Before going any further, do you have clear title to the bike in your name? The reason I ask is that I've known two people who got vehicles this way and after they'd got the vehicle fixed and running the owner came and got it because the mechanic never followed through with the mechanic's lien procedure to cancel the owner's title.
It would suck for you to fix this bike up for the owner for free.
That being said, you should fully disassemble the carbs to clean using Brakleen (red can) and compressed air, especially the air bleeds and jets. I did the spray and pray method twice to no avail before finally getting it right this way. 14,000 miles later and it still runs and starts like a champ.