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Message: [QUOTE="adam728, post: 1146797, member: 55463"] There's another kind of leak down where a constant psi is inputed to the leak down tester, then the pressure the cylinder holds is used to read the leak down. Example - if you regulate 100 psi to the tester and when opened to the cylinder it stays at a constant 90 psi you have 10% leak down. Downsides are that the measurement is much like a compression measurement, irrelavent without a previous reference. One engine might be healthy with 18% leak down, another might be half dead at 12%. The other downside is that it only tests with the cylinder in one position. So if there is damage on the cylinder wall half way through the stroke it will never show on a leak down unless you lock the engine in place mid-stroke. And on a 2 stroke you need to pull the reeds when doing this test (which isn't a common way to test 2 strokes). On a 4 stroke the crankcase is vented, so leakage past the rings can escape. On a 2 stroke reed-valve engine the crankcase should be fairly well sealed, so you'd be possibly applying a lot of pressure to the lower end. I agree that compression and leak downs don't tell the whole story. About a year ago my bike made 160-170 psi on the compression tester, yet had a 1" x 1/2" section of plating missing above the exhaust port. I ran the top end way too long and carbon was building in the ring lands and didn't allow them to move properly, so they started scraping the cylinder. You just can't beat a tear-down-and-measure session when you need to know how your engine is doing. :cool: [/QUOTE]

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