Do the self-extracting juicers (the Omega 4000 and the Miracle Ultramatic) process citrus fruits?

Yes, Citrus fruits, such as oranges, grapefruits, tangerines, have a bitter outer rind, and juice from a "whole" orange would be too bitter to drink. It also contains indigestible chemicals. One solution is to grate away (or peel) the outer rind (the orange coloring on the orange - it is best to leave the white "pith", as valuable nutrients are contained within the white area).