ok... the piece of paper I have used never has being suctioned. I use the starter motor. Admision valve prevents the suction.

If there is suction will be also "exhaustion"... right ? well, the plug/paper is not being spitted out either on exhaust stroke. Just blows out when the comnpression stroke comes in. God... I have made it many times!!!

Its like the ohms law! the flow will go throught the less resistant path! having any kind of plug, air will leave and get in just throught the valves. The minimal amount of suction or exhaustin over the plug won't affect it having the valves in working order.