Markovich wrote: ...It ran so strong that at 70 mph I rolled on the throttle and at around 4500 rpm it broke the clutch loose in 5th gear. Looks like I'll need to look into that as well. No slippage in other gears but I figure the engine is putting out some torque.
If not already done, would perform the clutch push-rod adjustment.
Clutch push rod adjustment -- Need and Purpose
[This is a 1 minute job from start to finish, maybe 2 minutes the first time.]
The FSM covering Z1 and KZ900 includes a periodic maintenance chart requiring clutch push rod adjustment at 2000 mile (3000 km) intervals and explains the need therefor in separate text, which explanation is quoted verbatim as follows:
Besides cable stretch, clutch plate wear also causes the clutch to go out of adjustment, with a decrease in push rod play. Due to this wear, the push rod gradually moves closer to the clutch release lever (at the lower end of the clutch cable) until it touches the adjusting screw. When the rod is touching the screw and therefore has no play, the clutch will not engage fully and clutch slippage will occur. Note that the clutch push rod does not necessarly have play just because the clutch hand lever has play, and so hand lever play alone cannot be used to determine whether or not the clutch requires adjustment.
The FSM covering '77-'79 KZ1000's includes a periodic maintenance chart requiring clutch adjustment at 5000 km intervals, and explains the need therefor in slightly different language, which explanation is quoted verbatim as follows:
Clutch plate wear also causes the clutch to go out of adjustment. This wear causes the play between the push rod and the adjusting screw to gradually diminish until the push rod touches the adjusting screw. When this play is lost, the clutch will not engage fully, causing the clutch to slip.
NOTE: Even though the proper amount of play exists at the clutch lever, clutch lever play alone cannot be used to determine whether or not the clutch requires adjustment.
end of excerpt
While the clutch may indeed be worn out with plates needing replacement, it isn't uncommon to overlook or be unaware of the clutch push rod adjustment feature, especially without a manual. It is worrisome to see anyone of unknown experience, without a manual or failing to consult a manual, proceed with clutch disassembly and plate replacement, without first being aware of and considering the clutch push rod adjustment.
Where both (1) clutch cable slack adjustment is perfect, and (2) clutch push rod adjustment is perfect, and the clutch still slips, suspicion is certainly toward the plates being worn and needing replacement. But other things can also cause slippage, even with proper adjustments and good plates, such as glazed plates or improper oil being used in the crankcase.
Good Fortune!