Forget the air chuck and use some cord / rope inside the spark plug hole and turn each cylinder upward on the compression stroke and hold th valve shut. Sometimes you have to wack the spring cap the release the valve keepers and that force can dislodge the valve sealing from the compressed air. I almost lost a valve one time into the cylinder after the compressed air spun the piston down and release the valve from it's seat. The rope won't do any damage and it's a fail proof tool. I have an air chuck tool also, but no longer use it and prefer the rope methode.