The main reason to perform a leak-down test is because two-cycle engines are not air-leak proof, and air sometimes enters the engine from places other than where it is supposed to. This results in fuel-to-air mixture changes, lean ratios, and eventual engine seizure. The #1 most common air leak problem areas are rubber intake manifolds. The rubber cracks, or the gasket between the manifold and the cylinder develops a leak. Sometimes the clamp on the carburetor and the manifold is loose. But let's look at finding the problem.

Tools:

To begin with, you'll need a pressure/vacuum Tester kit. Mityvac(tm) makes one, and you can find similar kits available through most OEM motorcycle parts departments, and at most motorcycle dealers. The kits usually come with all the parts and adapters needed to perform the test.

Pre-Test - Install the Test Kit:

1. Remove the exhaust pipe, and install the exhaust port adapter plug.
2. Remove the carburetor, and install the spark plug adapter plug, NOTE: Most test kits apply pressure into the engine via a carburetor manifold adapter. However, some kits utilize the spark plug hole to pressurize the engine. If this is not the case, do not remove the spark plug.
3. Pump up the pressure checker to make sure you have all the plugs installed properly. The plugs should not leak!!
4. Spray a soapy water solution around the test plugs. Look for air bubbles, as you would when testing an inner tube that has a small hole in it. (NOTE: Formula409, Fantastik, and Windex all work well for this step).

Performing the Leak-Down Test

1. Pump the tester up to the recommended gauge pressure.
2. Spray the soapy water solution around the intake manifold first.
3. Next, check the cylinder and head gasket.
4. Check the cases next, and pay close attention to where the cases join.
5. Remove the ignition cover, and spray around the seal. This seal commonly leaks, and causes engine seizure. NOTE: If the primary side seal leaks, the engine will smoke more than normal.
6. If you find a leak, repair it, and then run the test again. Many times, you will locate more than one leak. Making your engine air tight is part of the battle to making it run at it's best, and not break!