engine bogs on starting off in 1st gear, why?

Hi Guys.. I'm sure this has been discussed before.
78 KZ650C2. after engine is warmed up, when at a stop, when I pull away, the engine rpm seems to go flat, sometimes if I don't throttle up enough, it will die. When I crack the throttle while in neutral, I seem to detect a flat spot, before it revs up. I don't know much about the carbs (which are original, just rebuilt at dealer), but it seems like what occurs when a carb accelerator pump is not working properly. Maybe someone can direct me towards some relevant threads.

What are you using for air filtration?

throw a can of sea foam in the tank

bike has 16k on it - all original
stock air box - new filter
carbs 'rebuilt' at kawai dealer (K&L parts kits)
gas tank clean as a whistle - fresh 91 octane

when I open the throttle it goes flat for just a sec, before taking off.
Actually, a bit of a drop in rpm, so when engaging clutch, it easily wants to stall her out. If I rev it up an couple of times at the stop before engaging, I can prevent the bog out, but it's likely because rpms are higher. idle rpm 1200 some times looks like almost 15. On a few occasions, when I've come to a stop, after a bit, it will slowly drop down to the point where it has quit.

Would assure valve clearances within specs.

And sync carbs with engine running.

Pilot screws will be adjusted incident to sync and idle adjusting.

It stalling at pull-away persists, would clean carb pilot circuits.

And assure correct fuel levels via clear tube testing.

"Rebuilt" carbs might be only parts replacement, and might not include thoroughly cleaning pilot circuits and verifying correct fuel level.

Good Fortune!

just because the carbs were clearned, does not mean they are clean. it is common to have to reclean them.
u have a lean symptom. Could be dirty carbs, air leak?
i would follow Pattons recomedations.