I think you're looking in the wrong place.
If I'm reading you right, you have the lever all the way open, the action jams, and you can't close the bolt.
This is actually a mis-feed of the cartridge not an action jam.
What's usually happening is that a cartridge is feeding onto the cartridge lifter properly, but when the bolt starts to push it into the chamber, the bullet catches on the chamber mouth and jams the action open.
First, check to insure the upper receiver cartridge guide spring is in place.
This is a flat spring in the "roof" of the receiver.
In this exploded view, it's part number 27. Make sure it's there and not upside down.
It's not unusual for people to start fitting a scope and think the screw is a scope mount screw. Either they don't notice the guide spring and it falls out, or they get it back in upside down.
http://www.stevespages.com/ipb-marlin-39a.html
If your version has a cartridge guide on the right receiver section, part number 41, make sure it's clean and has proper spring tension.
Next, give the entire action a very thorough cleaning and fresh lube.
Last...... the classic....Try a different brand of ammo. Some bullets just don't feed well and get hung up easily. Try a better brand of ammo.