How long do you leave the choke on? How old are those plugs? Each bike has its own "personality" and some only need to be choked very briefly. Those plugs could probably be cleaned, but for a couple bucks a plug put some new one's and call it done. I would ask your mechanic how much he adjusted the mixture. It looks like you have the stock air cleaner so it shouldn't have needed much if any with just new pipes - if anything just a tweak to the mixture screw. Also check your air filter to make sure it isn't clogged.