How-To: Perform A Leak Down Test On A Motorcycle

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In this post I want to cover leak down testing and when it should be performed. If you want to build your own leak down tester, check out this post, which covers how to make one from parts you may already have. The engine in the video features a CB550 engine, check out the top end rebuild here.

 

Why Or When Should I Perform A Leak Down Test?

A leak down test should be performed after taking a compression test with low results, or a motorcycle that is equipped with a decompression mechanism. The decompression mechanism will yield lower results, so a leak down may be the only way to judge the condition of the engine. Also, it wouldn’t be a bad idea to take periodic leak down tests to simply check the engine’s health.

The Leak Down Formula

When I first made my leak down tester, and started testing engines, I started with the regulator set at 100psi. While this makes for easy math, it creates the problem of the motor rolling over from TDC due to the high pressure. This was especially true with single cylinder engines.

Since leak down rate is calculated as a percentage, I simply start with a lower regulator pressure. This eliminates the problem of the motor rolling over. This really helps me out since I’m usually alone in the garage. In the video above, I ended up with 22psi (pressure jumped 2psi once regulator was locked), and used the equation below to find percent leakage.

leak down test formula

So using the formula above, my initial pressure was 22psi, and at some point 16psi during testing. So the leakage rate is ((22-16)/22) X 100% = 27.27%.

 

Slightly Modified Leak Down Tester

Since I’ve posted on how to build the DIY tester, I have swapped out the gauge for a lower psi range unit. I went to a big box store plumbing section and found a 0-30psi gauge, which works perfect.

motorcycle leak down test how to

 

Where Can Leakage Occur?

When the engine is at TDC, the piston is at the top, valves closed, and pretty much no where for air to escape in a healthy engine. Here are areas to check when there is more than 5-10% leakage:

  • Intake valve. Listen for air leak or spray soapy water in port.
  • Exhaust valve. Listen for air leak or spray soapy water in port.
  • Head gasket. For liquid cooled models, open radiator cap and look for air bubbles rising. Spray exterior of engine where cylinder and head meet to look for leak.
  • Piston rings. Open oil filler cap and listen for air leaks.
  • Spark plug sealing gasket. Spray soapy water and look for leaks.

 

So What Is Considered Acceptable Leakage?

  • Leakage under 5% indicates engine in excellent condition.
  • Leakage up to 15% indicates engine in good condition.
  • Leakage above 15% indicates that engine components are worn.

 

How-To Perform A Motorcycle Leak Down Test

The process to perform this test is really easy. The engine should be COLD during testing.

    1. Bring engine to top dead center (TDC). Refer to your service manual for this procedure.
    2. Remove spark plug(s).
    3. Remove Schrader valve from compression tester hose and insert into plug hole.
    4. Hook up leak down tester to 40psi shop air supply.
    5. Regulate leak down tester pressure to 20psi.
    6. Connect compression hose to leak tester.
    7. Record pressure on leak down tester.
    8. Use a mechanics stethoscope to listen for leaks in air intake, exhaust, and crankcase. Open the oil fill cap and listen for air leaking past piston rings. Look under radiator cap for air bubbles indicating a bad head gasket.
    9. Use the formula below to calculate leakage rate %.

leak down test formula

Carbon Buildup

As demonstrated in the video, some engines may develop carbon buildup on the valves, and as a result have lower compression and larger leakage rates. During leak down testing, I took a piece of wood and lightly tapped on the exhaust valve. Immediately, the leakage rate decreased, indicating that the carbon was knocked loose and the valve was sealing better. Now this engine had to be taken apart to replace all the gaskets anyway, but feel free to experiment with products on the market that claim to remove carbon deposits, as they may get leakage back to an acceptable rate. I plan to review some of these products in the future.

 

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