rpm's slow to drop

I just got done rebuilding the carbs on my 77 KZ1000. The problem I have is the rpm's are slow to drop. If I blip the throttle, the rpm's don't drop as fast as I think they should. It is not the throttle cables because I can see the carb linkage close all the way and the rpm's are still slow to drop. I used 107.5 main jets and 17.5 pilots. Idle mixture screws are set at 1 1/4 turn out. I have synced the carbs. The bike runs great otherwise.

I tried tuning the idle mixture screws, but I could never hear a difference in idle rpm, so I left them at 1 1/4 turns out.

Any ideas??
Thanks

You could have a problem with your vacume lines. Mine were capped off when i got my bike, but the plugs were cracked. after replacing them the rpm's dropped like they should.