If you suspect your chainsaw engine has developed a leak, you can check and confirm this using a pressure/vacuum test. Quite often, the crank seals and O-rings get damaged due to fatigue and develop leaks which may prevent your engine from starting. In this post, I shall take you through the entire testing process while also recommending some useful pressure/vacuum testing tools for you.
Chainsaw pressure test kit:
A pressure/vacuum test helps diagnose and detect leaks at engine seals, exhaust ports, and head gaskets. It also helps determine the engine compression at cold start and check if the pistons are in good working condition. Typically, this compression value should be between 90-140 psi. A pressure test kit for chainsaws contains a manual pressure pump, connecting hoses, blocking plates, and a pressure gauge to measure the reading.
In the following sections, I will comprehensively explain the various tools you may need for the job, the step-by-step testing procedure, and how to perform such a test.
- 1 Why is a Pressure Test Needed?
- 2 Chainsaw Pressure Tester
- 3 Chainsaw Pressure Test Tools
- 4 How to Perform a Pressure/Vacuum Test on your Two-Stroke Engine?
- 5 How do I know if my chainsaw has good compression?
Why is a Pressure Test Needed?
Pressure or a vacuum test can help diagnose possible leakages in various components of your chainsaw. Typically, they are used to detect leakages in crank seals and O-rings. However, they are also effective in testing faulty exhaust ports, diaphragm needles, and fuel lines.
Your chainsaw’s engine and its carburetor need to be airtight. Possible leakage points in an engine may include air inlet/exhaust ports, gaskets, and crank seals. When the air leaks from the engine, the air to fuel ratio gets upset, causing the engine to overrev or stall. In some cases, it may not start at all, depending on how severe the leakage is.
Usually, the crank seals are highly susceptible to wear out and develop leakages. Essentially, it secures the crankshaft and ensures no air leaves the crankcase, thus maintaining the cylinder pressure. When an engine gets overheated or runs at full throttle, its bearings may get damaged and add additional fatigue to the seals and gaskets. The seals can puncture and cause leakage.
A pressure test aims to raise the cylinder pressure so that the air exiting the engine can be detected by suitable means (such as soap bubble formation). The reverse is done in a vacuum test where all the air is withdrawn from the chamber, and any rise in pressure indicates a leakage.
Except for one-way valves, most seals that don’t prevent air from leaking out don’t keep the air from entering across them either. Hence, a failure in a pressure test can also predict a failure in a subsequent vacuum test.
Chainsaw Pressure Tester
We use a pump (usually manual) that serves both functions to perform this test. i.e., creates both pressures as well as vacuum within a container.
1. Mityvac MV8500
From my own and many other user experiences, I can recommend purchasing the specially designed manual test kit for automotive work built by Mityvac.
The pump in this kit serves both pressure and vacuum functions. It also comes with a dial gauge which can be fixed at the pump inlet and indicates the pressure measured. Moreover, this kit also comes with multiple replacement parts such as adaptors, washers, rubber hoses, etc.
Some users have had issues with the pin that locks the piston. But overall, it is the most widely used, tried, and tested kit that serves multiple purposes and is durable compared to many of its counterparts.
2. Lil Red Barn Pressure and Vacuum Tester
The Lil Red Barn pressure tester is a kit designed specifically for engine pressure and vacuum testing. It comes with a standard hand pump, a dial gauge rubber hoses, adaptors, and some replacement parts. I don’t know this type well, and it isn’t as widely used as the Mityvac. But based on several other users, it seems like a decent product.
2. Stihl Pressure and Vacuum Testing Tool
Another such testing kit is developed by the leading machine tool manufacturer Stihl. This kit contains a mechanical hand pump which is much simpler in build. Also, it comes with a dial indicator, adaptors, hoses, and some replacement parts.
Chainsaw Pressure Test Tools
Your pressure testing kit must contain a manual pressure/vac pump, a pressure gauge, rubber hoses of various lengths, multiple adaptors, and blocking plates between the muffler and exhaust.
The abovementioned products contain almost all the tools needed to perform the job. I recommend that instead of engaging a mechanic to do this job for you regularly, you should invest in one of these toolkits. It can be used for pressure/vacuum testing of your engine and various other operations such as carburetor testing, port inspection, and fuel line examination.
