Why perform a leak down test?
A leak down test is performed after low and/or varying results (more than 10%) from a compression test, or to get an idea of the measurable amount of leakage a motorcycle engine may have. A leak down test may be your only option of assessing engine condition if your motorcycle has an automatic de-compression valve or mechanism. Most modern 4 stroke motocross bikes have this feature to ease starting.
What does a leak down test do?
A leak down test will pressurize the engine through the spark plug hole while at top dead center (TDC). When at TDC, all intake and exhaust valves will be closed, and the piston will be at the top of its stroke. The only way air pressure can escape is through the valves, piston rings, and head gasket.
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: DIY Leak down tester
The easiest and cheapest leak down tester may already be in your toolbox. You can take the hose from your compression tester and simply remove the Schrader valve from one end. The disadvantage with this method is that you can’t assign a value or percentage of leak in the system since no gauge is present.
I’ve decided to make my own leak down tester since some units can run well over $100. Below is a picture and video of how to make one. Everything can be purchased locally and will take you around 30min to build.
The key to the leak down tester is to make a restriction between the pressure regulator and pressure gauge. I used 5min epoxy and plugged the brass nipple about ¼-1/2” deep. After the epoxy dries, use a .040” (#60 drill bit) drill bit and drill through the epoxy.
You can also use the pressure gauge from your compression tester on a quick connect to save a bit of money. You should have a compression tester anyway if you have moved onto leak down testing.
How-To perform a motorcycle leak down test?
The process to perform this test is really easy. The engine should be COLD during testing.
- Bring engine to top dead center (TDC). Refer to your service manual for this procedure.
- Put motorcycle in high gear and have a helper apply rear brake.
- Remove spark plug(s). Remove Schrader valve from compression tester hose and insert into plug hole.
- Hook up leak down tester to 120psi air supply. Regulate leak down tester pressure to 100psi.
- Connect compression hose to leak tester. Record pressure on leak down tester.
- Use a mechanics stethoscope to listen for leaks in air intake, exhaust, and crankcase. Look under radiator cap for air bubbles indicating a bad head gasket.
NOTE: Do not have any hand tools attached to engine as it may rotate under pressure.
Leakage will be 100psi minus recorded pressure. For example, let’s say we get 90psi, 100psi -90psi equals 10psi, or 10% leakage.