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Chainsaw Pressure Test. Tips from a Professional

A pressure test effectively diagnoses the startup problems if your chainsaw doesn’t start even after multiple repairs. It can help determine and detect any leaks at the chainsaw’s crankcase, resulting in the issue. This article shall describe how to perform the pressure testing procedure for chainsaw engines and diagnose the associated faults without any mechanic’s aid.

Chainsaw pressure test.

A pressure test is commonly used to diagnose a leak in the chainsaw engine’s cylinder assembly, resulting in decreased compression. In this test, the cylinder is pressurized, and the seals and gaskets are checked for bubbles to pinpoint a leak. Usually, the crank seal is highly susceptible to fatigue and is most likely to develop a leakage. Other areas such as head gaskets, and inlet/outlet ports can also be troubleshot for faults in this method.

The following article explains the entire testing procedure and provides a holistic approach for fixing your chainsaw problems independently.

How do I know if my Chainsaw Crank Bearing is Bad?

A damaged chainsaw crank bearing causes radial play at the shaft. The chainsaw may even make metal noises when idling or at full throttle.

Crank bearings assist the rotational motion for shafts. They keep the shaft’s axis-aligned and eliminate any end play, thus ensuring a smooth rotation. A mechanical device like a chainsaw is subjected to considerable wear and tear, which fatigues its bearings and shafts. Over time, these bearings can become ineffective and need to get replaced.

The most obvious sign of a bearing failure is when the shaft exhibits end play. To check this, remove the outer casings to expose the clutch plate of your chainsaw on one side and the flywheel on the other. Bearings support the shaft at both ends.

First, check the clutch bearing by holding the shaft and pulling it up, down, and sideways. If the shaft has play in the radial direction, then the bearing needs replacement. For more accurate measurements, you can use a dial indicator to determine the amount of radial play. Typically, the value needs to be less than 0.002”. Similarly, check the shaft at the other side of the flywheel and determine if that bearing works fine.

Using a chainsaw with a damaged bearing exerts additional fatigue at the shaft and seals. The crank seal may get punctured and cause air to leak out of the chainsaw. The air leakage disrupts the carb setting and hampers the RPMs at a particular throttle setting. It may even cause startup problems for the engine by disturbing the engine’s compression.

How do you Check a Chainsaw Crank Seal?

The crank seal can be checked through a pressure or vacuum test. If air leakage occurs at the seal, then it is compromised and should be replaced at the earliest.

The crank seal secures the crankshaft and prevents the air from leaking in or out of the engine’s cylinder. Furthermore, the seal ensures that engine oil is fully available to lubricate the crankcase, and it doesn’t leak out as well.

To check a chainsaw crank seal, the cylinder is pressurized with air, and soap water are added to the seal. If bubbles form at the crank seal, the air is leaking out of it, and hence it is damaged. Also, perform the vacuum test to ensure if the cylinder holds the vacuum or not. In most cases, seals that fail the pressure test also fail the vacuum test.

The pressure testing procedure shall be explained in the following section.

How do you Test Crank Seals?

To test crank seals, the inlet port of the engine cylinder is connected to a pump and pressurized. This is followed by a spray of soap water at the crank seals. If the seals are damaged, air would leak out which would be indicated by the formation of bubbles.

Pressure/vacuum testing can help test not only faulty crank seals but also carburetor gaskets, exhaust ports, diaphragm needles, and fuel lines. The procedure is fairly simple to perform. All you need is a Mity vac hand pump along with rubber hoses to pressure the cylinder.

● Pressure Test:

The step-by-step procedure is explained as follows:

  • Step 1. Disassembly: Before beginning the test, you may need to disassemble certain parts such as the blade, the chain, and the outer casing which covers the clutch and flywheel. This is done to expose the crank seals and the base gaskets.
  • Step 2. Seal the ports: Now, you need to make sure that there are no escape points for the pressure in the cylinder. For this, insert a rubber strip between the muffler and the cylinder to prevent any air escape. Similarly, perform the same fitting at the inlet port between the carburetor and the cylinder. Ensure that the spark plug is connected and the decompression port is closed.
  • Step 3. Pressurize the cylinder: Connect the impulse line at the crankcase with the mity vac’s hose. Keep pumping the mity vac till the cylinder pressure reads at 7-10 psi at the dial. After that, leave the pump aside and note the dial reading, which should remain constant. Also, try rotating the crankshaft and note the pressure reading.
  • Step 4. Apply soapy water: Apply the soap water mixture at the crank seals and the base gasket. Make sure you apply enough mixture to form a small puddle at the seals. Now, notice if bubbles are formed continuously at the crank seal. This can be further confirmed by the pressure reading that would tend to drop.

