if its black smoke at high load/revs, sounds like its just way too rich. or its not burning the fuel you have properly, but you say compression is good? so makes me think whoever tuned it for you is a bit heavy handed on the fuel screw. try a test for me. hold the throttle so its at the point before it starts to blow smoke, now flatten it. is there any difference in power? probly not. all that extra fuel is doing nothing but going out the exhaust. you can probly do it yourself. back off the max fuel screw about half a turn (screw it anti-clockwise) until the smoke reduces. you cant hurt anything.