Put the cylinder to be tested at TDC. (Top Dead Center). A good tool for doing this is Jay Smith's TDC tool. Clamp Flywheel in place or use a suitible tool to keep it from spinning.(Four-cycle engines must be on the compression stroke).
Screw the hose into the spark plug hole of the cylinder to be tested. Be sure that it is tight enough to completely seal. Leakage here will give you a false reading other than the true leakdown.
Connect the hose to the tester and to your air supply. Ideally air supply should be between 110 and 125 psi (150 psi MAX) and be clean and moisture free.
Adjust the regulator until the regulated pressure (left) gauge reads 100 psi. (See Note A below if you air supply cannot maintain 100 psi on the regulated pressure (left) gauge.)
Adjust regulator to approximately 10 to 20 PSI. Turn engine over as follows. When testing a V-4, V-6, V-8 Outboard motor Turn Clockwise for Port side, Counter Clockwise for Starboard side. When you feel the tension become less as the piston nears TDC. At this point, increase the air pressure to 100 PSI. Be careful not to go past TDC or the engine will turn over.
At this point, the percentage of leakdown (right) gauge will display the leakdown as a percentage. For example 92 psi on the percentage of leakdown (right) gauge = 92% of regulated pressure or 8% leakage (100 - 92 = 8). See the chart below for leakage percentage. If you get an extremely low leakdown percentage and the motor seems to be running well check for a blockage in the precision restriction between the gauges.
For more detailed testing, slowly allow the crankshaft to rotate in the direction listed above. Watch percentage of leakdown (right) gauge for Leakdown readings as the piston travels down in the cylinder. This will test the entire length of the stroke. On two-cycle engines, you will see 100% leakdown when the top ring meets the exhaust port. On four-cycle engines, you will see 100 % leakdown when an intake or exhaust valve begins to open.
WARNING: It is important to lower the regulator
setting before disconnecting either air line to avoid rapid pressure
changes to
the gauges.
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NOTE A: If your air supply cannot maintain 100 psi on the regulated pressure (left) gauge, you may use a lower pressure. Use the chart below or divide the percentage of leakdown (right) gauge by the regulated pressure (left) gauge pressure for the %. |
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|
% leakage |
1% |
2% |
3% |
4% |
5% |
6% |
7% |
8% |
9% |
10% |
11% |
12% |
13% |
14% |
15% |
|
100 psi on regulated pressure (left) gauge |
99 |
98 |
97 |
96 |
95 |
94 |
93 |
92 |
91 |
90 |
89 |
88 |
87 |
86 |
85 |
|
85 psi on regulated pressure (left) gauge |
84 |
83 |
82 |
81 |
81 |
80 |
79 |
78 |
77 |
76 |
76 |
75 |
74 |
73 |
72 |
|
70 psi on regulated pressure (left) gauge |
69 |
69 |
68 |
67 |
66 |
66 |
65 |
64 |
64 |
63 |
62 |
61 |
61 |
60 |
59 |
|
Talk to your engine builder about what leakdown % should be expected on new and used motors. Most leakage is past the rings which, when worn, cannot seal the compression properly and costs you power. Once you have established a maximum leakage before rebuild, test your motor often to keep it running its best. |
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