder. accurate measurements can only be performed with a compressor.

  1. First of all, you need to study the technical documentation in relation to the specific type of internal combustion engine installed on the machine.
  2. Having determined the specific value of compression for the engine, it is necessary to start the engine and warm it up to the operating temperature (the cooling fan is turned on).
  3. Then the spark plugs are carefully twisted out. Next, you need to invite an assistant who will squeeze the gas pedal (an important condition for measuring compression) and crank the engine with a starter.
  4. If we talk about the device, the compressor has a special tip that fits snugly into the candle hole.
  5. After that, the engine scrolls with the starter for about 3-4 seconds. It is also important to consider that it is necessary to measure compression provided that the battery is charged and the starter itself is fully operational. The fact is that insufficient crankshaft speed can lead to a noticeable decrease in performance, however, this will not mean that compression has fallen so much in the engine.
  6. Then, when the readings were obtained, they need to be compared with the nominal for a particular engine.
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Video: How to Test Compression On a Chainsaw


It is also important to understand that it is not recommended to continue to operate an engine in which compression is reduced in one or more cylinders. The fact is that the parts in this cylinder will wear out even more, the engine itself will vibrate more and work unstably, unburned fuel will drain excessively into the oil pan, which will cause oil dilution, thereby increasing the overall wear of the internal combustion engine, etc.

Low compression and how to understand what the problem is: CPG, rings or valves

As already mentioned, a decrease in compression can occur for various reasons. In this case, quite often this leads to the occurrence, wear or destruction of the piston rings. To check the rings in place without disassembling the engine, you can use the available method of checking with engine oil. On hand you only need to have a candle key, some oil and a syringe.

  • After the problem cylinder has been identified and the compression readings are fixed, about 20-30 ml should be filled with a syringe through the candle hole. pure oil.
  • Then the hole must be closed and the engine cranked. This is necessary so that the grease spreads along the cylinder.
  • Next, the compression is repeated in this cylinder.

If the compression ratio after oil filling increases slightly, then problems with the valves (for example, burnout of the valve, etc.) are quite possible. Piston rings can also be severely damaged, as well as a cylinder head gasket.

The complete absence of any changes means that the tightness of the combustion chamber is broken due to any problems with the valve mechanism. It is possible that the valves do not fit tightly on the seats, that is, they must be adjusted, ground, etc. Also, cracks and breakdowns in the BC or cylinder head should not be ruled out.

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What is the result

As you can see, the need to measure compression can arise for various reasons. Often this procedure is used as part of a general diagnosis of the internal combustion engine. Measurements are also taken after the engine has been assembled after repair (both major and bulkheads). This is necessary to identify possible defects after assembly.

Also, if necessary, the device can be manufactured independently using a pressure gauge, tubes and various adapters. For this reason (especially owners of cars with solid mileage), it is recommended to have a separate, ready-made instrument for measuring in the car.

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