I have some experience in working on these older bikes and can tell you one thing, the wires go bad, and battery power is everything. Make sure the battery is good if any dought replace it. Now check to see if you are getting more than 11.9 volts to the coil withthe key on. The wires going to the coils can go bad. I had a similar problem with low rpm on my 1977 Kaw, yes its a KZ, as soon as you hit higher rpm it was OK at best, but lower rpms it was bouging. Finally test to see if you are getting 12.9 or higher at idle. At 4000 rpm you should be over 14.3v at the battery.
I actually ran a separate positive wire to my coils to a relay off the ignition and man did it get a kick in the pants Whow!
Shortly after I made my own ignition from two GM ignition moduals, It looks crude but a hell of a lot better then the old ignition. Now I just have to find out how to make my own coils
I'm not joking