TOPIC: '79 KZ400 ran well yesterday, really low idle today

Got the carbs professionally tuned a few months ago and it has been running great, until today when a buyer was pulling up to my house. With full choke it starts up, has a low idle, then dies a few minutes later. The plugs were pretty black so it has been running a little rich. I looked at the jets and they look pretty clean and i havent messed with the mixture screw since I got it back from the shop. Any ideas? Im pretty upset because im trying to sell it and this situation makes it seem as if the bike is a piece of crap when its just a random occurance. Any help would be great, thanks!

I see two options:
-the shop didnt tune it right + plugs are finally fouling
-you are using too much choke for too long