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 motocross bikes have this feature to ease starting.
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.
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.
The process to perform this test is really easy. The engine should be COLD during testing.
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.