Help with vacuum leak!

I'm having problems with my idle and I think it's a vacuum leak. If I take the hard plastic vacuum line that connects to the back of the upper intake off, the cars idle becomes steady and runs perfect. When I put my finger over the hole on the back of the intake where the hard plastic vacuum line was connected the car will start to loap. What does this mean?

I also had no luck trying to find a leak by spraying the ethenol carb stuff around the engine.

Thanks

TTT