If you don’t want to buy a ready-made tester kit, you can create your customized toolkit by getting the parts mentioned below:
1. Pressure/Vac Pump:
A hand-held pump with provisions for pressure and vacuum testing is a must for this job. Before buying one, ensure that it has the option of attaching a dial indicator. A good pump may cost you around 40$. Avoid buying cheaper versions as they break open after some use and aren’t too effective in pumping air.
2. Dial Indicator:
A pressure gauge in the form of a dial is typically provided along with a pump in most cases. If not, you should buy one separately to note the readings each time you perform a test.
Instead of using the ones provided in the testing kits, you can buy adaptors of your own choice. Adaptors can be of spark plug-to-hose, carb-to-hose, or gauge-to-hose types. Each type has its own dimensions, and you can buy based on the build of your chainsaw parts.
Our objective of using adaptors is to ensure connectivity of the rubber hoses with chainsaw components so that each joint is completely airtight.
4. Blocking Plates:
These plates must be inserted between the exhaust port and the muffler to seal any possible leakage sites. You can cut a 10″ x10″ bicycle inner tube as a sealing plate. Or, if convenient, you can create a customized aluminum plate based on your muffler’s dimensions.
5. Rubber Hoses:
Lastly, you should have a set of rubber hoses of various lengths that connect with the adaptors and the pressure pump. Each rubber hose should be at least 15-inches long.
How to Perform a Pressure/Vacuum Test on your Two-Stroke Engine?
To perform a pressure/vacuum test, seal your engine’s inlet and exhaust ports first. Then, connect the pump to the impulse line and pressurize the cylinder. Note the gauge pressure reading. If it drops, there exists a leakage somewhere.
Let’s go into the step-by-step method of a pressure test:
- Disassembly: Before starting the test, you may need to remove some parts, such as the blade, chain, and outer casing that covers the clutch and flywheel. The crank seals and base gaskets are exposed as a result of this.
- Shut the ports: You must now ensure that the pressure in the cylinder has no exit points. Place a rubber blocking plate between the muffler and the cylinder to prevent air from escaping. Similarly, fit the same fitting between the carburetor and the cylinder’s inlet port.
Check that the spark plug is in place and that the decompression valve is shut. Use adaptors to ensure proper connectivity with ports and rubber hoses where necessary.
- Pressurize the cylinder: Connect the Mityvac’s hose to the impulse line at the crankcase. If a suitable adaptor is present, you can attach it to the spark plug hole and connect the Mityvac to it instead.
Continue to pump the Mityvac (or another brand) until the cylinder pressure reads 7-10 psi on the dial. After that, set the pump aside and take note of the dial reading, which should be stable. Furthermore, try rotating the crankshaft and taking note of the pressure reading.
- Use soapy water: Apply the soap water mixture to the crank seals and the base gasket. Ensure to apply enough mixture to form a small puddle at the seals. Check to see if bubbles are continuously forming at the crank seal. This is supported by the pressure reading, which tends to fall.
It’s worth noting that the bubbles will vanish once the cylinder loses pressure and the dial reads zero. The rate at which the pressure diminishes indicates the condition of your crank seals.
A vacuum test achieves the same goal as a pressure test. Some seals and connectors only allow one-way flow through them. This means that air can enter the cylinder but cannot escape due to the seal, and vice versa. As a result, while the pressure test may yield a pass result in some cases, the engine may still leak somewhere.
The Mityvac or other brand tool is set to vacuum during the vacuum test. The pump lever is pulled until the pressure gauge reads 0 psi. For about 5 minutes, leave the cylinder as such. If the pressure does not rise significantly, it indicates that the seals and gaskets are in good condition.
How do I know if my chainsaw has good compression?
A compression testing gauge can be used to check chainsaw compression. The engine pressure should be at least 90 psi and no more than 140 psi.
Remove the spark plug and replace it with the compression testing gauge to check your engine’s compression. Pull your starter cable and record the pressure gauge value until the reading settles down. If the value is between 90psi and 140psi, it indicates that your engine is in good working order and produces adequate compression to power the saw.
If the value is less than 90 psi, perform the pressure/vacuum test described in this article to troubleshoot the fault. A value lower than 70 psi indicates a possible issue with the cylinder walls and piston head. The engine bogs down at a value lower than normal due to insufficient power and fails to maintain its RPMs.