Chapter 2: Basic Troubleshooting
17
Leak-down test
A leak-down test is the preferred method to test the
engine’s ability to compress the charge. It will also
show where pressure is leaking from.
To perform a leak-down test:
NOTE: A leak down test pressurizes the com-
bustion chamber with an external air source and
will allow the technician to listen for air “leaking “
at the valves, piston rings and the head gasket.
NOTE: These are general instructions. Read
and follow the instructions that came with the
tester before attempting to perform this test.
• If possible, run the engine for 3-5 minutes to
warm up the engine.
• Remove the spark plug and air filter.
• Find top dead center of the compression stroke.
CAUTION: If the engine is not centered at top
dead center, the engine will rotate when com-
pressed air is introduce to the combustion cham-
ber.
1. Find top dead center by following the steps
described in the valve lash section of Chapter 1:
Introduction
2. Thread the leak down tester adapter into the
spark plug hole. See Figure 2.1.
Figure 2.1
Leak-down tester
3. Connect tester to compressed air.
4. Adjust the regulator knob until the needle on the
gauge is in the yellow or set area of the gauge.
5. Connect the tester to the adapter.
NOTE: If the engine rotates it was not at top
dead center.
6. Check the reading on the gauge.
7. Compare the results to the following chart.
See Table 1.
Table 1:
Symptom Possible cause
Air escaping from
the breather
Worn cylinder or pis-
ton rings. Possible
blown head gasket
Air escaping from
the exhaust
Leaking exhaust valve
Air escaping from
the carburetor
Leaking intake valve
Gauge reading
low
Cylinder and piston
rigs are in good condi-
tion
Gauge reading
moderate
There is some wear in
the engine, but it is still
usable
Gauge reading
high
excessive wear of cyl-
inder and/or piston
rings. Engine should
be short blocked or it
could be a blown head
gasket.
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