Clocking the CAM and THUMB LEVER
To clock the cam, the thumb lever should be in a position so that it barely clears the under-side of the hammer, when the thumb lever would be sitting in the breech-closed position. To achieve this, loosen your set screw and then tighten the set-screw only to the degree it is just putting very minor resistance to the shaft. In this way you can rotate your parts to a proper position and they will remain in the position until you secure them. To move the thumb lever upward on the shaft, insert a wedge or old screw driver blade behind the top edge of the cam, up against the breech block by the spring cavity and hold it there while you rotate the lever upward to the proper position. Then tighten the set-screw to the correct degree. Then close the breech and determine if the cam has fully extended into it's recess or cradle in the rear of the breach. If the thumb lever is in the wrong position, it may rest against the lock-plate edge and not allow 100 percent engagement of the cam. It will look somewhat right. It will feel somewhat right. But that breech WILL come open on firing. You have to ensure the cam is resting in it's cradle all the way for this breech design to work. Move your hammer and check the clearance over the top of the thumb-lever. Adjust everything until you get it perfect. Tighten it down.
To test fire, you need to lash your rifle to an old tire with the butt resting either within or on the bead of the tire. Tie off the trigger and load and fire.
Dont close your breech on loose cam. The shaft will slip and you wont be able to get the breech open. This is a real problem. Dont get yourself into this position. Watch what you are doing.
If you dont know what you are doing, and cant tackle this operation, you need to take the rifle to a gunsmith so you dont injure yourself or someone else.