2-Stroke Pressure/Vacuum Leak-Down Test Information - Honda Motorcycle Service Manual

ENGINE TESTING
Squirting a little soapy water around the cylinder and head mating area will tell you if the head gasket is leaking to the outside
atmosphere. Checking for bubbles in the cooling system of a liquid-cooled machine will tell you whether or not the head
gasket is leaking into the cooling passages. The only thing this test won't tell you is the difference between a head gasket leak
into the adjacent cam chain (or gear) well, and a leak past the piston rings.
Be sure to follow the leak-down test tool manufacturer's instructions precisely when making this inspection
2-STROKE PRESSUREIVACUUM LEAK-DOWN TEST INFORMATION
Regular crankcase leak-down testing is much more important to the lifespan of a 2-stroke engine than a 4-stroke. Because the
engine relies on a very precise airlfuel mixture to ensure proper engine lubrication, the slightest air leak can lead to an engine
seizure. Consider these regularly scheduled tests as cheap insurance.
Pressurelvacuum tests on 2-stroke engines should always include both a pressure and a vacuum tests, both or which are per-
formed with essentially the same equipment.
Pressurelvacuum leak-down test equipment, specifically designed for 2-stroke engines, is commercially available through
various motorcycle and general tool sources.
A pressure/vacuum leak-down test tool consists of hand pressure/vacuum pump and various adaptors to seal your engine.
The test provides a clear indication of where a leak, or leaks, exist. Possible areas for leaks include anywhere downstream
of the carburetor until the mixture is ignited and forced out the exharst. Leaks can occur between the mating surfaces of
the crankcases
if
the gasket fails, If this gasket fails between the crankcase and the transmission, the mixture will become
much richer as transmission oil is slowly drawn into the engine. Similarly, a leaking crankshaft seal on the transmission
primary gear side will also consume transmission oil. Other air leaks include the cylinder base gasket, the magneto side
crankshaft seal, leaks between the reed valve assembly and its gaskets, and leaks in the carburetor mounting boot between
the carb and the reed valve.
Although some cylinders are more difficult t o seal for leakage down testing, such as those equipped with H.P.P. valves. the
general procedure that follows can be used on most two-stroke engines.
The first step in the testing prodecure is to remove the exhaust and to effectively seal the exhaust p o r t This is done by bolting
a piece of inner tube between the exhaust manifold and the exhaust port, with a plate made specifically t o bolt over the
exhaust port, backed by a rubber seat, or with some form of expandable rubber plug. NEXT the carburetor is removed and
a plug is clamped snugly in place where the carburetor was. This leaves only the seals and gaskets to show any defects they
may have. Then an attachment is inserted into the spark plug hole and pressure applied with a hand pump. Often a brake
bleeder pressure/vacuum tool is used for this purpose.
Spraying soapy water around the inlet tract, reed valve and crankcase mating areas will produce bubbles where there are
leaks.
The vacuum portion of the test ensures that the negative sealing characteristics of the crankshaft seals are adequate.
Be sure to follow the leak-down test tool manufacturer's instructions precisely when making this inspection
Date of Issue: March, 1995
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