If you get low readings or variable readings from a motorcycle’s compression test, you can perform a leak down test to further diagnose the bike’s potential compression issues.
Leak-down tests can also be used for motorcycles that feature decompression mechanisms (2-strokes).
What is a Leak Down Test?
A leak down test involves pressurizing the cylinders while the engine is a top dead center (TDC).
In this position all the valves will be closed, and the only places where pressure can escape is through:
- The valve seats.
- The piston rings.
- The head gasket.
How to do a Leak Down Test
To perform a leak down test, you’ll need:
- An air compressor.
- A leak down tester, which consists of a pressure regulator and pressure reading gauges.
Leak down tests should be performed when the motorcycle engine is cold.
To perform the leak down test:
- Bring the cylinder you’re testing to TDC position.
- Remove the spark plug.
- Hook up the air pressure supply and the leak test unit to the spark plug hole.
- Regulate the air pressure to 100 PSI.
- Listen and observe for any leaks.
- Record the pressure.
Leakage results can be measured as a percentage of: supplied air pressure – recorded air pressure, divided by supplied air pressure.
Leak Down Test Results
Results less than 5% leakage indicate that your engine is in near perfect condition.
Results up to 10-15% should still be good and serviceable.
Leakage percentage above 15% indicates that components are likely worn past serviceable limits. Unfortunately, your job just got a little bigger.
If you’ve recently rebuilt the engine, make sure you’ve torqued down the cylinder and head to the proper values in the proper sequence.