Its all right there in the title. I show you the procedure I use for testing the "air tightness" of chainsaw crankcases. This is one of those tests that would answer a lot of the mystery questions people have about why their saw runs weird. Fast idle? Big bog? Maybe it won't start at all. If you don't perform this test then you're just guessing.
check it out: https://youtu.be/wcGSrAsAhtM