Any sign of clutch slip? Any brake dragging-you can feel the front disc for heat after a long ride and spin the back wheel after things heat up to temp (again after a ride)
Could think about cam chain and guides...a bit of a weak point...theres a race spec mod you can easily do with the engine open.
Starter clutches a weakpoint in the 750/900 dohc...theres a R6 modification you can do..
If interested in the cam chain mod, let me know and ill send you a pdf...
Try Silvers USA.
Remember tho oil seals and gaskets that are NOS may have degraded if not stored well and if they haven't been produced for 20 or more years.....
The "Z" tends to be the first/second in the model line....similar to the 1979 CX500(z).
Tho' CMSNL lists the USA 1979CB750K as 1979CB750K(Z)....
https://www.cmsnl.com/honda-cb750k-750-four-k-1979-z-usa_model496/partslist/?modelguide_modelid=496
Touched on already-carb balance worth checking.
So (are you confident) its a "hanging idle or a binding issue??....does it return to normal idle without touching anything (eg. without touching the throttle)