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:
- Remove the
exhaust pipe, and install the exhaust port adapter plug.
- 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.
- Pump up the
pressure checker to make sure you have all the plugs installed properly. The plugs should
not leak!!
- 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
- Pump the
tester up to the recommended gauge pressure.
- Spray the
soapy water solution around the intake manifold first.
- Next, check
the cylinder and head gasket.
- Check the
cases next, and pay close attention to where the cases join.
- 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.
- 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!
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