[JAL]

Spraying oil into the cylinders is an old "mechanics trick" which helps the rings seal to improve the results on a compression check. For a true reading, don't do it. Also, allowing the engine to run will help rings seal, especially on two-strokes which burn a mixture of oil and gas.

For the most accurate test, check an engine that has not been run. I have always been slightly suspicious when checking out used outboards or boats when the owner has "warmed up" the motor just prior to my arrival.

There are two types of compression tests- one should do both since they give different results. One measures the pressure that is created while the engine is cranked (compression stroke). The second test measures how well a cylinder holds pressure (leak down test). Since an engine piston utilizes a set of rings on each cylinder for sealing purposes (with different functions), both tests give a better picture of the ring and cylinder wall conditions.

FYI, these tests require two seperate testing tools...