Motorcycle Questions? Ask a Mechanic.
Hi Daniel and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, proposed preliminary tries (if have not been done):
If the idle is relatively high and/or can not be adjusted/lowered, then most likely a vacuum leak; i.e. at the intake manifold coupling/carb holder possibly cracked and/or loose clamp.
On the other hand, if the idle is normal, then for a KZ400 Standard either: