Re: Orfice Size For Leak-Down Test
Hooty, I don't want to confuse you, but this is the way I have always understood and used compression and leak down tests.<br /><br />I must
first do a compression check to know that I have low compression, uneven compression, or losing compression at all. Hopefully I have some specs to go by. I only proceed to a leak down test if the compression is poor. A leak down test helps me pinpoint
where I'm losing compression, and on 4-strokes, how much that loss is.<br /><br />The reason I do a leak down is because the mere indication of poor compression alone does not tell me
why the compression is poor. It isn't always the rings. It could be a stuck open valve on a 4-stroke, or an air leak or bad seal on a 2-stroke. The leak down test helps me diagnose this without opening up the engine.<br /><br />But as I mentioned before, leak down tests for 4-strokes and 2-strokes are different. Even the tools are different. This is mainly because of the engine's valves.<br /><br />There is very little reason to pressurize the cylinder of a 2-stroke with compressed air and measure the pressure differential. There are no intake or exhaust valves to leak. Pressurizing the entire cylinder of a 2-stroke would cause leakage out the intake and exhaust ports. You can't hear the air leak out the crankcase tube....cause there isn't one. And if you were concerned with a cracked cylinder wall or head gasket you could simply do a pressure test....no need for the % differential leak down. A "wet"
compression test (which is assumed to have been done prior) already indicated the condition of the rings and cylinder. If the wet compression test brought up the compression significantly, I know I have ring/cylinder problems. But if it didn't bring it up, then the lost compression is going somewhere else.<br /><br />On a 2-stroke there aren't many places for the compression to go. Pretty much seals and gaskets. Doing the low pressure 2-stroke leak down test that I mentioned (hand pump through the intake plug) will pretty much pinpoint the leak. Under these lower pressures with the intake and exhaust openings plugged, even cylinder wall and head gasket failure can be found while rotating the engine and piston to different positions. You don't need 80 psi in the spark plug hole to do this.<br /><br />Now on a 4-stroke the leak can be a result of the valves. A prior "wet" compression check isn't going to be as definitive. The valves add more dimensions to the diagnosis since a wet compression check may also seal the valves if they were leaking. Thus we use the dual gauge differential leak down tester. With it we can not only record the % leakdown of each cylinder, but listen at the exhaust (exhaust valve leak), intake (intake valve leak), crankcase breather (rings/cylinder leak), adjacent cylinder (head gasket leak), or look in the radiator for bubbles. Since we have no way of plugging the intake and exhaust ports we move the crank position until both valves are closed. From here, and with the 4-stroke leak down tester, we can see if the pressure stabilizes or if it leaks some % within specifications. Some 4-strokes mignt have some type of leak down specs available from the factory. Two strokes aren't so lucky.<br /><br />As a note, all pistons/rings will leak somewhat under pressure. It is normal.<br /><br />Hooty, plugging the exhaust on a 2-stroke outboard can be a real bugger, but it beats tearing the engine down if you don't have to. Also be careful applying high pressure to the cylinder of a 2-stroke on the bench....the pressure can rotate the crankshaft and you'll have a wild thing on your hands. Tie it down. Don't blow the gaskets and seals out trying to do a high pressure differential test on a 2-stroke.<br /><br />Bottom line is I don't use a dual gauge differential leak down tester with any size orifice on a 2-stroke. I use a compression tester and a
2-stroke leak down tester. I'm sure everyone has their own way.
