And be sure to rotate the collar 90 degrees from where it is in your picture.
Why would you need to do that? The white piece is just a spacer, it doesn't make the seal and doesn't matter where the space goes. In the throat of the carb there should be a black rubber gasket that is formed to fit the end of your intake tube, THAT is the seal. Pull that black seal out of the carb and look at it, note how it is form fit to go right over the end of your intake tube, you need to get the intake into that gasket when the carb is mounted to get it to seal properly.... (make sure you put the gasket back before mounting the carb!)
To mount this carb first loosen the mounting screw quite a bit, use a flat blade to carefully spread the gap in the neck of the carb where the screw goes though. The carb should go right on now, if not, you can try it with the white spacer on the intake (rather than in the carb), but be careful with the white spacer cause they can be brittle and break. Once the carb is on the intake work it back until it bottoms out, then wiggle it around a bit while pressing it onto the intake so the intake seats that last little bit inside the gasket. Now, while pressing the carb firmly onto the intake tighten the screw.
I have never had my carb come loose or leak when mounted this way, never had to use any sealer, the rubber gasket in the throat does a real good job if mounted properly.
