[Kunama]

Sounds like it raises as it is tail heavy but drops from the vertical due to the fulcrum not being in line with the centerline. When the tube is pushed to the vertical there is more weight on one side of the horizontal axis (fulcrum) thus it drops but won't hold horizontal due to the the fulcrum being too high up on the tube.

The reason it has this 20-40degree range is due to the friction of the bearings, if you had actual roller bearings with near zero friction, the tube would always end up in the same orientation whether released from horizontal or vertical....

Edit: if you want to balance with weights, place the tube horizontal and move it in the tube holders till it balances. Then place the tube in vertical and add counterweights equidistant above and below the level of the pivot point till it balances vertically.

I would remake the bearings and place them correctly.....
Not sure if this all makes sense....

I like the look of the tube, nicely done!