If pressurizing cylinders, make sure each piston is at T.D.C., with valves closed and clearance between cams and shims. Leave one plug per cyl. installed. Then listen for air escaping and from where. I made a tester by brazing a fitting on a broken spark plug and hooking rubber hose connected to air chuck to charge each cyl..If you can breath through the cylinder with valves closed, that sounds like a major malfunction! If you blow into the cylinder through the spark plug hole where does the air go? Can you hear it escaping through the throttle bodies? Exhaust? Crankcase breather? That would help narrow down the problem.