First off, what size jets are in? The factory shop manual says #115 should be there. What condition are the float seals and seats in? Look at the tip, they are rubber and should NOT have a little groove in them from the seat! And the brass seats should be smooth. Stock K8 needles have no height grooves, and are not adjustable, Do the needles in there now have grooves? If so, try a higher groove. The higher the groove, the lower the needle sits. Unfortunately, the manual does not give me a needle length for the later years with the newer carbs. Your float height seems okay, to set them, Remove the bowls, and turn the carbs upside down. The floats should be parallel with the bowl sealing flange, and if you want to measure, they should be 12.5mm from the flange. Are the plugs fouling with gas? or oil? Black smoke usually means fuel, but my experience is if it is running rich enough to smoke, it is too rich to fire. is the air filter clean? try removing the paper element and see what happens then. Oh yeah, look at the tower the main jet and emulsion tube screw into. Do any of them have a crack? That will allow excess fuel in. You can fix it by cutting the bottom off of a spent 45 ACP shell and tapping it over the tower. It is ral easy to overtighten the tube and crack this. And lastly, when it is running, take a wet rag or paper towel and touch each header. You should hear the water hiss from the heat, any one that is cold is not firing. This could be just plug wires if only one, coil if both 1 and 3 or 2 and 4 are cold. I assume you are firing on all 4, since you did not mention any roughness.
The problem here is that so many things can cause this. Even though it is a bike, the same principles of fuel management from the car world apply, as all carb'd piston engines work the same way. Good luck!
