Why would you do a leakdown test vs. a compression test?

Doing a compression test may be necessary on some motors, but if possible I would recommend doing a leakdown test. 

To do a leakdown test, you need a leakdown tester and an air compressor.  To perform a leakdown test, follow manufacturers guidelines.  Keep in mind that with different leakdown testers, procedures and percentages may vary.

With a compression test, you can often get the reading you want by continuing to crank the motor until the correct number appears on the gage.  The leakdown test, in contrast, shows the percentage of air loss in the cylinder.  A healthy engine will show a reading of 5%-12%.  Readings of 13%-20% would indicate mild problems, and may be resolved with a decarbonizer.  Readings above 20% require further investigation.

Not only is a leakdown test more reliable, but it can also help to pinpoint the problem.  For example, if you hear the air leaking out of the exhaust, you could suspect the exhaust valve.  If it's leaking out of the intake, that could mean scored cylinders on a 2-stroke or the intake valve on a 4-stroke.  If you hear air coming out of the oil fill cap on a 4-stroke, this could indicate a scored cylinder or piston problem.