Notice that the bubbles would disappear after the cylinder runs out of pressure and the dial gives zero-rating. How fast the pressure runs out gives you an indication of how damaged your crank seals are.

● Vacuum Test:

A vacuum test serves the same objective as the pressure test. Some seals and connectors offer only one-way flow across them. This means that the air might be able to enter the cylinder, but the seal won’t allow it to leave and vice versa. Hence, it may be possible that in some cases, the pressure test might be giving a pass result, yet the engine might still leak somewhere.

In the vacuum test, the mity vac is turned to the vacuum setting. The pump lever is pulled until the dial shows a pressure of 0 psi. Keep the cylinder as it is for about 5 minutes. If the pressure doesn’t increase too significantly, the seals and gaskets are in an acceptable condition.

What causes Low Compression in a Chainsaw?

Low compression can be due to two main factors. It can either be due to a faulty crank seal or head gasket. Or it can be due to scored cylinder walls and damaged piston rings.

Low compression in the engine is characterized by a problematic starting procedure. In most cases, the saw doesn’t start or stalls when started. In some cases, it shows trouble idling or runs erratically. When checked, the fuel lines and spark plug(s) work perfectly. When the cylinder is subjected to a pressure test in the above cases, it is mostly the cause of the problem.

● Damaged Seals:

As explained in the previous sections, a ruptured or damaged seal disturbs the cylinder pressure and affects its normal working. A pressure/vacuum test can further confirm which seals are compromised. After identification, the seals must be replaced promptly so as not to aggravate the problem.

● Expert’s View Low Compression in a Chainsaw:

– Piston Wear:

When the above-performed diagnosis fails to work, you probably need to go to the next level by getting your engine taken apart. This could reveal if the fault lies in the pistons or cylinders. I have experienced that piston rings are at fault and a replacement fixes the issue in most cases. In some cases, the piston skirt gets worn out due to the continuous rocking of the piston against the cylinder walls.

– Cylinder Walls Scoring:

Cylinder walls scoring occurs when the tolerances between the piston and cylinder are high. As a result, the piston rocks sideways. Another common issue I’ve seen people face is cylinder wall scoring. This results from an inadequate piston to cylinder lubrication. I’ve seen that most people are unaware of the fuel and oil separation due to ethanol in fuel.

The oil gets separated from the fuel before entering into the engine. As a result, the increased friction causes scoring of the cylinder walls and ends up hampering the engine performance. Hence, if you can manage, prefer ethanol-free fuel, especially for small two-stroke engines.

What lubricates the crank on a chainsaw?

In almost all gas chainsaw engines, the crank is lubricated by the oil mixed in the fuel. Unlike four-stroke engines, which house the lubricant in the crankcase, these two-stroke engines utilize fuel premixed with the lube oil in an appropriate ratio (typically 50:1).

The fuel to oil mixture is also known as a premix. To prepare the fuel-oil mix, you should take a separate container, ensure it is clean, and pour the fuel into it. Set the premix ratio as per your owner’s manual. The most commonly used value is 50:1. You may need a lower mixing ratio (more oil) for older engines such as 30:1.

Afterward, measure the appropriate amount of oil to be added to the fuel to obtain the desired proportion. Finally, add the premix into your gas tank and start your engine again. In four-stroke engines, the engine oil is added separately in the crankcase. Please ensure that you always choose the engine oils of the SAE-30 class. Synthetic oils which provide a multi-grade performance should be preferred over single-grade lubricants.

How much compression should my chainsaw have?

The recommended value for chainsaw compression lies between 90 and 110 psi. If the value is lower than 90, a crank seal is faulty. A value less than 70 indicates damage to either the piston or the cylinder walls.

Chainsaw compression impacts the engine power directly. When the piston reaches the top dead center, the air fuel is compressed to a high temperature and pressure. The pressurized mixture delivers power in the next stroke by pushing the piston to the bottom dead center. The pressure built in the compression stroke thus controls the power delivered in the power stroke.

This compression is affected when the engine has a leak or when the piston or cylinder walls get damaged. Hence, the engine shows difficulty starting or maintaining its RPMs while running.

How do I know if my chainsaw has good compression?

Chainsaw compression can be checked utilizing a compression testing gauge. The pressure value in the engine should be at least 90 psi and at most 140 psi.

To check your engine’s compression, remove the spark plug and attach the compression testing gauge there instead. Next, pull your starter cable and note the pressure gauge value until it becomes stable. If the value lies between 90psi-140 psi, it shows that your engine is healthy and produces enough compression to power the saw. If the value is less than 90 psi, you should perform the pressure/vacuum test as explained in this article to troubleshoot the fault.