Do You Know The Leak down Test?

In the beginning, we should know the reason why we should perform a leak down test? Actually, a leak-down test is performed after low and/or varying results which should be more than 10% from a compression test. Or you can 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 decompression valve, motorcycle air valve, or mechanism. As we all know, most modern 4 stroke motocross bikes have this feature.

Then, we should know what a leak-down test is. To be concrete, a leak-down test will pressurize the engine through the spark plug hole while at the top dead center (TDC). During the TDC period, all intake and motorcycle 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 motorbike valves, piston rings, and head gasket.

So do you know the standards about what is considered acceptable leakage? Generally speaking, leakage under 5% indicates that the engine is in excellent condition. Leakage up to 15% indicates that the engine is in good condition. However, if the leakage is above 15%, it indicates that engine components are worn and old.

To be frank, you can get the easiest and cheapest leak-down tester in your toolbox as you can take the hose from your compression tester and simply remove the valve like the bike fuel valve from one end. The disadvantage with this method is that people can’t assign a value or percentage of a leak in the system as no gauge is present.

Now, we should understand how to build the motorcycle leak-down tester. Without any doubt, the key to the leak-down tester is to restrict the pressure regulator and pressure gauge of your motorcycle. What’s more, you can also use the pressure gauge from your compression tester on a quick connect to save money as this way is cheaper than another. You would better own a compression tester in case you may need a leak-down test one day. 

How to perform a motorcycle leak-down test is a really very important and complex question for many people. Hence, the process to perform the motorcycle leak-down test is really easy. You should keep in mind that the engine should be cold as well as motorcycle inner tubes during testing to avoid any emergencies.

You should bring the motorcycle engine to the top dead center and refer to your service manual for this procedure. Then, you need to put the motorcycle in high gear and have a rear arashi brake. You also need to remove spark plugs as well as valves such as motorcycle wheel valves from the compression tester hose and insert them into the plughole. You need to connect the compression hose to the leak tester and record pressure on the leak down tester. It is better for you to use a mechanics stethoscope to listen for leaks in the air intake, exhaust, and crankcase. Finally, you should make sure your motorcycle is in a good condition such as a good aluminum motorcycle ramp so as to smoothly finish this test.