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How to do a Motorcycle Leak Down Test

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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. 

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