It deals with how well the cylinder is sealing and nothing else. Leakages over 20 generally indicate internal repairs are required.
Dan 39 S Motorcycle Leak Down Test
Acceptable leak down percentage. If the pressure comes up significantly the problem may be worn. Even for normally aspirated engines respectable leakage numbers would be anywhere from 8 to 12 percent with a variation between cylinders of 4 to 5 percent but its possible the variation could. Racing engines would be in the 1 10 range for top performance although this number can vary. So the great thing about the cylinder leak down test is that. The cylinder leak down test reading is expressed in a percentage. The tester is connected to an air source and is attached to the engine through the spark plug hole.
Leak down readings of up to 20 are usually acceptable. If you find one or more cylinders are below the 10 percent range of the others squirt a little oil down the cylinders and try again. One test you could try is to pump a shot or two of engine oil into a cylinder crank the engine over for a dozen rotations or so and try the compression test again. Consequently good cylinder leakdown readings should be below 5 8. Doing a leak down test is a fairly simple matter. In simplified terms if air is pumped into the cylinder at 100 psi and the gauge reads 97 psi then the leak down percentage is 3.
Ideally a baseline number should be taken on a fresh engine and recorded. To get back to your question as long as the cranking compression is relatively equal i wouldnt worry much about the 40 percent leakage. Furthermore the readings are not affected cam timing or even engine cranking speed.
Gallery of Acceptable Leak Down Percentage
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