Kawasaki z400 1975 (Engine 79) engine problems

HiI have come to a dead end and i really need a advice here :-)My setup:Kawasaki z400 1975 but engin swaped with an z400 from 79Eletronic egnition mounted Mikuni carbs New wrieings all round. During the build i have had the same problem all along. One cylinder seems to burn just fine but the other one are a mess. Like it almost dosent run on the second cylinder. The sparkplug are becomming black very fast and the exauhst are smoky - at least more than the other one. I guess that the problem could be the valves but i did ajust them to begin with so i am really in distress. Is it possible that a lifter or valve are bended but not noisy? In the worst case scenario i have to swap engine - but witch ones fits a frame from 75?Thanks for you time/Andreas Fredslund Denmark 

Plugs going black and smokey exhaust sounds like excess fuel to me - I'm more inclined to point a finger at the carburettor that is causing your issues.

Having said this, the easiest way to check that the engine is OK is with a compression test which is quick and easy if you have or know someone with a compression tester. That will let you know if there's something wrong with the rings, lifters, or the valves. If that's OK it's not a bad idea to check the valve clearances, but if your compression is fine it's unlikely that it's the valves causing the issues you have.

On the carbs, the first thing I'd check is the float level and make sure it remains constant and at the right height when the petcock is set to 'prime'. You can also check that the pilot screws are set correctly and much the same on both sides. Much more than that and it would be easiest to take them off, clean them, and inspect the slides, jets, and diaphragms.

Thanks for your quick reply. Its only the left cylinder i cant figure out. The plug on the right are just fine. Compression on both are spot on - the carbs have been adjusted several times and every possible position on the air screw has been tested. 
I think we used like 100 hours on trying different setup with the carbs. One perfect one shit. (btw the carbs are all new)
I did a lot of testing the float levels early on and i dont think its due to wrong float levels. 
I had the bike in a stand so i could easily work on the carbs without the fuel tank was in the way al the time.