Throttle needle?

Im having an issue currently with my KZ400. When I start the bike, it will fully rev up in Neutral and sounds great. Once I kick it into 1st or even 2nd gear the bike bogs down like it cant breathe or something and just wont go. Its like its choking itself out. Is this a Carb issue or is something else going on. The carbs had a Full rebuild when the bike was being rebuilt a few months ago.

Stock air box or pot filter, some more info/pics about your bike/engine enviroment are nessesary.
Any other Basic maintance done to the engine, carb cleaning, ignition timing, valve clearance, and btw, get the Service Manual at the articles tab?

Also do a clutch adjustment. This involves not just the cable, but also adjustment of the free play in the disengagement mechanism on the engine. The procedure should be in the factory service manual: kzrider.com/service-manuals

What year is your KZ400?