Your fuel mixer looks complete to me. There is a circular ring screwed into place and something dark in the middle. To me it is a very typical type of fuel mixer, not carby, no float bowl. If a bowl was to be attached to that position it would have to be open to the rest of the carby and is what I base my opinion on it being a much simpler mixer.
Remove it, strip and clean it but take a picture at every stage to show you how it goes together. I feel that you may have a blockage, maybe the needle and seat or a jet or gallery. If you do take it apart show us the detailed pics so help can be offered.
The theory of a fuel mixer rather than carby is simplistic, you adjust the amount of fuel going into the engine by opening the big thumb screw rather than being netered by idle and main jets. With a tank fitted above your mixer there is no need for a bowl and float but should have a needle n seat.