Since new, our '93 CB750 Nighthawks were reluctant to start if they sat unused more than 4 - 6 wks., and very reluctant to start if the carbs were drained. We discovered decades ago that they start immediately if the carbs are primed by applying vacuum to the vacuum port of the petcocks for a half-minute or so, to fill the float bowls. There is no 'prime' position on these petcocks like on some vintage petcocks. Once running, they start cold very easily for many days afterward. They're jetted lean from the factory, so it doesn't take much of a drop of fuel level in the float bowls to make cold starts cantankerous.
Looks like your KZ750 has a vacuum petcock. If it doesn't have a 'prime' position, consider trying this:
With the petcock in the 'RUN' or 'RES' position, apply vacuum to the petcock's vacuum port for 15 ~ 30 seconds to make sure the float bowls are full. Doesn't take much - 3" ~ 5" Hg should be plenty if you have a vacuum pump. You may be able to accomplish the same thing by temporarily removing the vacuum hose from the petcock's vacuum port, sliding a clean piece of vacuum hose on to it, and applying vacuum by sucking on the free end, like trying to draw thick milkshake up a straw. Make sure the vacuum supply hose is reattached before starting. Agreed it sounds sounds weird, but works for us.
Good Ridin'
slmjim & Z1BEBE