Service won't work on carbs without the origional air box...

[seestheday]

yep, what Shiney said.

However, if you're not feeling up to it, the guy that wrote this manual will work on your carbs:
http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revE.pdf

You will just have to send them to him.  Price seems very reasonable to me.