I have a cns style carb, altho it is not a cns brand.
i too had the same problem. the carb. would work it's way backwards, and fall off the intake manifold.
I used some 100 grit sandpaper to rough up the intake tube, and the collar in the carb that goes over the intake tube (mine has a plastic collar in the carb.)
this worked but the carb is still a little loose and i can move it side to side with some effort. So far, it has not fallen off of the intake tube, and it does not have an air leak.
I'm using the stock intake tube, so i don't know how this would work for you since you are using the billit intake with the o-ring on it.