Had a similar problem a while back. The problem with those little Hitachi carbs is that they are getting old . Being old they have been on an off a few times and stripped downn etc.

What happens is the main body warps as does the air horn.That causes an internal vacuum leak between the body and the air horn. When that happens the power valve remains open at all times and produces the rich running you are experiencing.
Run a straight edge over the two mating surfaces and look for a gap. Make sure you place the straight edge on the ears where the screws are inserted. They are particularly prone to warpage.
What you need to do is strip the body down to the bare body and run the top surface over a surface plate with some 180 wet and dry and some kero . That will bring it back to a flat surface.

Similarly with the air horn you can to the same thing. However this is a bit trickier as you have the lugs for the float and the spring loaded plunger for the power valve in the way . What I did was to make a surface plate out of a piece of 100mm by 8mm plate about 150mm long and made some slots in it so that I could move the air horn back and forth about an inch. I then simply place a piece of similar wet and dry over the plate, punched holes in the wet and dry and then sat the air horn on it, aligning the dangly bits with the holes and started rubbing until it was flat. That should sort it out.